**Meet the editor**

Pietro Mandracci was born in Torino in 1970. He attained an MD in Physics at the Torino University in 1996 and a PhD in Electron Devices at the Trento University in 2001. From 2017 he has been an associate professor at Politecnico di Torino. During his career he has worked on the simulation of solar cells, on the fabrication of UV detectors based on amorphous semiconductors, on

the design and fabrication of multilayer structures for photonic devices, as well as on the synthesis of advanced thin-film materials and nanostructures. His present research interests deal with thin-film technology and nanotechnology, structural, optical, and electrical characterization of materials and nanostructures, as well as their application to electronic and photonic devices, micromechanical systems, and biomedicine.

Contents

**Preface VII**

**Section 1 CVD Fundamentals 1**

Bartasyte

**Section 2 CVD of Thin Films 53**

**Application 55** Marta Januś

Tatsuhiko Aizawa

**Section 3 CVD of 2D Materials 95**

**Graphene 115** Phuong V. Pham

Chapter 1 **Spatial Atomic Layer Deposition 3**

Carmen Jiménez and Daniel Bellet

Chapter 2 **Direct Liquid Injection Chemical Vapor Deposition 29**

Chapter 3 **DLC Layers Created Using CVD Techniques and Their**

**High-Density Plasma Oxidation 75**

**Chemical Vapor Deposition 97**

Chapter 6 **Atmospheric Pressure Chemical Vapor Deposition of**

Chapter 4 **Controlled Post-treatment of Thick CVD-Diamond Coatings by**

Chapter 5 **Large-Area Synthesis and Growth Mechanism of Graphene by**

Chen Wang, Kizhanipuram Vinodgopal and Gui-Ping Dai

David Muñoz-Rojas, Viet Huong Nguyen, César Masse de la Huerta,

Vincent Astié, Cyril Millon, Jean-Manuel Decams and Ausrine

## Contents

## **Preface XI**

#### **Section 1 CVD Fundamentals 1**


## **Section 4 CVD of Nanofibers 135**

Chapter 7 **Chemical Vapor Deposition of Helical Carbon Nanofibers 137** Yoshiyuki Suda

Preface

the synthesis of nanodimensional structures.

valves, are reviewed in detail.

processes involved in the different techniques.

graphene layers, and carbon nanofibers (CNFs), is described.

Chemical vapor deposition (CVD) techniques have played a major role in the development of modern technology since the start of the microelectronics industry, thanks to their ability to synthesize materials of high purity and high quality. In the last decade the rise of nano‐ technology has further improved the importance of CVD technology, thanks to the ability of techniques such as atomic layer deposition (ALD) and vapor liquid solid growth to achieve

This book presents some of the most interesting developments in CVD technology, consider‐ ing both the fundamentals of novel processes and the application of CVD techniques to the synthesis of the new generation of thin films and nanomaterials. A description of particular‐ ly advanced techniques, such as spatial atomic layer deposition (SALD), direct liquid injec‐ tion chemical vapor deposition (DLI-CVD), as well as electron cyclotron resonance chemical vapor deposition (ECR-CVD), is given in considerable detail. Moreover, the application of CVD techniques to materials of great interest, such as diamond-like carbon (DLC) coatings,

The book is divided into four sections, the first of which is dedicated to the description of the fundamentals of particularly advanced CVD techniques, while the following sections are de‐ voted to the application of CVD techniques to specific types of materials: thin films, two-di‐ mensional materials, and nanofibers. The first chapter describes the SALD technique, a variation of ALD in which precursors are continuously supplied in different locations and kept apart by an inert gas region or zone. Film growth is achieved by exposing the substrate to the locations containing the different precursors. Because the purge step is eliminated, the process becomes faster, more versatile, easier, and cheap to scale up. In addition, it can be performed at

The second chapter is devoted to DLI-CVD, which, enabling the usage of solid and liquid precursors, has proven to be one of the most versatile CVD processes to meet the require‐ ments for industrial application. The requirements to be met by the precursors suitable for DLI-CVD, the different classes of available precursors, as well as the models used to de‐ scribe the evaporation process are overviewed in the chapter. Then, different liquid delivery devices used in DLI-CVD, such as capillary tubes, syringes, aerosol delivery systems, and

The third chapter deals with a general description of the CVD growth of DLC thin films, which are known and used in various branches of technology and medicine. A review of the most important growth methods is provided, describing the role of the physicochemical

ambient pressure and even in the open air while not compromising deposition rate.

## Preface

**Section 4 CVD of Nanofibers 135**

**VI** Contents

Yoshiyuki Suda

Chapter 7 **Chemical Vapor Deposition of Helical Carbon Nanofibers 137**

Chemical vapor deposition (CVD) techniques have played a major role in the development of modern technology since the start of the microelectronics industry, thanks to their ability to synthesize materials of high purity and high quality. In the last decade the rise of nano‐ technology has further improved the importance of CVD technology, thanks to the ability of techniques such as atomic layer deposition (ALD) and vapor liquid solid growth to achieve the synthesis of nanodimensional structures.

This book presents some of the most interesting developments in CVD technology, consider‐ ing both the fundamentals of novel processes and the application of CVD techniques to the synthesis of the new generation of thin films and nanomaterials. A description of particular‐ ly advanced techniques, such as spatial atomic layer deposition (SALD), direct liquid injec‐ tion chemical vapor deposition (DLI-CVD), as well as electron cyclotron resonance chemical vapor deposition (ECR-CVD), is given in considerable detail. Moreover, the application of CVD techniques to materials of great interest, such as diamond-like carbon (DLC) coatings, graphene layers, and carbon nanofibers (CNFs), is described.

The book is divided into four sections, the first of which is dedicated to the description of the fundamentals of particularly advanced CVD techniques, while the following sections are de‐ voted to the application of CVD techniques to specific types of materials: thin films, two-di‐ mensional materials, and nanofibers. The first chapter describes the SALD technique, a variation of ALD in which precursors are continuously supplied in different locations and kept apart by an inert gas region or zone. Film growth is achieved by exposing the substrate to the locations containing the different precursors. Because the purge step is eliminated, the process becomes faster, more versatile, easier, and cheap to scale up. In addition, it can be performed at ambient pressure and even in the open air while not compromising deposition rate.

The second chapter is devoted to DLI-CVD, which, enabling the usage of solid and liquid precursors, has proven to be one of the most versatile CVD processes to meet the require‐ ments for industrial application. The requirements to be met by the precursors suitable for DLI-CVD, the different classes of available precursors, as well as the models used to de‐ scribe the evaporation process are overviewed in the chapter. Then, different liquid delivery devices used in DLI-CVD, such as capillary tubes, syringes, aerosol delivery systems, and valves, are reviewed in detail.

The third chapter deals with a general description of the CVD growth of DLC thin films, which are known and used in various branches of technology and medicine. A review of the most important growth methods is provided, describing the role of the physicochemical processes involved in the different techniques.

In the fourth chapter, research concerning CVD diamond coatings post-treated by plasma oxidation, with the aim of allowing the recycling of an original tungsten carbide with cobalt mother tool substrate, is presented. A developed radio-frequency/direct current plasma oxi‐ dation system is stated together with a hollow cathode device to intensify the oxygen ion and electron densities with quantitative plasma diagnosis equipment. Plasma oxidation ash‐ ing conditions are optimized by this quantitative diagnosis toward the perfect ashing of dia‐ mond films with fewer residuals and less tool edge damage. The geometric effect of tool teeth structure on this ashing process is discussed by *in situ* monitoring of plasma, and an engineering solution to this ashing process is proposed for industrial applications.

The fifth chapter presents a review of the preparation methods of high-quality graphene de‐ veloped in recent years, considering the effect of experimental parameters, challenges of preparation of high-quality graphene, and timeline of graphene production. Finally, devel‐ opment trends and application prospects of CVD graphene are described.

The sixth chapter deals with the atmospheric pressure chemical vapor deposition (APCVD) of graphene, which is one of the best synthesis methods due to very low diffusitivity coeffi‐ cient and a critical step for graphene-based device fabrication. High-temperature APCVD processes are being recognized in many technologies, such as solid-state electronic devices, high-quality epitaxial semiconductor films, as well as metal oxide semiconductor transis‐ tors. In this chapter, recent advances in APCVD-based graphene synthesis and related ap‐ plications are addressed.

The seventh chapter deals with helical carbon nanofibers (HCNFs), including carbon nano‐ coils (CNCs), carbon nanotwists, and multiwalled carbon nanotubes, which can be synthe‐ sized by CVD. HCNFs are predicted to have a high mechanical strength and hence are expected to have a use in nanodevices such as electromagnetic wave absorbers and sensors. For nanodevice applications, it is necessary to synthesize HCNFs in high yield and purity. The chapter focuses on CNCs, and describes its history, expected application, and synthesis method. Finally, it introduces the authors' recent studies on the improvement of the purity of CNCs by improving CVD conditions.

> **Prof. Pietro Mandracci** Department of Applied Science and Technology Politecnico di Torino Torino, Italy

**Section 1**

**CVD Fundamentals**

**Section 1**

**CVD Fundamentals**

In the fourth chapter, research concerning CVD diamond coatings post-treated by plasma oxidation, with the aim of allowing the recycling of an original tungsten carbide with cobalt mother tool substrate, is presented. A developed radio-frequency/direct current plasma oxi‐ dation system is stated together with a hollow cathode device to intensify the oxygen ion and electron densities with quantitative plasma diagnosis equipment. Plasma oxidation ash‐ ing conditions are optimized by this quantitative diagnosis toward the perfect ashing of dia‐ mond films with fewer residuals and less tool edge damage. The geometric effect of tool teeth structure on this ashing process is discussed by *in situ* monitoring of plasma, and an

The fifth chapter presents a review of the preparation methods of high-quality graphene de‐ veloped in recent years, considering the effect of experimental parameters, challenges of preparation of high-quality graphene, and timeline of graphene production. Finally, devel‐

The sixth chapter deals with the atmospheric pressure chemical vapor deposition (APCVD) of graphene, which is one of the best synthesis methods due to very low diffusitivity coeffi‐ cient and a critical step for graphene-based device fabrication. High-temperature APCVD processes are being recognized in many technologies, such as solid-state electronic devices, high-quality epitaxial semiconductor films, as well as metal oxide semiconductor transis‐ tors. In this chapter, recent advances in APCVD-based graphene synthesis and related ap‐

The seventh chapter deals with helical carbon nanofibers (HCNFs), including carbon nano‐ coils (CNCs), carbon nanotwists, and multiwalled carbon nanotubes, which can be synthe‐ sized by CVD. HCNFs are predicted to have a high mechanical strength and hence are expected to have a use in nanodevices such as electromagnetic wave absorbers and sensors. For nanodevice applications, it is necessary to synthesize HCNFs in high yield and purity. The chapter focuses on CNCs, and describes its history, expected application, and synthesis method. Finally, it introduces the authors' recent studies on the improvement of the purity

**Prof. Pietro Mandracci**

Politecnico di Torino

Torino, Italy

Department of Applied Science and Technology

engineering solution to this ashing process is proposed for industrial applications.

opment trends and application prospects of CVD graphene are described.

plications are addressed.

VIII Preface

of CNCs by improving CVD conditions.

**Chapter 1**

**Provisional chapter**

**Spatial Atomic Layer Deposition**

**Spatial Atomic Layer Deposition**

DOI: 10.5772/intechopen.82439

In conventional atomic layer deposition (ALD), precursors are exposed sequentially to a substrate through short pulses while kept physically separated by intermediate purge steps. Spatial ALD (SALD) is a variation of ALD in which precursors are continuously supplied in different locations and kept apart by an inert gas region or zone. Film growth is achieved by exposing the substrate to the locations containing the different precursors. Because the purge step is eliminated, the process becomes faster, being indeed compatible with fast-throughput techniques such as roll-to-roll (R2R), and much more versatile and easier and cheap to scale up. In addition, one of the main assets of SALD is that it can be performed at ambient pressure and even in the open air (i.e., without using any deposition chamber at all), while not compromising the deposition rate. In the present chapter, the fundamentals of SALD and its historical development are presented. Then, a succinct description of the different engineering approaches to SALD developed to date is provided. This is followed by the description of the particular fluid dynamics aspects and the engineering challenges associated with SALD. Finally, some of the applications

in which the unique assets of SALD can be exploited are described.

**Keywords:** chemical vapor deposition, spatial atomic layer deposition (SALD), atmospheric pressure, in-line processing, thin films, transparent conductive materials,

The ALD technique was patented in 1977 by Suntola and Antson [1]. It is based on the selfterminating, surface-limited reactions of volatile precursor molecules with a substrate. As

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

David Muñoz-Rojas, Viet Huong Nguyen,

David Muñoz-Rojas, Viet Huong Nguyen, César Masse de la Huerta, Carmen Jiménez

Daniel Bellet

and Daniel Bellet

**Abstract**

fluid dynamics

**1. Introduction**

César Masse de la Huerta, Carmen Jiménez and

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.82439

#### **Spatial Atomic Layer Deposition Spatial Atomic Layer Deposition**

David Muñoz-Rojas, Viet Huong Nguyen, César Masse de la Huerta, Carmen Jiménez and Daniel Bellet David Muñoz-Rojas, Viet Huong Nguyen, César Masse de la Huerta, Carmen Jiménez and Daniel Bellet

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.82439

#### **Abstract**

In conventional atomic layer deposition (ALD), precursors are exposed sequentially to a substrate through short pulses while kept physically separated by intermediate purge steps. Spatial ALD (SALD) is a variation of ALD in which precursors are continuously supplied in different locations and kept apart by an inert gas region or zone. Film growth is achieved by exposing the substrate to the locations containing the different precursors. Because the purge step is eliminated, the process becomes faster, being indeed compatible with fast-throughput techniques such as roll-to-roll (R2R), and much more versatile and easier and cheap to scale up. In addition, one of the main assets of SALD is that it can be performed at ambient pressure and even in the open air (i.e., without using any deposition chamber at all), while not compromising the deposition rate. In the present chapter, the fundamentals of SALD and its historical development are presented. Then, a succinct description of the different engineering approaches to SALD developed to date is provided. This is followed by the description of the particular fluid dynamics aspects and the engineering challenges associated with SALD. Finally, some of the applications in which the unique assets of SALD can be exploited are described.

DOI: 10.5772/intechopen.82439

**Keywords:** chemical vapor deposition, spatial atomic layer deposition (SALD), atmospheric pressure, in-line processing, thin films, transparent conductive materials, fluid dynamics

#### **1. Introduction**

The ALD technique was patented in 1977 by Suntola and Antson [1]. It is based on the selfterminating, surface-limited reactions of volatile precursor molecules with a substrate. As

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

a result, ALD offers unique assets, namely, precise film thickness control to the nanometer, high-quality materials even at low temperature (thanks to the specific highly reactive nature of ALD precursors), and crack-free, compact and conformal film deposition even over highaspect-ratio features. In this first patent, both the temporal and the spatial approaches were already proposed. In the former, precursors are injected in consecutive pulses separated by purge steps (and thus separated in time) [2–5]. In the later, precursors are supplied in different locations and it is the substrate that moves from one location to another (thus separated in space, see **Figure 1**) [6–9].

In all the enclosures proposed in Suntola's first patent, the reaction takes place inside deposition chambers that operate under vacuum. Curiously enough, although the three first reactors proposed by Suntola were spatial, followed by two temporal reactors (**Figure 2**), ALD has traditionally been developed only based on temporal approaches, both industrially and at the laboratory level, until recently. Some years afterward, in a new patent from 1983, Suntola introduced the idea of using an inert gas flow to separate the different precursors, as an alternative to purging under vacuum [10]. He again applied this principle to both temporal and spatial reactors (**Figure 3**). This was supported by an analytical study of precursor diffusion across the inert gas flow as a function of reactor design and gas flow rate and pulse time. The analysis showed that it was possible to conceive SALD reactors that could operate at ambient pressure without the need of unrealistic inert gas barrier flows, making atmospheric pressure SALD (AP-SALD) possible.

the SALD field has experienced a significant growth, as shown by the increase in the number of publications dealing with SALD. In addition, many patents have been published since 1983 due to the appropriateness of the AP-SALD approach for application in industry and mass production. The first one by D. Levy (from Kodak) [12] describes a close-proximity, open-air reactor (see Section 2). Rather quickly, SALD has indeed reached industrial commercialization, and several companies are currently developing, fabricating, and/or selling SALD equip-

**Figure 3.** Two of the enclosures proposed by Suntola in his patent from 1983. Top: temporal approach and bottom:

**Figure 2.** Two of the enclosures proposed by Suntola in his patent from 1977. Left: spatial approach and right: temporal

Spatial Atomic Layer Deposition

5

http://dx.doi.org/10.5772/intechopen.82439

ment, both for industrial production and for academic research [13].

approach.

spatial approach.

Thus, from the chemical point of view, spatial ALD is equivalent to temporal ALD, and therefore, self-terminating, surface-limited reactions occur. As a consequence, SALD also offers a highly precise growth per cycle, the ability to conformally coat high-aspect-ratio features, and the possibility to deposit high-quality films at lower temperatures than with chemical vapor deposition (CVD). But in addition to retaining the unique assets of ALD, SALD can be up to two orders of magnitude faster. Finally, the possibility to perform SALD at atmospheric pressure (AP-SALD), and even in the open air (without using any deposition chamber at all), makes it cheaper and easier to scale up since complex and expensive vacuum processing is not required.

Although the first patent involving atmospheric-pressure SALD dates from 1983, the first report applying the SALD approach was published only in 2004 [11]. After this initial report,

**Figure 1.** Schema of conventional ALD (left) and of SALD (right).

a result, ALD offers unique assets, namely, precise film thickness control to the nanometer, high-quality materials even at low temperature (thanks to the specific highly reactive nature of ALD precursors), and crack-free, compact and conformal film deposition even over highaspect-ratio features. In this first patent, both the temporal and the spatial approaches were already proposed. In the former, precursors are injected in consecutive pulses separated by purge steps (and thus separated in time) [2–5]. In the later, precursors are supplied in different locations and it is the substrate that moves from one location to another (thus separated

In all the enclosures proposed in Suntola's first patent, the reaction takes place inside deposition chambers that operate under vacuum. Curiously enough, although the three first reactors proposed by Suntola were spatial, followed by two temporal reactors (**Figure 2**), ALD has traditionally been developed only based on temporal approaches, both industrially and at the laboratory level, until recently. Some years afterward, in a new patent from 1983, Suntola introduced the idea of using an inert gas flow to separate the different precursors, as an alternative to purging under vacuum [10]. He again applied this principle to both temporal and spatial reactors (**Figure 3**). This was supported by an analytical study of precursor diffusion across the inert gas flow as a function of reactor design and gas flow rate and pulse time. The analysis showed that it was possible to conceive SALD reactors that could operate at ambient pressure without the need of unrealistic inert gas barrier flows, making atmospheric pressure

Thus, from the chemical point of view, spatial ALD is equivalent to temporal ALD, and therefore, self-terminating, surface-limited reactions occur. As a consequence, SALD also offers a highly precise growth per cycle, the ability to conformally coat high-aspect-ratio features, and the possibility to deposit high-quality films at lower temperatures than with chemical vapor deposition (CVD). But in addition to retaining the unique assets of ALD, SALD can be up to two orders of magnitude faster. Finally, the possibility to perform SALD at atmospheric pressure (AP-SALD), and even in the open air (without using any deposition chamber at all), makes it cheaper and easier to scale up since complex and expensive vacuum processing is not required. Although the first patent involving atmospheric-pressure SALD dates from 1983, the first report applying the SALD approach was published only in 2004 [11]. After this initial report,

in space, see **Figure 1**) [6–9].

4 Chemical Vapor Deposition for Nanotechnology

SALD (AP-SALD) possible.

**Figure 1.** Schema of conventional ALD (left) and of SALD (right).

**Figure 2.** Two of the enclosures proposed by Suntola in his patent from 1977. Left: spatial approach and right: temporal approach.

the SALD field has experienced a significant growth, as shown by the increase in the number of publications dealing with SALD. In addition, many patents have been published since 1983 due to the appropriateness of the AP-SALD approach for application in industry and mass production. The first one by D. Levy (from Kodak) [12] describes a close-proximity, open-air reactor (see Section 2). Rather quickly, SALD has indeed reached industrial commercialization, and several companies are currently developing, fabricating, and/or selling SALD equipment, both for industrial production and for academic research [13].

**Figure 3.** Two of the enclosures proposed by Suntola in his patent from 1983. Top: temporal approach and bottom: spatial approach.

## **2. Engineering approaches to SALD**

The spatial approach has proven to be a very versatile one from the engineering point of view since many reactors have been reported to date. Nonobe et al. reported a spatial horizontal quartz hot-wall type reactor for the growth of HfO<sup>2</sup> films on 10 × 10 mm silicon substrates [11]. HfCl4 (evaporated at 433 K) and O<sup>2</sup> are introduced in opposite sides of the reaction zone using a purified N<sup>2</sup> gas flow. A purified N<sup>2</sup> gas curtain is used to prevent precursors' mixing. Finally, the substrate is oscillated between the different zones by a computer-controlled system. Despite the spatial distribution of the precursors, the precursors are supplied in a temporal fashion, since pulses are delivered to the substrate once it is in place.

substrate passing underneath the various spatially separated slits in the outer cylinder. With

Levy et al. from Kodak designed a very neat approach to AP-SALD in 2008 by [17]. In their approach, an injector manifold head is used to supply the different precursors along parallel channels. The precursors are kept separated by adjacent channels supplying an inert gas flows. Effective precursor separation is attained with practical gas flows (up to thousands of sccm (standard cubic centimeters per minute)) by placing the substrate close to the deposition head (around 50 microns). A relative motion between the head and the substrate replicates the ALD cycles yielding film growth (**Figure 6**). As a result, the system works at atmospheric pressure and even operates in the open air, that is, without using a deposition chamber. In the first model, the injector head was placed on top of the substrate. Later, a different design by Kodak was made in which the head lays at the bottom (the outlets facing upward), with the substrate oscillating on top. Later, other groups, such as the Laboratoire des Matériaux et du Génie Physique (LMGP) in Grenoble (**Figure 6**), have designed similar "close-proximity" SALD setups [18, 19]. In situ monitoring of film thickness has been performed by Yersak et al. taking advantage of the open-air environment offered by such AP-SALD approach. In their work, a reflectometer is placed in series with the injection module in order to monitor film

The close proximity approach, and the effective precursor separation that it provides, has been used in other reactors. TNO (Netherlands Organisation for Applied Scientific Research) has developed a cylindrical reactor consisting of separate zones exposing the precursors one by one to a circular substrate that moves underneath the precursor injector [21]. Between and around the reaction zones, shields of inert gas separate the precursor flows. Under the right operating conditions, these gas shields act as gas bearings, facilitating virtually frictionless movement between reactor and substrate. The group in TNO has also used this type of reactor to do plasma-activated SALD [22, 23]. The same group has also developed an R2R type of reactor based on the close proximity approach similar to the one developed by George's

**Figure 5.** SALD reactor developed at the University of Colorado. Reproduced from Sharma, K. et al., J. Vac. Sci. Technol.

deposition rates can even be achieved by adding more modules.

thickness while depositing on a web.to (**Figure 7**) [20].

A 33, 01A132 (2015), with the permission of the American Vacuum Society.

films can be grown at a rate of 2 Å/s, for rotation speeds of 175 RPM. Higher

Spatial Atomic Layer Deposition

7

http://dx.doi.org/10.5772/intechopen.82439

this reactor, Al2

O3

Since in most systems, the precursors are continuously fed into the reactor, SALD has also been referred to as continuous ALD. This is the case of the R2R reactor designed by Lotus Applied Technology [14]. In it, a web substrate is moved between reactor zones containing the different precursors, which are separated by a purge zone (**Figure 4**). Differential pressure and pumping are used to prevent precursor migration into the purge zone. Process pressure is generally about 2 mbar. Web speed can reach tens of meters per minute. The reactor has also been configured for plasma-based ALD mode since at high web speeds, it is found that water desorption is not fast enough, thus producing an anomalous growth (see Section 3 for more details).

The ASTRal group from the Lappeenranta University of technology developed a circular reactor for flexible substrates [15]. The design consists of a cylindrical drum to which the flexible substrate is attached. It then rotates inside a reaction chamber containing different precursors and purge zones. The process is equivalent to conventional ALD since the obtained growth per cycle saturates as the precursor flow rate is increased, reaching approximately 1 Å/cycle for the trimethylaluminum (TMA)/water process for the deposition of Al2 O3 at 100 °C. Prof. Steven M. George's group from the University of Colorado has also developed a modular rotating cylindrical reactor [16]. The design is based on two concentric cylinders. The outer cylinder is fixed and contains several slits that can accept a wide range of modules that attach from the outside (**Figure 5**). The modules can easily move between the various slot positions to perform precursor dosing, purging, or pumping. The inner cylinder rotates with the flexible

**Figure 4.** Low-pressure SALD reactor proposed by Lotus Applied Technology. Reproduced from Dickey, E. et al. J. Vac. Sci. Technol. A 30, 021502 (2012), with the permission of the American Vacuum Society.

substrate passing underneath the various spatially separated slits in the outer cylinder. With this reactor, Al2 O3 films can be grown at a rate of 2 Å/s, for rotation speeds of 175 RPM. Higher deposition rates can even be achieved by adding more modules.

**2. Engineering approaches to SALD**

6 Chemical Vapor Deposition for Nanotechnology

[11]. HfCl4

using a purified N<sup>2</sup>

quartz hot-wall type reactor for the growth of HfO<sup>2</sup>

gas flow. A purified N<sup>2</sup>

(evaporated at 433 K) and O<sup>2</sup>

The spatial approach has proven to be a very versatile one from the engineering point of view since many reactors have been reported to date. Nonobe et al. reported a spatial horizontal

ing. Finally, the substrate is oscillated between the different zones by a computer-controlled system. Despite the spatial distribution of the precursors, the precursors are supplied in a

Since in most systems, the precursors are continuously fed into the reactor, SALD has also been referred to as continuous ALD. This is the case of the R2R reactor designed by Lotus Applied Technology [14]. In it, a web substrate is moved between reactor zones containing the different precursors, which are separated by a purge zone (**Figure 4**). Differential pressure and pumping are used to prevent precursor migration into the purge zone. Process pressure is generally about 2 mbar. Web speed can reach tens of meters per minute. The reactor has also been configured for plasma-based ALD mode since at high web speeds, it is found that water desorption is

not fast enough, thus producing an anomalous growth (see Section 3 for more details).

for the trimethylaluminum (TMA)/water process for the deposition of Al2

The ASTRal group from the Lappeenranta University of technology developed a circular reactor for flexible substrates [15]. The design consists of a cylindrical drum to which the flexible substrate is attached. It then rotates inside a reaction chamber containing different precursors and purge zones. The process is equivalent to conventional ALD since the obtained growth per cycle saturates as the precursor flow rate is increased, reaching approximately 1 Å/cycle

Steven M. George's group from the University of Colorado has also developed a modular rotating cylindrical reactor [16]. The design is based on two concentric cylinders. The outer cylinder is fixed and contains several slits that can accept a wide range of modules that attach from the outside (**Figure 5**). The modules can easily move between the various slot positions to perform precursor dosing, purging, or pumping. The inner cylinder rotates with the flexible

**Figure 4.** Low-pressure SALD reactor proposed by Lotus Applied Technology. Reproduced from Dickey, E. et al. J. Vac.

Sci. Technol. A 30, 021502 (2012), with the permission of the American Vacuum Society.

temporal fashion, since pulses are delivered to the substrate once it is in place.

films on 10 × 10 mm silicon substrates

O3

at 100 °C. Prof.

are introduced in opposite sides of the reaction zone

gas curtain is used to prevent precursors' mix-

Levy et al. from Kodak designed a very neat approach to AP-SALD in 2008 by [17]. In their approach, an injector manifold head is used to supply the different precursors along parallel channels. The precursors are kept separated by adjacent channels supplying an inert gas flows. Effective precursor separation is attained with practical gas flows (up to thousands of sccm (standard cubic centimeters per minute)) by placing the substrate close to the deposition head (around 50 microns). A relative motion between the head and the substrate replicates the ALD cycles yielding film growth (**Figure 6**). As a result, the system works at atmospheric pressure and even operates in the open air, that is, without using a deposition chamber. In the first model, the injector head was placed on top of the substrate. Later, a different design by Kodak was made in which the head lays at the bottom (the outlets facing upward), with the substrate oscillating on top. Later, other groups, such as the Laboratoire des Matériaux et du Génie Physique (LMGP) in Grenoble (**Figure 6**), have designed similar "close-proximity" SALD setups [18, 19]. In situ monitoring of film thickness has been performed by Yersak et al. taking advantage of the open-air environment offered by such AP-SALD approach. In their work, a reflectometer is placed in series with the injection module in order to monitor film thickness while depositing on a web.to (**Figure 7**) [20].

The close proximity approach, and the effective precursor separation that it provides, has been used in other reactors. TNO (Netherlands Organisation for Applied Scientific Research) has developed a cylindrical reactor consisting of separate zones exposing the precursors one by one to a circular substrate that moves underneath the precursor injector [21]. Between and around the reaction zones, shields of inert gas separate the precursor flows. Under the right operating conditions, these gas shields act as gas bearings, facilitating virtually frictionless movement between reactor and substrate. The group in TNO has also used this type of reactor to do plasma-activated SALD [22, 23]. The same group has also developed an R2R type of reactor based on the close proximity approach similar to the one developed by George's

**Figure 5.** SALD reactor developed at the University of Colorado. Reproduced from Sharma, K. et al., J. Vac. Sci. Technol. A 33, 01A132 (2015), with the permission of the American Vacuum Society.

**Figure 6.** Close-proximity SALD approaches. Left: Schemes of the initial SALD systems reported by Kodak. Right: system developed at the LMGP.

They have developed a 27-m long, 4-mm internal diameter tube reactor, and the substrate nanopar-

**Figure 8.** Tubular SALD reactor for coating nanoparticles. Reproduced from van Ommen, J. R. et al., J. Vac. Sci. Technol.

The high throughput and scalability of AP-SALD has already been harnessed, and industrial and laboratory equipment is already commercialized. Solaytec (www.solaytec.com), a spin-off from TNO, developed a linear modular close-proximity reactor specially designed to

similar approach. Beneq has as a well-developed, large area R2R industrial coating SALD system (WCS 600), along with the laboratory-scale SALD systems (TFS 200R, R11). A large-scale, large area SALD reactor has recently been presented for the coating of large flat substrates. Applied Materials and Jusung Engineering also propose SALD reactors. SALD is also being used to functionalize commercial products (e.g., plastics) using home-built equipment.

From the chemical point of view, SALD is equivalent to ALD and in order to have layer-bylayer surface limited growth, the surface to be coated must be saturated, and any excess precursor that stays physisorbed or laying above the surface should be removed in the purge zone. Since in SALD, the precursors are supplied continuously, efficient separation is a key issue and the reactors must be designed so that the inert flow barriers are effective. As we have seen in the previous section, this has been achieved using various engineering approaches. In order to evaluate the effectiveness of a particular reactor and the effect of the different conditions on the

The first main advantage of SALD with respect to ALD is that processing can be much faster. The maximum deposition rate that is achievable depends of course on the type of reactor

passivation layers on silicon solar cells. Levitech (www.levitech.nl) proposes a

into the tube from a vessel in which they are

Spatial Atomic Layer Deposition

9

http://dx.doi.org/10.5772/intechopen.82439

flow (typically called a fluidized bed). The particles are carried inside

. Different sections of the reactor tube contain the different precursor

in this case) are fed at about 1 g min−<sup>1</sup>

A 33, 021513 (2015), with the permission of the American Vacuum Society.

**3. SALD fluid dynamics and modeling**

process, modeling is frequently used, as shown below.

ticles (TiO<sup>2</sup>

deposit Al2

suspended in an upward N2

and inert gas purge zones.

O3

the tube at velocities of 5 ms−<sup>1</sup>

group in Colorado (described above), but in this case, the gas-bearing principle is used (the web floated onto the gas flows coming out of the inner cylinder). There are indeed several patents concerning substrate floatation [24], and this approach has been used for the Solaytec and Levitech commercial systems (see below).

Finally, Ruud van Ommen et al. from the Delft University of Technology have developed a tubular SALD reactor capable of coating nanoparticles thanks to pneumatic transport (**Figure 8**) [25].

**Figure 7.** Implementation of in situ film thickness monitoring to an in-line AP-SALD reactor. Reproduced from Yersak, A. S. eta l., J. Vac. Sci. Technol. A 32, 01A130 (2014), with the permission of the American Vacuum Society.

**Figure 8.** Tubular SALD reactor for coating nanoparticles. Reproduced from van Ommen, J. R. et al., J. Vac. Sci. Technol. A 33, 021513 (2015), with the permission of the American Vacuum Society.

They have developed a 27-m long, 4-mm internal diameter tube reactor, and the substrate nanoparticles (TiO<sup>2</sup> in this case) are fed at about 1 g min−<sup>1</sup> into the tube from a vessel in which they are suspended in an upward N2 flow (typically called a fluidized bed). The particles are carried inside the tube at velocities of 5 ms−<sup>1</sup> . Different sections of the reactor tube contain the different precursor and inert gas purge zones.

The high throughput and scalability of AP-SALD has already been harnessed, and industrial and laboratory equipment is already commercialized. Solaytec (www.solaytec.com), a spin-off from TNO, developed a linear modular close-proximity reactor specially designed to deposit Al2 O3 passivation layers on silicon solar cells. Levitech (www.levitech.nl) proposes a similar approach. Beneq has as a well-developed, large area R2R industrial coating SALD system (WCS 600), along with the laboratory-scale SALD systems (TFS 200R, R11). A large-scale, large area SALD reactor has recently been presented for the coating of large flat substrates. Applied Materials and Jusung Engineering also propose SALD reactors. SALD is also being used to functionalize commercial products (e.g., plastics) using home-built equipment.

## **3. SALD fluid dynamics and modeling**

**Figure 7.** Implementation of in situ film thickness monitoring to an in-line AP-SALD reactor. Reproduced from Yersak,

group in Colorado (described above), but in this case, the gas-bearing principle is used (the web floated onto the gas flows coming out of the inner cylinder). There are indeed several patents concerning substrate floatation [24], and this approach has been used for the Solaytec

**Figure 6.** Close-proximity SALD approaches. Left: Schemes of the initial SALD systems reported by Kodak. Right:

Finally, Ruud van Ommen et al. from the Delft University of Technology have developed a tubular SALD reactor capable of coating nanoparticles thanks to pneumatic transport (**Figure 8**) [25].

and Levitech commercial systems (see below).

system developed at the LMGP.

8 Chemical Vapor Deposition for Nanotechnology

A. S. eta l., J. Vac. Sci. Technol. A 32, 01A130 (2014), with the permission of the American Vacuum Society.

From the chemical point of view, SALD is equivalent to ALD and in order to have layer-bylayer surface limited growth, the surface to be coated must be saturated, and any excess precursor that stays physisorbed or laying above the surface should be removed in the purge zone. Since in SALD, the precursors are supplied continuously, efficient separation is a key issue and the reactors must be designed so that the inert flow barriers are effective. As we have seen in the previous section, this has been achieved using various engineering approaches. In order to evaluate the effectiveness of a particular reactor and the effect of the different conditions on the process, modeling is frequently used, as shown below.

The first main advantage of SALD with respect to ALD is that processing can be much faster. The maximum deposition rate that is achievable depends of course on the type of reactor used, but ultimately, it always depends on the properties of the different precursors and the reaction rate (i.e., reaction kinetics). The time that the substrate is in contact with the precursor flow must be long enough for achieving a complete reaction (i.e., surface saturation). If we take the reaction between TMA and water as an example, the time scale for the TMA halfreaction can be estimated by

reaction can be estimated by

$$t\_{\rm HR} = \frac{A \chi d\_{mm} \rho\_{A|\Omega\_{\parallel}}}{c\_{\rm gr} (M\_{\Lambda\_{\parallel}^{\prime}} / M\_{\rm TM}) \oslash\_{\rm TM}}\tag{1}$$

is a time constant. So, the excess of water molecules absorbed on the substrate as a function of

discussion, the amount of excess growth (i.e., anomalous CVD growth due to remaining physisorbed water in the substrate) is obtained by combining the water accumulation on the surface while the substrate is in the precursor zone and the desorption taking place in the purging zone:

∆*G* = *bIs*(1 − exp( − *t*/*τ*)) exp(−*t*/*τ*) (8)

where b is a constant. Thus, temperature, precursor concentration, and speed need to be carefully adjusted in order to ensure minimal excess of absorbed molecules and effective purg-

The second advantage of SALD is that it can be easily performed at atmospheric pressure and in the open air. For that, the precursors need to be isolated from the atmosphere by the inert gas flow. In its patent from 1983 [10], Suntola developed the equations allowing to estimate the flows that would be required in order to ensure precursor separation and isolation from the ambient. The equations can be applied to temporal ALD reactors and spatial ones, after proper modification taking into account the particular design of each reactor. The equations developed allowed him to predict the viability

An important parameter affecting the SALD process is reaction temperature, since it can affect effective precursor separation and the kinetics of the different reactions and processes (such as desorption) taking place. As a consequence, the effective precursor separation and the maximum achievable speed for a particular reactor and reaction can vary for different deposition temperatures. For example, Pan et al. have performed the simulations of a reactor based on the manifold head injector approach [27]. They have used the following set of equations

*dt* <sup>+</sup> <sup>∇</sup><sup>∙</sup> (*ρ<sup>V</sup>*

→*V* →

) + ∇∙ (*ρ ci*

<sup>→</sup>(*E* + *P*)] = ∇∙

) + ∇∙ (*ρV*

→

*V* →

[

is the velocity vector, P is the static pressure, *ρg*

) = −∇*P* + ∇∙ *τ*

) = −∇∙ *J* →

*k* ∇ *T* − ∑ *i hi J* → *<sup>h</sup>*,*<sup>i</sup>* <sup>+</sup> (*<sup>τ</sup>* ~ ∙ *V* →)

~ + *ρg* <sup>→</sup> + *F*

~

of atmospheric pressure SALD without the need for unrealistically high inert gas flows.

*<sup>m</sup>* (7)

http://dx.doi.org/10.5772/intechopen.82439

Spatial Atomic Layer Deposition

11

O3

) = 0 (9)

<sup>→</sup> (10)

] (12)

are the gravi-

*<sup>m</sup>*,*<sup>i</sup>* + *Ri* (11)

<sup>→</sup> and *F* →

is the stress tensor, ci is the local

<sup>m</sup>). From the above

growth obtained

time spent in the precursor zone can be approximated as

*Q*<sup>0</sup> = *Is*(1 − *exp*(−1/*τ*)) *for t* > *t*

where tm is the time required for having a monolayer (thus Q0 = 0 for t < t

ing. Maydannik et al. used this analytical approach to fit anomalous Al<sup>2</sup>

experimentally between 100 and 150 °C [26].

for their simulations:

\_\_*<sup>d</sup>*

\_\_*<sup>d</sup>*

where ρ is the density, *V*

\_\_*<sup>d</sup>*

*<sup>d</sup>*\_\_\_

*dt <sup>d</sup>*(*ρ<sup>V</sup>* →

*dt <sup>d</sup>*(*E*) <sup>+</sup> <sup>∇</sup>[*<sup>V</sup>*

tational body force and external body forces, respectively, *τ*

→

*dt <sup>d</sup>*(*<sup>ρ</sup> ci*

with A being the area available for deposition, χdmono being the self-limiting film thickness, ρAl2O3 the density of the deposited layer, cgs the stoichiometric coefficient, MAl2O3 and MTMA are the molar masses of Al2 O3 and TMA, and ∅TMA the precursor mass flow rate. Poodt et al. used this approach to estimate a time scale of a few milliseconds for their reactor [21].

Substrate speed thus needs to be adjusted in order to ensure surface saturation. At the same time, speed may also be limited by the desorption kinetics of precursor molecules physisorbed on the substrate. Water, for example, is known to desorb increasingly slower as temperature diminishes (meaning than in conventional ALD, purging steps become much longer, i.e., 30 s and even minutes), and plasma oxidation is a better choice for low temperatures [14]. The rate of desorption is proportional to the surface concentration, Cs, and on the probability that water molecules desorb. Taking the binding energy of absorbed molecules from the Boltzmann equation, the desorption rate per molecule can be expressed as:

$$R\_d = L \exp\left(-\frac{qE}{kT}\right) \tag{2}$$

where L is a constant including the molecular vibrational frequency, E is the molecule binding energy (eV), q is the electronic charge, k is the Boltzmann's constant (JK−<sup>1</sup> ), and T is the temperature (K). The rate of molecules accumulation is then given as:

$$\mathbf{dCs}/dt = \mathbf{Is} - \mathbf{CsR}d\tag{3}$$

where I is the impingement rate of water molecules and s is the sticking coefficient. I can be expressed as

$$I = \frac{P \, N\_A}{\sqrt{2\pi MkT}}\tag{4}$$

where NA is the Avogadro's number, M is the molar mass (kg), and P is the partial pressure of the water vapor. Eq. (3) can thus be solved as

$$C\_s = I\kappa \tau \left(1 - \exp\left(-\frac{t}{\overline{\tau}}\right)\right) \tag{5}$$

where

$$\tau = \frac{1}{R\_{\perp}} = \frac{1}{L} \exp\left(\frac{qE}{kT}\right) \tag{6}$$

is a time constant. So, the excess of water molecules absorbed on the substrate as a function of time spent in the precursor zone can be approximated as

used, but ultimately, it always depends on the properties of the different precursors and the reaction rate (i.e., reaction kinetics). The time that the substrate is in contact with the precursor flow must be long enough for achieving a complete reaction (i.e., surface saturation). If we take the reaction between TMA and water as an example, the time scale for the TMA half-

*HR* <sup>=</sup> *<sup>A</sup>*<sup>χ</sup> *dmono <sup>ρ</sup>Al*<sup>2</sup>

this approach to estimate a time scale of a few milliseconds for their reactor [21].

Boltzmann equation, the desorption rate per molecule can be expressed as:

ing energy (eV), q is the electronic charge, k is the Boltzmann's constant (JK−<sup>1</sup>

temperature (K). The rate of molecules accumulation is then given as:

*O*3

with A being the area available for deposition, χdmono being the self-limiting film thickness, ρAl2O3 the density of the deposited layer, cgs the stoichiometric coefficient, MAl2O3 and MTMA are

Substrate speed thus needs to be adjusted in order to ensure surface saturation. At the same time, speed may also be limited by the desorption kinetics of precursor molecules physisorbed on the substrate. Water, for example, is known to desorb increasingly slower as temperature diminishes (meaning than in conventional ALD, purging steps become much longer, i.e., 30 s and even minutes), and plasma oxidation is a better choice for low temperatures [14]. The rate of desorption is proportional to the surface concentration, Cs, and on the probability that water molecules desorb. Taking the binding energy of absorbed molecules from the

*qE*\_\_\_

where L is a constant including the molecular vibrational frequency, E is the molecule bind-

where I is the impingement rate of water molecules and s is the sticking coefficient. I can be

√ \_\_\_\_\_\_\_

*Rd* = \_\_1 *<sup>L</sup>* exp(

where NA is the Avogadro's number, M is the molar mass (kg), and P is the partial pressure of

*qE*\_\_\_

*O*3 \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ *cgs*(*MAl*<sup>2</sup>

and TMA, and ∅TMA the precursor mass flow rate. Poodt et al. used

*kT*) (2)

*dCs/dt = Is – CsRd* (3)

<sup>2</sup>*MkT* (4)

*τ*)) (5)

*kT*) (6)

), and T is the

(1)

/*MTMA*) ∅*TMA*

reaction can be estimated by

10 Chemical Vapor Deposition for Nanotechnology

the molar masses of Al2

expressed as

where

*t*

O3

*Rd* = *Lexp*(−

*<sup>I</sup>* <sup>=</sup> \_\_\_\_\_\_ *<sup>P</sup> NA*

*Cs* <sup>=</sup> *Is*(<sup>1</sup> <sup>−</sup> exp(−\_\_*<sup>t</sup>*

the water vapor. Eq. (3) can thus be solved as

*τ* = \_\_\_1

$$Q\_0 = \operatorname{Is} \tau \{ 1 - \exp(-1/\tau) \} \text{ for } t \ge t\_m \tag{7}$$

where tm is the time required for having a monolayer (thus Q0 = 0 for t < t <sup>m</sup>). From the above discussion, the amount of excess growth (i.e., anomalous CVD growth due to remaining physisorbed water in the substrate) is obtained by combining the water accumulation on the surface while the substrate is in the precursor zone and the desorption taking place in the purging zone:

$$
\Delta G = bIsr(1 - \exp\{-t/\tau\}) \exp\{-t/\tau\} \tag{8}
$$

where b is a constant. Thus, temperature, precursor concentration, and speed need to be carefully adjusted in order to ensure minimal excess of absorbed molecules and effective purging. Maydannik et al. used this analytical approach to fit anomalous Al<sup>2</sup> O3 growth obtained experimentally between 100 and 150 °C [26].

The second advantage of SALD is that it can be easily performed at atmospheric pressure and in the open air. For that, the precursors need to be isolated from the atmosphere by the inert gas flow. In its patent from 1983 [10], Suntola developed the equations allowing to estimate the flows that would be required in order to ensure precursor separation and isolation from the ambient. The equations can be applied to temporal ALD reactors and spatial ones, after proper modification taking into account the particular design of each reactor. The equations developed allowed him to predict the viability of atmospheric pressure SALD without the need for unrealistically high inert gas flows.

An important parameter affecting the SALD process is reaction temperature, since it can affect effective precursor separation and the kinetics of the different reactions and processes (such as desorption) taking place. As a consequence, the effective precursor separation and the maximum achievable speed for a particular reactor and reaction can vary for different deposition temperatures. For example, Pan et al. have performed the simulations of a reactor based on the manifold head injector approach [27]. They have used the following set of equations for their simulations:

$$\frac{d\rho}{dt} + \nabla \cdot \left(\rho \vec{V}\right) = 0\tag{9}$$

$$\frac{d}{dt}d\left(\rho \vec{V}\right) + \nabla \cdot \left(\rho \vec{V} \vec{V}\right) = -\nabla P + \nabla \cdot \vec{\tau} + \rho \vec{g} + \vec{F} \tag{10}$$

$$\frac{d}{dt}d\left(\rho \ c\_{\parallel}\right) + \nabla \cdot \left(\rho \ c\_{\parallel} \vec{V}\right) = -\nabla \cdot \vec{f}\_{m,l} + R\_{\parallel} \tag{11}$$

$$\frac{d}{dt}d(\rho E) + \nabla \left[\vec{V}(\rho E + P)\right] = \nabla \cdot \left[k \,\nabla \, T - \sum\_{i} h\_{i} \vec{f}\_{i,l} + \left(\vec{\tau} \cdot \vec{V}\right)\right] \tag{12}$$

where ρ is the density, *V* → is the velocity vector, P is the static pressure, *ρg* <sup>→</sup> and *F* → are the gravitational body force and external body forces, respectively, *τ* ~ is the stress tensor, ci is the local molar fraction of species *i, Ri* is the net rate of production of species *i* by chemical reaction, *J* → *m*,*i* is the mass diffusive flux of mixture species *i*, *k* is the material thermal conductivity, *hi* is the enthalpy of mixture species *i*, and *J* → *h*,*i* is the energy diffusive flux of mixture species *i*.

The results of the modeling show that temperatures higher than 250 °C accelerate the diffusive mass transport, thus inducing precursor intermixing. On the other hand, high temperature also increases the film deposition rate and results in a higher saturated growth per cycle (GPC) level. The simulation also showed that the chemical deposition process is highly affected by the flow and concentration of precursors. Surface reaction kinetics of the in-line spatial ALD are thus a function of deposition temperature, flow conditions, reactive surface sites, and precursor distributions.

In another example, Deng et al. have presented a numerical model for atmospheric SALD process, based again on an in-line (i.e., manifold injector head) reactor [28]. The effect of inert gas flow rate, carrier gas flow rate, and precursor concentration (i.e., partial pressure or mass fraction) is addressed. The inert gas flow rate obtained from the model agrees well with the experimental values. Simulation results show that the precursor concentration is the determinant factor governing minimal residual time; the optimum precursor usage is inversely proportional to the carrier gas flow rate. Thus, for a constant carrier gas flow, the optimal precursor usage and the precursor mass fraction form a monotonic decreasing relationship. The gap between the gas head and the substrate, regardless of its specific structures in any SALD system, leads to a low Péclet number, thus implying that precursor diffusion plays a more important role than convection.

Finally, due to the relative movement between the substrate and the different gas zones, the possibility of carrying some precursor molecules from one precursor zone to the second precursor zone across the inert gas barrier is not negligible. This precursor entrainment is very much related to the sample speed and to the inert gas flow rate. The movement between a surface and a gas gives rise to a boundary layer at the surface having a thickness inversely proportional to the square root of the relative velocity [15, 26]. The relationship between layer thickness, *δ*, and substrate speed (Vs) can be expressed as:

$$
\delta \sim \sqrt{\frac{1}{V\_\ast}} \tag{13}
$$

For close-proximity systems, the gap between injector and sample is very important in order to ensure no mixing of the precursors in the gas phase. Nevertheless, for systems for which the gap can be adjusted mechanically, there is also the possibility to perform the deposition in a pseudo-CVD (or spatial CVD, SCVD) mode, that is, allowing a partial mixing of precursors immediately above the substrate. For samples not having complicated or highaspect-ratio features, not having CVD is not an issue and the obtained films are still dense, uniform, and the thickness obtained is proportional to the number of cycles [29]. In order to control the deposition mode, computational fluid dynamic approaches (CFD) can be used to simulate the process (**Figure 9**). It is also important to consider that the properties and the morphologies of the films obtained using the two different modes (SALD vs. SCVD) are

Spatial Atomic Layer Deposition

13

http://dx.doi.org/10.5772/intechopen.82439

The possibility of having precursor molecules being entrained from the region where they are being injected to the region where the other precursor is being injected has also to be considered. This is due to the relative movement of the substrate and the injector or precursor zones, and the existence of a boundary layer [15, 19, 20, 30–32]. Again, CFD approaches can be used to predict whether precursor entrainment is to be expected for a set of conditions with a specific SALD system. For example, while many close-proximity systems operate with gap

**Figure 9.** Comparison between SCVD and SALD modes. Scheme of a close-proximity head having two precursor channels and different gaps between head and sample: (a) 150 μm gap, no mixing in the gas phase, (b) 700 μm gap, resulting in precursor mixing above the sample surface, and (c) and (d) show a CFD stationary simulation of the two different modes. A whole representation of the head was used to simulate the behavior of the gases, but only two precursor channels are shown. A cross section of the head with precursor, inert and evacuation channels is shown in which the normalized concentration of the different precursors is given. Below, normalized precursor concentration

profiles calculated along the substrate line are shown.

usually different [9].

The dependence of the concentration of precursor in the boundary layer is proportional to *δ*, which in turn is proportional to the square root of the substrate residence time (residence time being inversely proportional to substrate velocity). Since precursor concentration falls from maximum to 0 once the substrate enters a purge zone, the total quantity of precursor in the boundary layer can be expressed as G = (1/2)Cs δ, where Cs is the concentration at the substrate surface. G diminishes as the substrate progresses in the purge zone. The flux of precursor out of the boundary layer will depend on the concentration gradient:

$$F(t) = -D\frac{dC}{dx} \approx -D\frac{C\_s}{V\_s} \approx -D\frac{2G}{V\_s^2} \tag{14}$$

where D is the diffusion coefficient of the precursor. A similar approach to the one used above to evaluate the excess growth due to excess precursor physisorption can be used here to evaluate anomalous growth rate due to precursor entrainment.

For close-proximity systems, the gap between injector and sample is very important in order to ensure no mixing of the precursors in the gas phase. Nevertheless, for systems for which the gap can be adjusted mechanically, there is also the possibility to perform the deposition in a pseudo-CVD (or spatial CVD, SCVD) mode, that is, allowing a partial mixing of precursors immediately above the substrate. For samples not having complicated or highaspect-ratio features, not having CVD is not an issue and the obtained films are still dense, uniform, and the thickness obtained is proportional to the number of cycles [29]. In order to control the deposition mode, computational fluid dynamic approaches (CFD) can be used to simulate the process (**Figure 9**). It is also important to consider that the properties and the morphologies of the films obtained using the two different modes (SALD vs. SCVD) are usually different [9].

molar fraction of species *i, Ri*

enthalpy of mixture species *i*, and *J*

12 Chemical Vapor Deposition for Nanotechnology

is the net rate of production of species *i* by chemical reaction, *J*

is the energy diffusive flux of mixture species *i*.

is the mass diffusive flux of mixture species *i*, *k* is the material thermal conductivity, *hi* is the

The results of the modeling show that temperatures higher than 250 °C accelerate the diffusive mass transport, thus inducing precursor intermixing. On the other hand, high temperature also increases the film deposition rate and results in a higher saturated growth per cycle (GPC) level. The simulation also showed that the chemical deposition process is highly affected by the flow and concentration of precursors. Surface reaction kinetics of the in-line spatial ALD are thus a function of deposition temperature, flow conditions, reactive surface sites, and precursor distributions.

In another example, Deng et al. have presented a numerical model for atmospheric SALD process, based again on an in-line (i.e., manifold injector head) reactor [28]. The effect of inert gas flow rate, carrier gas flow rate, and precursor concentration (i.e., partial pressure or mass fraction) is addressed. The inert gas flow rate obtained from the model agrees well with the experimental values. Simulation results show that the precursor concentration is the determinant factor governing minimal residual time; the optimum precursor usage is inversely proportional to the carrier gas flow rate. Thus, for a constant carrier gas flow, the optimal precursor usage and the precursor mass fraction form a monotonic decreasing relationship. The gap between the gas head and the substrate, regardless of its specific structures in any SALD system, leads to a low Péclet number, thus implying that precursor diffusion plays a more important role than convection.

Finally, due to the relative movement between the substrate and the different gas zones, the possibility of carrying some precursor molecules from one precursor zone to the second precursor zone across the inert gas barrier is not negligible. This precursor entrainment is very much related to the sample speed and to the inert gas flow rate. The movement between a surface and a gas gives rise to a boundary layer at the surface having a thickness inversely proportional to the square root of the relative velocity [15, 26]. The relationship between layer

> \_\_\_ \_\_1 *Vs*

The dependence of the concentration of precursor in the boundary layer is proportional to *δ*, which in turn is proportional to the square root of the substrate residence time (residence time being inversely proportional to substrate velocity). Since precursor concentration falls from maximum to 0 once the substrate enters a purge zone, the total quantity of precursor in the

surface. G diminishes as the substrate progresses in the purge zone. The flux of precursor out

*dx* <sup>≈</sup> <sup>−</sup>*<sup>D</sup>*

where D is the diffusion coefficient of the precursor. A similar approach to the one used above to evaluate the excess growth due to excess precursor physisorption can be used here

δ, where Cs

*C*\_\_*s Vs*

≈ −*D* \_\_\_ <sup>2</sup>*<sup>G</sup> Vs*

→ *h*,*i*

thickness, *δ*, and substrate speed (Vs) can be expressed as:

of the boundary layer will depend on the concentration gradient:

to evaluate anomalous growth rate due to precursor entrainment.

*<sup>δ</sup>* <sup>~</sup> <sup>√</sup>

boundary layer can be expressed as G = (1/2)Cs

*F*(*t*) = −*D* \_\_\_ *dC*

→ *m*,*i*

(13)

is the concentration at the substrate

<sup>2</sup> (14)

The possibility of having precursor molecules being entrained from the region where they are being injected to the region where the other precursor is being injected has also to be considered. This is due to the relative movement of the substrate and the injector or precursor zones, and the existence of a boundary layer [15, 19, 20, 30–32]. Again, CFD approaches can be used to predict whether precursor entrainment is to be expected for a set of conditions with a specific SALD system. For example, while many close-proximity systems operate with gap

**Figure 9.** Comparison between SCVD and SALD modes. Scheme of a close-proximity head having two precursor channels and different gaps between head and sample: (a) 150 μm gap, no mixing in the gas phase, (b) 700 μm gap, resulting in precursor mixing above the sample surface, and (c) and (d) show a CFD stationary simulation of the two different modes. A whole representation of the head was used to simulate the behavior of the gases, but only two precursor channels are shown. A cross section of the head with precursor, inert and evacuation channels is shown in which the normalized concentration of the different precursors is given. Below, normalized precursor concentration profiles calculated along the substrate line are shown.

**Figure 10.** Spatial distribution of (a) precursor velocity and (b) precursor concentration above the substrate for the deposition of ZnO from diethyl zinc (DEZ). Here, only the cross-section views including a DEZ inlet and two exhaust inlets are shown. The blue color corresponds to low velocity/precursor concentration, while red or yellow color corresponds to high velocity/precursor concentration.

sizes in the order of 100 μm, Pan et al. have shown that injector-substrate gaps of as a much as 1.5 mm can be used without having precursor cross-talk [27]. **Figure 10** shows a simulation of the effect of substrate speed in the gas velocity and precursor concentration for the closeproximity SALD system at LMGP [9, 33]. As it can be seen, with the conditions used for the simulation, no precursor entrainment takes place between different precursor regions. The evaluation of precursor entrainment has also been monitored experimentally [30]. Finally, it has recently been demonstrated that atmospheric pressure SALD can be used to coat porous substrates with high throughput as long as high precursor partial pressures and molar flows can be reached [34].

## **4. Taking advantage of SALD**

To date, several materials including intrinsic, doped and mixed oxides, metals and recently organic coatings, have been produced using SALD reactors, as detailed in **Table 1**. The applications range from active and passive components for thin film transistors (TFTs), barriers passivation layers to active components for new-generation solar cells. Some examples are briefly described next.

results prompted the creation of the spinoff company Solaytec (see above) [21]. The same group has dedicated efforts to deposit transparent conductive oxides (TCO) layers and more complex oxides for application in photovoltaic devices [51, 56, 84, 85]. Prof. Driscoll's group was the first one to apply the SALD technique for the deposition of active layers for new-generation solar cells [52, 71–74]. For example, Muñoz-Rojas et al. showed that a 15-nm thick amorphous TiO<sup>2</sup> layer can act as an efficient hole blocking layer in bulk heterojunction solar cells [72]. Thanks to the high quality offered by the SALD technique, an extremely thin (yet crack and pinhole free) layer can be used. And as a result, low deposition temperatures can be used, since the lower con-

rates are two orders of magnitude faster than for other low-temperature, atmospheric deposition methods (**Figure 11**). Such high throughput characteristic of SALD is of high relevance for

ited by SALD can have a beneficial impact on the stability of hybrid perovskite solar cells [62, 69].

organic-based PV technology. Recently, it has also been shown that SnO<sup>x</sup>

**Material References**

Cu2

Nb2

Doped oxides ZnO:N [46, 55, 63, 67]

Zn1 –x Mgx

Mixed oxides IGZO [79]

Metals Ag [23]

Organic materials Polyamide [81]

O<sup>3</sup> [14–16, 19–21, 26, 31, 35–50] ZnO [18, 36, 39, 45, 46, 51–67]

Spatial Atomic Layer Deposition

15

http://dx.doi.org/10.5772/intechopen.82439

SnO<sup>x</sup> [68, 69] TiO<sup>2</sup> [14, 18, 70–72]

O<sup>5</sup> [71] NiO<sup>x</sup> [91] ZrO<sup>2</sup> [92, 93] MoO<sup>x</sup> [94]

Al:ZnO [36, 56, 75]

In:ZnO [51, 77] ZnO:S [58] TiO2–xCl2x [78]

CIGS [80]

Pt [25]

O [46, 64, 76]

O [46, 73, 74]

Intrinsic metal oxides HfO<sup>2</sup> [11] Al2

is compensated by having a very thin layer. In addition, deposition

and ZnO films depos-

ductivity of amorphous TiO<sup>2</sup>

**Table 1.** Materials deposited by SALD to date.

The first application of SALD, from Kodak, involved the deposition of ZnO and Al<sup>2</sup> O3 layers as components in TFTs [17]. The same group has published different studies on this line, including the use of growth inhibitors for the deposition of patterned films, and thus showing that selected area deposition is also possible with spatial ALD [36, 39, 82, 83]. The group from TNO used their rotary reactor to deposit Al2 O3 passivation layers for silicon solar cells, and the good


**Table 1.** Materials deposited by SALD to date.

**Figure 10.** Spatial distribution of (a) precursor velocity and (b) precursor concentration above the substrate for the deposition of ZnO from diethyl zinc (DEZ). Here, only the cross-section views including a DEZ inlet and two exhaust inlets are shown. The blue color corresponds to low velocity/precursor concentration, while red or yellow color

sizes in the order of 100 μm, Pan et al. have shown that injector-substrate gaps of as a much as 1.5 mm can be used without having precursor cross-talk [27]. **Figure 10** shows a simulation of the effect of substrate speed in the gas velocity and precursor concentration for the closeproximity SALD system at LMGP [9, 33]. As it can be seen, with the conditions used for the simulation, no precursor entrainment takes place between different precursor regions. The evaluation of precursor entrainment has also been monitored experimentally [30]. Finally, it has recently been demonstrated that atmospheric pressure SALD can be used to coat porous substrates with high throughput as long as high precursor partial pressures and molar flows

To date, several materials including intrinsic, doped and mixed oxides, metals and recently organic coatings, have been produced using SALD reactors, as detailed in **Table 1**. The applications range from active and passive components for thin film transistors (TFTs), barriers passivation layers to active components for new-generation solar cells. Some examples are briefly described next.

components in TFTs [17]. The same group has published different studies on this line, including the use of growth inhibitors for the deposition of patterned films, and thus showing that selected area deposition is also possible with spatial ALD [36, 39, 82, 83]. The group from TNO

O3

passivation layers for silicon solar cells, and the good

layers as

The first application of SALD, from Kodak, involved the deposition of ZnO and Al<sup>2</sup>

O3

corresponds to high velocity/precursor concentration.

14 Chemical Vapor Deposition for Nanotechnology

can be reached [34].

**4. Taking advantage of SALD**

used their rotary reactor to deposit Al2

results prompted the creation of the spinoff company Solaytec (see above) [21]. The same group has dedicated efforts to deposit transparent conductive oxides (TCO) layers and more complex oxides for application in photovoltaic devices [51, 56, 84, 85]. Prof. Driscoll's group was the first one to apply the SALD technique for the deposition of active layers for new-generation solar cells [52, 71–74]. For example, Muñoz-Rojas et al. showed that a 15-nm thick amorphous TiO<sup>2</sup> layer can act as an efficient hole blocking layer in bulk heterojunction solar cells [72]. Thanks to the high quality offered by the SALD technique, an extremely thin (yet crack and pinhole free) layer can be used. And as a result, low deposition temperatures can be used, since the lower conductivity of amorphous TiO<sup>2</sup> is compensated by having a very thin layer. In addition, deposition rates are two orders of magnitude faster than for other low-temperature, atmospheric deposition methods (**Figure 11**). Such high throughput characteristic of SALD is of high relevance for organic-based PV technology. Recently, it has also been shown that SnO<sup>x</sup> and ZnO films deposited by SALD can have a beneficial impact on the stability of hybrid perovskite solar cells [62, 69].

uniform along the pores. Coating of the AAO was used to evaluate the conditions for coating porous substrates. They then used optimized conditions to coat porous Li battery electrodes.

Chen et al. have developed a system in which two injector heads are combined for the deposi-

ness and overall composition could be turned from crystalline to amorphous by increasing the number of bilayers (i.e., by reducing the thickness of each individual bilayer). Optical transmittance of the laminates increases in the visible range with the number of bilayers. The refractive index of laminates increases with decreasing bilayer thickness, thus demonstrating the possibility for tuning. The conductivity of nanolaminates is higher than for ZnO films due to an increase in carrier density. On the other hand, mobility decreases with respect to

Recent reports from Muñoz-Rojas and coworkers have shown that SALD is an ideal technique to enhance the stability of transparent electrodes based on metallic nanowires (MNWs). In these electrodes, metallic nanowires are deposited by different low-cost, scalable methods, such as spray coating, to obtain a random network. Above a certain areal mass density of the electrode (i.e., amount of nanowires), percolation of the nanowires occurs and the electrode becomes conductive. Yet, since most of the space is empty, the electrode remains transparent. The best electrodes based on AgNWs are comparable and even better than the stateof-the-art indium tin oxide (ITO, the most commonly used transparent conductive oxide) electrodes [87–89]. In addition, electrodes based on MNWs are flexible, which in combination with the soft deposition methods used to fabricate the networks, and make this technology

**Figure 12.** Left: SEM and TEM images of bare and ZnO-coated AgNWs. Right: failure voltage of AgNWs networks versus ZnO coating thickness. The insets correspond to SEM images of a network in which the wires have started spheroidizing due to thermal and electrical instabilities (top), and to a coated network in which the nanowires are intact after being subjected to electrical stress (bottom). The discontinuous line is obtained using a simple model that shows that the enhanced stability of the coated networks is due to hindered Ag atomic diffusion (see reference for details) [61].

Other recent studies show the benefits of SALD for the field of energy storage [45, 78].

SALD coating used was observed to enhance the capacity stability of the electrodes.

nanolaminates [18]. They show that nanolaminates with the same thick-

Spatial Atomic Layer Deposition

17

http://dx.doi.org/10.5772/intechopen.82439

The Al2

O3

tion of ZnO/TiO<sup>2</sup>

intrinsic ZnO layers.

**Figure 11.** Top: TEM images of TiO<sup>2</sup> amorphous films deposited at 100 °C. Bottom: cell efficiency compared with equivalent cells from the literature in terms of blocking layer fabrication time [72].

SALD has also been used for LEDs, in some cases to deposit active ZnO layers for polymer and hybrid perovskite-based diodes [54, 76], and in another case using Al2 O3 as permeation barrier for flexible organic LEDs [49]. Other authors have also demonstrated the suitability of SALD for depositing barrier and encapsulation layers both on plastic and paper substrates [14, 31, 40, 48].

K. Sharma et al. used their cylindrical reactor (the University of Colorado, see Section 2) to study the deposition of ZnO on 25 μm thick flexible anodized aluminum oxide (AAO) templates [86]. They used DEZ and ozone as precursors, and the reaction temperature was set at 50 °C. The results show that pores with 100 nm of diameter can be conformally coated for substrate speeds ≤10 RPM. This corresponds to an aspect ratio of 250, and the corresponding residence time is 48 ms. For faster substrate speeds or smaller pores, the films were not uniform along the pores. Coating of the AAO was used to evaluate the conditions for coating porous substrates. They then used optimized conditions to coat porous Li battery electrodes. The Al2 O3 SALD coating used was observed to enhance the capacity stability of the electrodes. Other recent studies show the benefits of SALD for the field of energy storage [45, 78].

Chen et al. have developed a system in which two injector heads are combined for the deposition of ZnO/TiO<sup>2</sup> nanolaminates [18]. They show that nanolaminates with the same thickness and overall composition could be turned from crystalline to amorphous by increasing the number of bilayers (i.e., by reducing the thickness of each individual bilayer). Optical transmittance of the laminates increases in the visible range with the number of bilayers. The refractive index of laminates increases with decreasing bilayer thickness, thus demonstrating the possibility for tuning. The conductivity of nanolaminates is higher than for ZnO films due to an increase in carrier density. On the other hand, mobility decreases with respect to intrinsic ZnO layers.

Recent reports from Muñoz-Rojas and coworkers have shown that SALD is an ideal technique to enhance the stability of transparent electrodes based on metallic nanowires (MNWs). In these electrodes, metallic nanowires are deposited by different low-cost, scalable methods, such as spray coating, to obtain a random network. Above a certain areal mass density of the electrode (i.e., amount of nanowires), percolation of the nanowires occurs and the electrode becomes conductive. Yet, since most of the space is empty, the electrode remains transparent. The best electrodes based on AgNWs are comparable and even better than the stateof-the-art indium tin oxide (ITO, the most commonly used transparent conductive oxide) electrodes [87–89]. In addition, electrodes based on MNWs are flexible, which in combination with the soft deposition methods used to fabricate the networks, and make this technology

SALD has also been used for LEDs, in some cases to deposit active ZnO layers for polymer and

for flexible organic LEDs [49]. Other authors have also demonstrated the suitability of SALD for depositing barrier and encapsulation layers both on plastic and paper substrates [14, 31, 40, 48]. K. Sharma et al. used their cylindrical reactor (the University of Colorado, see Section 2) to study the deposition of ZnO on 25 μm thick flexible anodized aluminum oxide (AAO) templates [86]. They used DEZ and ozone as precursors, and the reaction temperature was set at 50 °C. The results show that pores with 100 nm of diameter can be conformally coated for substrate speeds ≤10 RPM. This corresponds to an aspect ratio of 250, and the corresponding residence time is 48 ms. For faster substrate speeds or smaller pores, the films were not

O3

amorphous films deposited at 100 °C. Bottom: cell efficiency compared with

as permeation barrier

hybrid perovskite-based diodes [54, 76], and in another case using Al2

equivalent cells from the literature in terms of blocking layer fabrication time [72].

**Figure 11.** Top: TEM images of TiO<sup>2</sup>

16 Chemical Vapor Deposition for Nanotechnology

**Figure 12.** Left: SEM and TEM images of bare and ZnO-coated AgNWs. Right: failure voltage of AgNWs networks versus ZnO coating thickness. The insets correspond to SEM images of a network in which the wires have started spheroidizing due to thermal and electrical instabilities (top), and to a coated network in which the nanowires are intact after being subjected to electrical stress (bottom). The discontinuous line is obtained using a simple model that shows that the enhanced stability of the coated networks is due to hindered Ag atomic diffusion (see reference for details) [61].

compatible with plastic and other soft substrates. Finally, electrodes based on MNWs are more transparent to the infrared part of the spectrum than the transparent conductive oxides (TCO). Despite these advantages, electrodes made of MNWs suffer from thermal, electrical, and chemical instabilities. A solution to increase the stability of the nanowires is to coat them with a thin oxide film, which prevents the diffusion of metal atoms in the wires and the reaction of the same with the atmosphere. ALD is a perfect technique for that since the coatings obtained are dense and crack free. But conventional ALD is usually performed in vacuum, and growth rate is too low. SALD on the other hand can be processed in the open air and offers a high deposition throughput. The close-proximity system at the LMGP has thus been used to coat MNW electrodes. In a first example, ZnO coatings of different thicknesses were deposited on an AgNWs electrode. The oxide coating resulted in a better electrical and thermal stability of the electrodes, and the improvement observed was directly proportional to the thickness of the oxide layer (**Figure 12**) [61]. In another example, Al2 O3 coatings were used to protect CuNWs electrode. The encapsulation of the wires with the Al2 O3 layer prevented the oxidation of the wires upon heating in the atmosphere, which allowed the fabrication of stable transparent heaters [44].

**Acknowledgements**

**Conflict of interest**

**Author details**

and Daniel Bellet

**References**

1.1940727

DOI: 10.1021/cr900056b

The authors declare no conflict of interest.

David Muñoz-Rojas acknowledges funding through the Marie Curie Actions (FP7/2007-2013, grant agreement no. 631111). DMR also acknowledges funding from the European Union's Horizon 2020 FETOPEN-1-2016-2017 research and innovation programme under grant agreement 801464. The authors thank the Agence Nationale de Recherche (ANR, France) for funding via the projects INDEED (no. ANR-15-CE05-0019) and DESPATCH (no. ANR-16-CE05-0021). This project has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement no. 801464. Viet Huong Nguyen thanks the "ARC Energy Auvergne-Rhône Alpes" for the economic support for Ph.D. grants. César Masse de la Huerta acknowledges and thanks the funding received by CONACYT, Mexico. This work is supported by the French National Research Agency in the framework of the "Investissements d'avenir" program (ANR-15-IDEX-02) through the project Eco-SESA. This project was also

Spatial Atomic Layer Deposition

19

http://dx.doi.org/10.5772/intechopen.82439

financially supported by the "Carnot Énergies du futur" Institute (Aldash Project).

David Muñoz-Rojas\*, Viet Huong Nguyen, César Masse de la Huerta, Carmen Jiménez

[1] Suntola TS, Antson J, Method for Producing Compound Thin Films. US 4,058,430. 1977 [2] George SM. Atomic layer deposition: An overview. Chemical Reviews. 2010;**110**:111-131.

[3] Leskelä M, Ritala M, Nilsen O. Novel materials by atomic layer deposition and molecular layer deposition. MRS Bulletin. 2011;**36**:877-884. DOI: 10.1557/mrs.2011.240

[4] Puurunen RL. Surface chemistry of atomic layer deposition: A case study for the trimethylaluminum/water process. Journal of Applied Physics. 2005;**97**:121301. DOI: 10.1063/

\*Address all correspondence to: david.munoz-rojas@grenoble-inp.fr

Université Grenoble Alpes, CNRS, Grenoble INP, LMGP, Grenoble, France

#### **5. Conclusions**

Although already patented in 1977 at the same time than temporal ALD, spatial ALD has only been developed with the beginning of the twenty-first century. The possibility to perform ALD in much faster deposition rate, at atmospheric pressure and even in the open air has converted SALD in to a technique that is gaining much attention and momentum. SALD has also made the transition from laboratory to industrial scale, and different commercial systems are already available both for laboratory and production scale, while home-made systems are also used. SALD is a very flexible deposition technology that allows a high degree of design freedom, as shown by the increasing number of reactors being developed. Because in SALD, precursors are continuously being injected, efficient separation by the inert gas flow/zone needs to be ensured. The analytical study of fluid dynamics and modeling are thus commonly used during the design of reactors and to evaluate optimum deposition conditions.

Many materials have been already deposited using SALD. Initial works focused on Al2 O3 and ZnO. Later on, other binary oxides such as Cu<sup>2</sup> O, TiO<sup>2</sup> , or Nb2 O5 have been developed, together with the deposition of more complex oxides (including doping and mixes oxides), metals, and even organic films, in which is the first example of spatial molecular layer deposition (SMLD) [90]. The main applications of SALD to date have been for the deposition of components for TFTs, solar cells, and LEDs and for the deposition of barrier and encapsulation layers. SALD has been demonstrated on flexible substrates such as paper or plastic and even on features with high aspect ratios.

The combination of the unique assets of ALD with faster deposition rates and air processing, design flexibility, and easy scalability are expected to make SALD one of the main thin-film deposition techniques in the coming years.

## **Acknowledgements**

compatible with plastic and other soft substrates. Finally, electrodes based on MNWs are more transparent to the infrared part of the spectrum than the transparent conductive oxides (TCO). Despite these advantages, electrodes made of MNWs suffer from thermal, electrical, and chemical instabilities. A solution to increase the stability of the nanowires is to coat them with a thin oxide film, which prevents the diffusion of metal atoms in the wires and the reaction of the same with the atmosphere. ALD is a perfect technique for that since the coatings obtained are dense and crack free. But conventional ALD is usually performed in vacuum, and growth rate is too low. SALD on the other hand can be processed in the open air and offers a high deposition throughput. The close-proximity system at the LMGP has thus been used to coat MNW electrodes. In a first example, ZnO coatings of different thicknesses were deposited on an AgNWs electrode. The oxide coating resulted in a better electrical and thermal stability of the electrodes, and the improvement observed was directly proportional

O3

O3

coatings were

layer pre-

O3 and

have been developed, together

to the thickness of the oxide layer (**Figure 12**) [61]. In another example, Al2

tion of stable transparent heaters [44].

18 Chemical Vapor Deposition for Nanotechnology

ZnO. Later on, other binary oxides such as Cu<sup>2</sup>

deposition techniques in the coming years.

with high aspect ratios.

**5. Conclusions**

used to protect CuNWs electrode. The encapsulation of the wires with the Al2

vented the oxidation of the wires upon heating in the atmosphere, which allowed the fabrica-

Although already patented in 1977 at the same time than temporal ALD, spatial ALD has only been developed with the beginning of the twenty-first century. The possibility to perform ALD in much faster deposition rate, at atmospheric pressure and even in the open air has converted SALD in to a technique that is gaining much attention and momentum. SALD has also made the transition from laboratory to industrial scale, and different commercial systems are already available both for laboratory and production scale, while home-made systems are also used. SALD is a very flexible deposition technology that allows a high degree of design freedom, as shown by the increasing number of reactors being developed. Because in SALD, precursors are continuously being injected, efficient separation by the inert gas flow/zone needs to be ensured. The analytical study of fluid dynamics and modeling are thus commonly

used during the design of reactors and to evaluate optimum deposition conditions.

Many materials have been already deposited using SALD. Initial works focused on Al2

O, TiO<sup>2</sup>

with the deposition of more complex oxides (including doping and mixes oxides), metals, and even organic films, in which is the first example of spatial molecular layer deposition (SMLD) [90]. The main applications of SALD to date have been for the deposition of components for TFTs, solar cells, and LEDs and for the deposition of barrier and encapsulation layers. SALD has been demonstrated on flexible substrates such as paper or plastic and even on features

The combination of the unique assets of ALD with faster deposition rates and air processing, design flexibility, and easy scalability are expected to make SALD one of the main thin-film

, or Nb2

O5

David Muñoz-Rojas acknowledges funding through the Marie Curie Actions (FP7/2007-2013, grant agreement no. 631111). DMR also acknowledges funding from the European Union's Horizon 2020 FETOPEN-1-2016-2017 research and innovation programme under grant agreement 801464. The authors thank the Agence Nationale de Recherche (ANR, France) for funding via the projects INDEED (no. ANR-15-CE05-0019) and DESPATCH (no. ANR-16-CE05-0021). This project has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement no. 801464. Viet Huong Nguyen thanks the "ARC Energy Auvergne-Rhône Alpes" for the economic support for Ph.D. grants. César Masse de la Huerta acknowledges and thanks the funding received by CONACYT, Mexico. This work is supported by the French National Research Agency in the framework of the "Investissements d'avenir" program (ANR-15-IDEX-02) through the project Eco-SESA. This project was also financially supported by the "Carnot Énergies du futur" Institute (Aldash Project).

## **Conflict of interest**

The authors declare no conflict of interest.

## **Author details**

David Muñoz-Rojas\*, Viet Huong Nguyen, César Masse de la Huerta, Carmen Jiménez and Daniel Bellet

\*Address all correspondence to: david.munoz-rojas@grenoble-inp.fr

Université Grenoble Alpes, CNRS, Grenoble INP, LMGP, Grenoble, France

## **References**


[5] Knez M, Nielsch K, Niinistö L. Synthesis and surface engineering of complex nanostructures by atomic layer deposition. Advanced Materials. 2007;**19**:3425-3438. DOI: 10.1002/ adma.200700079

[18] Chen R, Lin J-L, He W-J, Duan C-L, Peng Q, Wang X-L, et al. Spatial atomic layer deposi-

[19] Ryan Fitzpatrick P, Gibbs ZM, George SM. Evaluating operating conditions for continuous atmospheric atomic layer deposition using a multiple slit gas source head. Journal of Vacuum Science & Technology A—Vacuum Surfaces and Films. 2012;**30**:01A136. DOI:

[20] Yersak AS, Lee YC, Spencer JA, Groner MD. Atmospheric pressure spatial atomic layer deposition web coating with in situ monitoring of film thickness. Journal of Vacuum Science & Technology A—Vacuum Surfaces and Films. 2014;**32**:01A130. DOI: 10.1116/1.4850176 [21] Poodt P, Lankhorst A, Roozeboom F, Spee K, Maas D, Vermeer A. High-speed spatial atomic-layer deposition of aluminum oxide layers for solar cell passivation. Advanced

[22] Poodt P, Kniknie B, Branca A, Winands H, Roozeboom F. Patterned deposition by plasma enhanced spatial atomic layer deposition. Physica Status Solidi (RRL)—Rapid

[23] van den Bruele FJ, Smets M, Illiberi A, Creyghton Y, Buskens P, Roozeboom F, et al. Atmospheric pressure plasma enhanced spatial ALD of silver. Journal of Vacuum Science & Technology A—Vacuum Surfaces and Films. 2015;**33**:01A131. DOI: 10.1116/1.4902561

[24] Levy DH. Deposition System and Method Using a Delivery Head Separated Froma

[25] van Ommen JR, Kooijman D, de Niet M, Talebi M, Goulas A. Continuous production of nanostructured particles using spatial atomic layer deposition. Journal of Vacuum Science & Technology A—Vacuum Surfaces and Films. 2015;**33**:021513. DOI: 10.1116/1.4905725 [26] Maydannik PS, Kaariainen TO, Cameron DC. Continuous atomic layer deposition: Explanation for anomalous growth rate effects. Journal of Vacuum Science & Technology

[27] Pan D, Jen T-C, Yuan C. Effects of gap size, temperature and pumping pressure on the fluid dynamics and chemical kinetics of in-line spatial atomic layer deposition of

[28] Deng Z, He W, Duan C, Chen R, Shan B.Mechanistic modeling study on process optimization and precursor utilization with atmospheric spatial atomic layer deposition. Journal of Vacuum Science & Technology A—Vacuum Surfaces and Films. 2016;**34**:01A108. DOI:

[29] Hoye RLZ, Muñoz-Rojas D, Musselman KP, Vaynzof Y, MacManus-Driscoll JL. Synthesis and modeling of uniform complex metal oxides by close-proximity atmospheric pressure chemical vapor deposition. ACS Applied Materials & Interfaces. 2015;**7**:

. International Journal of Heat and Mass Transfer. 2016;**96**:189-198. DOI: 10.1016/j.

Substrate by Gas Pressure. US 2009/0130858, US 2010/0311130 Al. 2009

A—Vacuum Surfaces and Films. 2012;**30**:01A122. DOI: 10.1116/1.3662861

Surfaces and Films. 2016;**34**:051502. DOI: 10.1116/1.4955289

Materials. 2010;**22**:3564-3567. DOI: 10.1002/adma.201000766

Research Letters. 2011;**5**:165-167. DOI: 10.1002/pssr.201004542

nanolaminates. Journal of Vacuum Science & Technology A—Vacuum

Spatial Atomic Layer Deposition

21

http://dx.doi.org/10.5772/intechopen.82439

tion of ZnO/TiO<sup>2</sup>

10.1116/1.3664765

Al2 O3

ijheatmasstransfer.2016.01.034

10684-10694. DOI: 10.1021/am5073589

10.1116/1.4932564


[18] Chen R, Lin J-L, He W-J, Duan C-L, Peng Q, Wang X-L, et al. Spatial atomic layer deposition of ZnO/TiO<sup>2</sup> nanolaminates. Journal of Vacuum Science & Technology A—Vacuum Surfaces and Films. 2016;**34**:051502. DOI: 10.1116/1.4955289

[5] Knez M, Nielsch K, Niinistö L. Synthesis and surface engineering of complex nanostructures by atomic layer deposition. Advanced Materials. 2007;**19**:3425-3438. DOI: 10.1002/

[6] Poodt P, Cameron DC, Dickey E, George SM, Kuznetsov V, Parsons GN, et al. Spatial atomic layer deposition: A route towards further industrialization of atomic layer deposition. Journal of Vacuum Science & Technology A—Vacuum Surfaces and Films. 2012;**30**:010802.

[7] Muñoz-Rojas D, MacManus-Driscoll J. Spatial atmospheric atomic layer deposition: A new laboratory and industrial tool for low-cost photovoltaics. Materials Horizons. 2014;**1**:

[8] Musselman KP, Uzoma CF, Miller MS.Nanomanufacturing: High-throughput, cost-effective deposition of atomic scale thin films via atmospheric pressure spatial atomic layer deposition. Chemistry of Materials. 2016;**28**:8443-8452. DOI: 10.1021/acs.chemmater.6b03077

[9] Muñoz-Rojas D, Nguyen VH, Masse de la Huerta C, Aghazadehchors S, Jiménez C, Bellet D. Spatial Atomic Layer Deposition (SALD), an emerging tool for energy materials. Application to new-generation photovoltaic devices and transparent conductive materials. Comptes Rendus Physique. 2017;**1**:1-10. DOI: 10.1016/j.crhy.2017.09.004

[10] Suntola TS, Pakkala AJ, Lindfors SG. Apparatus for Performing Growth of Compound

[13] Muñoz-Rojas D, Maindron T, Esteve A, Piallat F, Kools J, Decamps M. Speeding up the

[14] Dickey E, Barrow WA. High rate roll to roll atomic layer deposition, and its application to moisture barriers on polymer films. Journal of Vacuum Science & Technology A—

[15] Maydannik PS, Kääriäinen TO, Cameron DC. An atomic layer deposition process for moving flexible substrates. Chemical Engineering Journal. 2011;**171**:345-349. DOI: 10.1016/j.

[16] Sharma K, Hall RA, George SM. Spatial atomic layer deposition on flexible substrates using a modular rotating cylinder reactor. Journal of Vacuum Science & Technology A—

[17] Levy DH, Freeman D, Nelson SF, Cowdery-Corvan PJ, Irving LM. Stable ZnO thin film transistors by fast open air atomic layer deposition. Applied Physics Letters. 2008;**92**:192101.

unique assets of atomic layer deposition. Materials Today Chemistry. 2018

Vacuum Surfaces and Films. 2012;**30**:021502. DOI: 10.1116/1.3678486

Vacuum Surfaces and Films. 2015;**33**:01A132. DOI: 10.1116/1.4902086

nano-films by atomic layer

under atmospheric pressure. Solid State Sciences. 2004;**6**:

adma.200700079

DOI: 10.1116/1.3670745

20 Chemical Vapor Deposition for Nanotechnology

314-320. DOI: 10.1039/c3mh00136a

Thin Films. US 4,389,973. 1983

deposition using HfCl4

cej.2011.03.097

DOI: 10.1063/1.2924768

[11] Nonobe S, Takahashi N, Nakamura T. Preparation of HfO<sup>2</sup>

and O<sup>2</sup>

1217-1219. DOI: 10.1016/j.solidstatesciences.2004.07.030

[12] Levy DH. Process for Atomic Layer Deposition. US 7,413,982 B2. 2008


[30] Maydannik PS, Kääriäinen TO, Lahtinen K, Cameron DC, Söderlund M, Soininen P, et al. Roll-to-roll atomic layer deposition process for flexible electronics encapsulation applications. Journal of Vacuum Science and Technology A. 2014;**32**:51603. DOI: 10.1116/1.4893428

[42] Poodt P, Illiberi A, Roozeboom F.The kinetics of low-temperature spatial atomic layer deposition of aluminum oxide. Thin Solid Films. 2013;**532**:22-25. DOI: 10.1016/j.tsf.2012.10.109

[43] Mousa MBM, Ovental JS, Brozena AH, Oldham CJ, Parsons GN. Modeling and experimental demonstration of high-throughput flow-through spatial atomic layer deposition

[44] Celle C, Cabos A, Fontecave T, Laguitton B, Benayad A, Guettaz L, et al. Oxidation of copper nanowire based transparent electrodes in ambient conditions and their stabilization by encapsulation: Application to transparent film heaters. Nanotechnology. 2018;**29**:085701

[45] Yersak AS, Sharma K, Wallas JM, Dameron AA, Li X, Yang Y, et al. Spatial atomic layer deposition for coating flexible porous Li-ion battery electrodes Spatial atomic layer deposition for coating flexible porous Li-ion battery electrodes. Journal of Vacuum Science and

[46] Musselman KP, Muñoz-Rojas D, Hoye RLZ, Sun H, Sahonta S-L, Croft E, et al. Rapid openair deposition of uniform, nanoscale, functional coatings on nanorod arrays. Nanoscale

[47] Franke S, Baumkötter M, Monka C, Raabe S, Caspary R, Johannes H-H, et al. Alumina films as gas barrier layers grown by spatial atomic layer deposition with trimethylaluminum and different oxygen sources. Journal of Vacuum Science & Technology A—

[48] Ali K, Ali J, Mehdi SM, Choi K-H, An YJ. Rapid fabrication of Al2O3 encapsulations for organic electronic devices. Applied Surface Science. 2015;**353**:1186-1194. DOI: 10.1016/j.

[49] Choi H, Shin S, Jeon H, Choi Y, Kim J, Kim S, et al. Fast spatial atomic layer deposition of

[50] Ali K, Choi K-H. Low-temperature roll-to-roll atmospheric atomic layer deposition of

[51] Illiberi A, Scherpenborg R, Roozeboom F, Poodt P. Atmospheric spatial atomic layer deposition of In-doped ZnO. ECS Journal of Solid State Science and Technology.

[52] Hoye RLZ, Muñoz-Rojas D, Iza DC, Musselman KP, MacManus-Driscoll JL. High performance inverted bulk heterojunction solar cells by incorporation of dense, thin ZnO layers made using atmospheric atomic layer deposition. Solar Energy Materials & Solar

[53] Illiberi A, Scherpenborg R, Theelen M, Poodt P, Roozeboom F. On the environmental stability of ZnO thin films by spatial atomic layer deposition. Journal of Vacuum Science & Technology A—Vacuum Surfaces and Films. 2013;**31**:061504. DOI: 10.1116/1.4816354

thin films. Langmuir. 2014;**30**:14195-14203. DOI: 10.1021/la503406v

 at low temperature (<100 °C) as a gas permeation barrier for flexible organic lightemitting diode displays. Journal of Vacuum Science & Technology A—Vacuum Surfaces

Vacuum Surfaces and Films. 2017;**35**:01B117. DOI: 10.1116/1.4971173

Technology A. 2018;**36**:031517. DOI: 10.1116/1.5022077

Technology A. 2018;**36**:01A123. DOI: 10.1116/1.5006670

Horizons. 2017;**2**:110-117. DOI: 10.1039/C6NH00197A

and Films. 2016;**34**:01A121. DOI: 10.1116/1.4934752

Cells. 2013;**116**:197-202. DOI: 10.1016/j.solmat.2013.04.020

2014;**3**:P111-P114. DOI: 10.1149/2.002405jss

coatings on textiles at atmospheric pressure. Journal of Vacuum Science and

Spatial Atomic Layer Deposition

23

http://dx.doi.org/10.5772/intechopen.82439

of Al2 O3

apsusc.2015.07.032

Al2 O3

Al2 O3


[42] Poodt P, Illiberi A, Roozeboom F.The kinetics of low-temperature spatial atomic layer deposition of aluminum oxide. Thin Solid Films. 2013;**532**:22-25. DOI: 10.1016/j.tsf.2012.10.109

[30] Maydannik PS, Kääriäinen TO, Lahtinen K, Cameron DC, Söderlund M, Soininen P, et al. Roll-to-roll atomic layer deposition process for flexible electronics encapsulation applications. Journal of Vacuum Science and Technology A. 2014;**32**:51603. DOI:

[31] Maydannik PS, Plyushch A, Sillanpää M, Cameron DC, Cameron DC. Spatial atomic layer

[32] Levy DH, Nelson SF, Freeman D. Oxide electronics by spatial atomic layer deposition. Journal of Display Technology. 2009;**5**:484-494. Available from: http://jdt.osa.org/abstract.

[33] Nguyen VH, Resende J, Jiménez C, Deschanvres J, Carroy P, Muñoz D, et al. Deposition of ZnO based thin films by atmospheric pressure spatial atomic layer deposition for application in solar cells. Journal of Renewable and Sustainable Energy. 2017;**9**:021203.

[34] Poodt P, Mameli A, Schulpen J, Roozeboom WMMEK, Roozeboom F. Effect of reactor pressure on the conformal coating inside porous substrates by atomic layer deposition. Journal of Vacuum Science and Technology A. 2017:021502. DOI: 10.1116/1.4973350 [35] Suh S, Park S, Lim H, Choi Y-J, Seong Hwang C, Joon Kim H, et al. Investigation on spatially separated atomic layer deposition by gas flow simulation and depositing Al<sup>2</sup>

Journal of Vacuum Science & Technology A—Vacuum Surfaces and Films. 2012;**30**:051504.

aluminum-doped ZnO using poly(vinyl pyrrolidone). Chemistry of Materials. 2014;**26**:

thin films on PET substrates. Chemical Vapor Deposition. 2014;**20**:1-8. DOI:

[37] Ali K, Choi K-H, Muhammad NM. Roll-to-roll atmospheric atomic layer deposition

[38] Poodt P, Knaapen R, Illiberi A, Roozeboom F, van Asten A. Low temperature and rollto-roll spatial atomic layer deposition for flexible electronics. Journal of Vacuum Science & Technology A—Vacuum Surfaces and Films. 2012;**30**:01A142. DOI: 10.1116/1.3667113

[39] Levy DH, Ellinger CR, Nelson SF. Metal-oxide thin-film transistors patterned by print-

[40] Lahtinen K, Maydannik P, Johansson P, Kääriäinen T, Cameron DC, Kuusipalo J. Utilisation of continuous atomic layer deposition process for barrier enhancement of extrusion-coated paper. Surface and Coatings Technology. 2011;**205**:3916-3922. DOI: 10.1016/j.surfcoat.2011.

[41] Poodt P, van Lieshout J, Illiberi A, Knaapen R, Roozeboom F, van Asten A. On the kinetics of spatial atomic layer deposition. Journal of Vacuum Science & Technology A—Vacuum

ing. Applied Physics Letters. 2013;**103**:043505. DOI: 10.1063/1.4816322

Surfaces and Films. 2013;**31**:01A108. DOI: 10.1116/1.4756692

[36] Ellinger CR, Nelson SF. Selective area spatial atomic layer deposition of ZnO, Al<sup>2</sup>

electronics encapsulation. Journal of Vacuum Science and Technology A. 2015;**33**:031603.

O and O<sup>3</sup>

oxidant chemistry for flexible

O3 films.

> O3 , and

10.1116/1.4893428

22 Chemical Vapor Deposition for Nanotechnology

DOI: 10.1116/1.4914079

DOI: 10.1063/1.4979822

DOI: 10.1116/1.4737123

10.1002/cvde.201407126

of Al2 O3

02.009

1514-1522. DOI: 10.1021/cm402464z

deposition: Performance of low temperature H2

cfm?URI=jdt-5-12-484 [Accessed: Jun 27, 2012]


[54] Hoye RLZ, Chua MR, Musselman KP, Li G, Lai M-L, Tan Z-K, et al. Enhanced performance in fluorene-free organometal halide perovskite light-emitting diodes using tunable, low electron affinity oxide electron injectors. Advanced Materials. 2015;**20**: 1414-1419. DOI: 10.1002/adma.201405044

[66] Hoye RLZ, Muñoz-Rojas D, Nelson SF, Illiberi A, Poodt P, Roozeboom F, et al. Research update: Atmospheric pressure spatial atomic layer deposition of ZnO thin films: Reactors,

[67] Musselman KP, Albert-Seifried S, Hoye RLZ, Sadhanala A, Muñoz-Rojas D, MacManus-Driscoll JL, et al. Improved exciton dissociation at semiconducting polymer:ZnO donor: acceptor interfaces via nitrogen doping of ZnO. Advanced Functional Materials. 2014;**24**:

[68] Hoffmann L, Theirich D, Schlamm D, Hasselmann T, Pack S, Brinkmann KO, et al. Atmospheric pressure plasma enhanced spatial atomic layer deposition of SnOx as conductive gas diffusion barrier. Journal of Vacuum Science and Technology A. 2018;**36**:01A112.

[69] Hoffmann L, Brinkmann KO, Malerczyk J, Rogalla D, Becker T, Theirich D, et al. Spatial atmospheric pressure atomic layer deposition of tin oxide as an impermeable Electron extraction layer for perovskite solar cells with enhanced thermal stability. ACS Applied

[70] Aghaee M, Maydannik PS, Johansson P, Kuusipalo J, Creatore M, Homola T, et al. Low

[71] Armstrong CL, Price MB, Muñoz-Rojas D, Davis NJKL, Abdi-Jalebi M, Friend RH, et al. Influence of an inorganic interlayer on exciton separation in hybrid solar cells. ACS

[72] Muñoz-Rojas D, Sun H, Iza DC, Weickert J, Chen L, Wang H, et al. High-speed atmo-

[73] Marin AT, Muñoz-Rojas D, Iza DC, Gershon T, Musselman KP, MacManus-Driscoll JL. Novel atmospheric growth technique to improve both light absorption and charge collec-

[74] Muñoz-Rojas D, Jordan M, Yeoh C, Marin AT, Kursumovic A, Dunlop L, et al. Growth

[75] Fullager DB, Boreman GD, Ellinger CD, Hofmann T. Broadband optical properties of aluminium zinc oxide thin films prepared by spatial atomic layer deposition. Thin Solid

[76] Hoye RLZ, Musselman KP, Chua MR, Sadhanala A, Raninga RD, Friend RH, et al. Bright and efficient blue polymer light emitting. Journal of Materials Chemistry C. 2015;**3**:

layer deposition (AALD) at 225 °C and below. AIP Advances. 2012;**2**:042179. DOI: 10.1063/

for inverted bulk heterojunction solar cells. Progress in Photovoltaics: Research and

O thin film solar cells. Advanced Functional Materials. 2013;**23**:3413-3419.

Vacuum Science & Technology A—Vacuum Surfaces and Films. 2015;**33**:041512. DOI:

films. Journal of

Spatial Atomic Layer Deposition

25

http://dx.doi.org/10.5772/intechopen.82439

blocking layers at 100 °C

O) films by fast atmospheric atomic

Materials & Interfaces. 2018;**10**:6006-6013. DOI: 10.1021/acsami.7b17701

temperature temporal and spatial atomic layer deposition of TiO<sup>2</sup>

Nano. 2015;**9**:11863-11871. DOI: 10.1021/acsnano.5b05934

Applications. 2013;**21**:393-400. DOI: 10.1002/pip.2380

Films. 2018;**653**:267-273. DOI: 10.1016/j.tsf.2018.03.047

9327-9336. DOI: 10.1039/C5TC01581B

spheric atomic layer deposition of ultra thin amorphous TiO<sup>2</sup>

mobility, p-type Copper(I)oxide (Cu2

doping, and devices. APL Materials. 2015;**3**:040701. DOI: 10.1063/1.4916525

3562-3570 DOI: 10.1002/adfm.201303994

DOI: 10.1116/1.5006781

10.1116/1.4922588

tion in ZnO/Cu<sup>2</sup>

 V−<sup>1</sup> s−<sup>1</sup>

of 5 cm2

1.4771681

DOI: 10.1002/adfm.201203243


[66] Hoye RLZ, Muñoz-Rojas D, Nelson SF, Illiberi A, Poodt P, Roozeboom F, et al. Research update: Atmospheric pressure spatial atomic layer deposition of ZnO thin films: Reactors, doping, and devices. APL Materials. 2015;**3**:040701. DOI: 10.1063/1.4916525

[54] Hoye RLZ, Chua MR, Musselman KP, Li G, Lai M-L, Tan Z-K, et al. Enhanced performance in fluorene-free organometal halide perovskite light-emitting diodes using tunable, low electron affinity oxide electron injectors. Advanced Materials. 2015;**20**:

[55] Ehrler B, Musselman KP, Böhm ML, Morgenstern FSF, Vaynzof Y, Walker BJ, et al. Preventing interfacial recombination in colloidal quantum dot solar cells by doping the

[56] Illiberi A, Scherpenborg R, Wu Y, Roozeboom F, Poodt P. Spatial atmospheric atomic

[57] Illiberi A, Roozeboom F, Poodt P. Spatial atomic layer deposition of zinc oxide thin films. ACS Applied Materials & Interfaces. 2012;**4**:268-272. DOI: 10.1021/am2013097

[58] Frijters CH, Poodt P, Illiberi A. Atmospheric spatial atomic layer deposition of Zn(O,S) buf-

[59] Nelson SF, Levy DH, Tutt LW, Burberry M. Cycle time effects on growth and transistor characteristics of spatial atomic layer deposition of zinc oxide. Journal of Vacuum Science & Technology A—Vacuum Surfaces and Films. 2012;**30**:01A154. DOI: 10.1116/1.3670878

[60] Nguyen VH, Gottlieb U, Valla A, Muñoz D, Bellet D, Muñoz-Rojas D. Electron tunneling through grain boundaries in transparent conductive oxides and implications for electrical conductivity: The case of ZnO:Al thin films. Materials Horizons. 2018;**5**:715-726. DOI:

[61] Khan A, Nguyen VH, Muñoz-Rojas D, Aghazadehchors S, Jiménez C, Nguyen ND, et al. Stability enhancement of silver nanowire networks with conformal ZnO coatings deposited by atmospheric pressure spatial atomic layer deposition. ACS Applied Materials &

[62] Najafi M, Zardetto V, Zhang D, Koushik D, Dorenkampe M, Creatore M, et al. Highly efficient semitransparent p-i-n planar perovskite solar cells by atmospheric pressure spatial atomic layer deposited ZnO. Solar RRL. 2018;**2**:1800147. DOI: 10.1002/solr.201800147

[63] Dunlop L, Kursumovic A, MacManus-Driscoll JL. Reproducible growth of p-type ZnO:N using a modified atomic layer deposition process combined with dark annealing. Applied

[64] Hoye RLZ, Ehrler B, Böhm ML, Muñoz-Rojas D, Altamimi RM, Alyamani AY, et al. Improved open-circuit voltage in ZnO-PbSe quantum dot solar cells by understanding and reducing losses arising from the ZnO conduction band tail. Advanced Energy

[65] Nandakumar N, Dielissen B, Garcia-Alonso D, Liu Z, Gortzen R, Kessels WMME, et al. Resistive intrinsic ZnO films deposited by ultrafast spatial ALD for PV applications. IEEE Journal of Photovoltaics. 2015;**5**:1462-1469. DOI: 10.1109/JPHOTOV.2015.2438644

Interfaces. 2018;**10**:19208-19217. DOI: 10.1021/acsami.8b03079

Physics Letters. 2008;**93**:172111. DOI: 10.1063/1.3000604

Materials. 2014;**4**:1301544. DOI: 10.1002/aenm.201301544

Zn1-xO. ACS Applied Materials & Interfaces. 2013;**5**:13124-13128.

solar cells. Solar Energy Materials & Solar Cells. 2016;**155**:356-361.

metal oxide. ACS Nano. 2013;**7**:4210-4220. DOI: 10.1021/nn400656n

1414-1419. DOI: 10.1002/adma.201405044

layer deposition of Alx

24 Chemical Vapor Deposition for Nanotechnology

DOI: 10.1021/am404137e

fer layer for Cu(In,Ga)Se2

10.1039/C8MH00402A

DOI: 10.1016/j.solmat.2016.06.016


[77] Illiberi A, Frijters C, Ruth M, Bremaud D, Poodt P, Roozeboom F, et al. Atmospheric spatial atomic layer deposition of ZnOS buffer layers for flexible Cu(In,Ga)Se<sup>2</sup> solar cells. Journal of Vacuum Science and Technology A. 2018;**36**:051511. DOI: 10.1116/1.5040457

[89] Sannicolo T, Lagrange M, Cabos A, Celle C, Simonato J, Bellet D. Metallic nanowirebased transparent electrodes for next generation flexible devices : A review. Small.

[90] Gregorczyk K, Knez M. Progress in materials science hybrid nanomaterials through molecular and atomic layer deposition: Top down, bottom up , and in-between approaches to new materials. Progress in Materials Science. 2016;**75**:1-37. DOI: 10.1016/j.pmatsci.2015.06.004 [91] Zhao B, Lee LC, Yang L, Pearson AJ, Lu H, She X, et al. In-situ atmospheric deposition of ultra-smooth nickel oxide for efficient perovskite solar cells. ACS Applied Materials &

[92] Creyghton Y, Illiberi A, Mione A, Boekel W. van, Debernardi N, Seitz M, et al. Plasma-

[93] Mione MA, Katsouras I, Creyghton Y, van Boekel W, Maas J, Gelinck G, et al. Atmospheric pres-

[94] Gregory G, Wilson M, Ali H, Davis KO. Thermally Stable Molybdenum Oxide Hole-Selective Contacts Deposited using Spatial Atomic Layer Deposition. 2018 IEEE 7th World Conference Photovolt. Energy Convers. (WCPEC)(A Jt. Conf. 45th IEEE PVSC, 28th PVSEC 34th EU PVSEC) (pp. 2006-2009). IEEE. 2006-2009 (2018). DOI: 10.1109/PVSC.2018.8547343

O<sup>3</sup>

and ZrO<sup>2</sup>

for low-temperature, large-area applications. ECS

Y. Creyghton. ECS

Spatial Atomic Layer Deposition

27

http://dx.doi.org/10.5772/intechopen.82439

2016;**12**:6052-6075. DOI: 10.1002/smll.201602581

Interfaces. 2018. DOI: 10.1021/acsami.8b15503

sure plasma enhanced spatial ALD of ZrO<sup>2</sup>

Transation. 2016;**75**:11-19

enhanced atmospheric-pressure spatial ALD of Al2

Journal of Solid State Science and Technology. 2017;**6**:N243-N249


[89] Sannicolo T, Lagrange M, Cabos A, Celle C, Simonato J, Bellet D. Metallic nanowirebased transparent electrodes for next generation flexible devices : A review. Small. 2016;**12**:6052-6075. DOI: 10.1002/smll.201602581

[77] Illiberi A, Frijters C, Ruth M, Bremaud D, Poodt P, Roozeboom F, et al. Atmospheric spatial atomic layer deposition of ZnOS buffer layers for flexible Cu(In,Ga)Se<sup>2</sup>

[78] Moitzheim S, Balder JE, Poodt P, Unnikrishnan S, De Gendt S, Vereecken PM. Chlorine

[79] Illiberi A, Cobb B, Sharma A, Grehl T, Brongersma H, Roozeboom F, et al. Spatial atmo-

[80] Illiberi A, Grob F, Frijters C, Poodt P, Ramachandra R, Winands H, et al. High rate (~7 nm/s),

[81] Higgs DJ, Dumont JW, Sharma K, George SM. Spatial molecular layer deposition of polyamide thin films on flexible polymer substrates using a rotating cylinder reactor. Journal of Vacuum Science & Technology A—Vacuum Surfaces and Films. 2018;**36**:01A117 [82] Ellinger CR, Nelson SF. Design freedom in multilayer thin-film devices. ACS Applied

[83] Nelson SF, Ellinger CR, Levy DH. Improving yield and performance in ZnO thin-film transistors made using selective area deposition. ACS Applied Materials & Interfaces.

[84] Illiberi A, Poodt P, Bolt P-J, Roozeboom F. Recent advances in atmospheric vapor-phase deposition of transparent and conductive zinc oxide. Chemical Vapor Deposition. 2014;

[85] Illiberi A, Scherpenborg R, Poodt P, Roozeboom F. (Invited) spatial atomic layer deposition of transparent conductive oxides. ECS Transactions. 2013;**58**:105-110. DOI: 10.1149/

[86] Sharma K, Routkevitch D, Varaksa N, George SM. Spatial atomic layer deposition on

[87] Bellet D, Lagrange M, Sannicolo T, Aghazadehchors S, Nguyen VH, Langley DP, et al. Transparent electrodes based on silver nanowire networks: From physical considerations towards device integration. Materials (Basel). 2017;**10**:570. DOI: 10.3390/ma10060570 [88] Langley D, Giusti G, Mayousse C, Celle C, Bellet D, Simonato J-P. Flexible transparent conductive materials based on silver nanowire networks: A review. Nanotechnology.

battery electrodes. Journal of Vacuum Science & Technology A—Vacuum Surfaces and

flexible porous substrates: ZnO on anodic aluminum oxide films and Al<sup>2</sup>

cells with efficiency beyond 15%. Progress in Photovoltaics: Research and Applications.

Gay Zn<sup>z</sup>

of Materials. 2017;**29**:10007-10018. DOI: 10.1021/acs.chemmater.7b03478

atmospheric pressure deposition of ZnO front electrode for Cu(In,Ga)Se<sup>2</sup>

Materials & Interfaces. 2015;**7**:3671-3675. DOI: 10.1021/am508071y

Materials & Interfaces. 2015;**7**:4675-4684. DOI: 10.1021/am508088p

doping of amorphous TiO<sup>2</sup>

26 Chemical Vapor Deposition for Nanotechnology

spheric atomic layer deposition of Inx

2013;**21**:1559-1566. DOI: 10.1002/pip.2423

2015;**7**:2754-2759. DOI: 10.1021/am5077638

**20**:234-242. DOI: 10.1002/cvde.201400056

Films. 2016;**34**:01A146. DOI: 10.1116/1.4937728

2013;**24**:452001. DOI: 10.1088/0957-4484/24/45/452001

05810.0105ecst

Journal of Vacuum Science and Technology A. 2018;**36**:051511. DOI: 10.1116/1.5040457

for increased capacity and faster Li+

solar cells.

thin-film solar

O3

on Li ion


O for thin films transistors. ACS Applied


**Chapter 2**

**Provisional chapter**

**Direct Liquid Injection Chemical Vapor Deposition**

**Direct Liquid Injection Chemical Vapor Deposition**

DOI: 10.5772/intechopen.80244

Thin film technology, based on different chemical and physical methods, enabled miniaturization, co-integration, and amelioration of the performance of the devices. Chemical vapor deposition (CVD) systems ensure high productivity and demonstrate excellent film uniformity (up to 12 inch wafers) and repeatability with high throughput for a variety of different films of oxides, nitrides, metals, chalcogenides, etc. In the last two decades, direct liquid injection (DLI)-CVD enabling the usage of solid and liquid precursors has proven to be one of the most versatile CVD process to meet industrial requirements. In this chapter, the requirements to the precursors suitable for DLI-CVD, different classes of available precursors, and models used to describe the evaporation are overviewed. Then, different liquid delivery devices used in DLI-CVD such as capillary tubes, syringes, aero-

**Keywords:** direct liquid injection, DLI-CVD, evaporation, precursors, injectors, thin

In the last decades, thin film technology has been integrated with many large area applications in electronics, information processing/storage, telecommunications, LED lighting/displays, solar energy harvesting, etc. Thin film technology enabled miniaturization, co-integration, and amelioration of the performance of the devices. Many different chemical and physical deposition methods were developed in order to satisfy industrial needs and requirements in different application fields. There is no universal technique for the processing of functional coatings. For each application, more than one technique is being considered at industrial level and the final choice depends on the specifications and on the maturity and readiness level of each one. The industry

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

Vincent Astié, Cyril Millon,

and Ausrine Bartasyte

**Abstract**

films

**1. Introduction**

Jean-Manuel Decams and Ausrine Bartasyte

Vincent Astié, Cyril Millon, Jean-Manuel Decams

sol delivery systems, and valves are reviewed in detail.

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.80244

#### **Direct Liquid Injection Chemical Vapor Deposition Direct Liquid Injection Chemical Vapor Deposition**

DOI: 10.5772/intechopen.80244

Vincent Astié, Cyril Millon, Jean-Manuel Decams and Ausrine Bartasyte Vincent Astié, Cyril Millon, Jean-Manuel Decams and Ausrine Bartasyte

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.80244

#### **Abstract**

Thin film technology, based on different chemical and physical methods, enabled miniaturization, co-integration, and amelioration of the performance of the devices. Chemical vapor deposition (CVD) systems ensure high productivity and demonstrate excellent film uniformity (up to 12 inch wafers) and repeatability with high throughput for a variety of different films of oxides, nitrides, metals, chalcogenides, etc. In the last two decades, direct liquid injection (DLI)-CVD enabling the usage of solid and liquid precursors has proven to be one of the most versatile CVD process to meet industrial requirements. In this chapter, the requirements to the precursors suitable for DLI-CVD, different classes of available precursors, and models used to describe the evaporation are overviewed. Then, different liquid delivery devices used in DLI-CVD such as capillary tubes, syringes, aerosol delivery systems, and valves are reviewed in detail.

**Keywords:** direct liquid injection, DLI-CVD, evaporation, precursors, injectors, thin films

#### **1. Introduction**

In the last decades, thin film technology has been integrated with many large area applications in electronics, information processing/storage, telecommunications, LED lighting/displays, solar energy harvesting, etc. Thin film technology enabled miniaturization, co-integration, and amelioration of the performance of the devices. Many different chemical and physical deposition methods were developed in order to satisfy industrial needs and requirements in different application fields. There is no universal technique for the processing of functional coatings. For each application, more than one technique is being considered at industrial level and the final choice depends on the specifications and on the maturity and readiness level of each one. The industry

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

prefers conservative approaches in terms of available subcontracting environment, which makes that process modification requires much higher performance, durability, and lower cost than actually implemented solutions, the safety/environmental issues are considered, as well.

ensured high productivity and demonstrated excellent film uniformity (up to 12 inch wafers) and repeatability with high throughput for a variety of different films including semiconductors

genides, and noble metals. Despite industrial success of CVD in the manufacture of thin films and its flexibility, offered by chemistry, CVD is still an expensive method compared to the wet

In the CVD process, gaseous or evaporated precursors are transported to the heated substrate where decompositions or other type of reactions take place. However, not all precursors are under gaseous form at atmospheric pressure and ambient temperature (see **Figure 1**). In fact, only about 10% of the industrially available precursors are gaseous (e.g., trimethylborane

form (powders or crystals) (80%). About 45% of elements can be grown by using liquid precursors and the precursors of 5% elements are only liquid. Most available precursors exhibit safety hazards and must be carefully operated and stored. The main disadvantages of classical MOCVD systems are the difficulty to control the chemistry and the evaporation of the solid/ liquid precursors and therefore the composition and structure of coating (especially those of multicomponents coatings), which affect the reproducibility. It makes difficult the control of the film thickness and growth rate, which affects crystalline orientation, surface morphology, and film density. Nevertheless, chemistry of deposition process might be a potential advan-

**Figure 1.** Physical states of commercially available precursors at ambient temperature for each element in the periodic

table (gas in green, liquid in red, and solid in gold). www.sciencenotes.org

, LaNiO<sup>3</sup>

, AlN, etc.), rare-earth oxides, ceramic materials (e.g. TiN,

, etc.). The vast majority of precursors are available in solid

Cu<sup>3</sup> O<sup>7</sup>−<sup>x</sup>

Direct Liquid Injection Chemical Vapor Deposition http://dx.doi.org/10.5772/intechopen.80244

), chalco-

31

, etc.), superconductors (e.g. YBa2

(Si, III–V, etc.), dielectrics (e.g. SiO2

chemistry techniques.

, etc.), ferroelectrics (e.g. PbTiO<sup>3</sup>

, trimethylsilane Si(CH<sup>3</sup>

)3

tage for precision manufacturing of films, if used in a controlled manner.

ZrO2

B(CH<sup>3</sup> )3

There are two basic technologies, used for the synthesis of functional coatings: dry methods (depositions in vacuum conditions) and wet chemistry. The liquid-phase techniques, such as ink-jet printing, aerosol/spray deposition, sol-gel method, spin-coating, or dip-coating, are widely used in the coating technology due to their scalability and low cost [1, 2]. The usage of liquids/solvents in the wet techniques may induce degradation of the properties of the bottom layers and substrates and may introduce the impurities/defects in the heterostructure, which may be a critical limitation to device performance in the applications such as electronics [2]. Moreover, the drying of the solvent can also lead to the inhomogeneous distribution of the solute—"coffee stain" effect [3]. It is very challenging to grow the homogeneous coatings on 3D substrates with high aspect features by means of wet techniques due to dewetting and surface tension issues on the sidewalls and edges [2]. There is a considerable effort to be made to solve the strong drawbacks and limitations to the types of substrates of the wet techniques. Moreover, these methods usually provide poor reproducibility and non-uniform compositional distributions in the coatings. Thus, dry methods are highly preferred for the controlled synthesis of functional coatings. However, due to the strong advantages (low cost and easy up-scaling), wet techniques are able to meet actual specifications and thus take lead in "technology competition" with regard to dry techniques.

Different physical and chemical vapor deposition methods are used for the dry synthesis of the functional films at laboratory scale [4]. Physical vapor deposition techniques, such as pulsed laser deposition (PLD), magnetron sputtering, etc. offer in most cases a direct link between the composition of the gas phase and that of the coating [5], they may require less tuning and for this reason they are able to provide to the academia and the industry with "real samples" easier and faster than chemical vapor deposition techniques. However, these techniques will face issues in the stoichiometry adjustment in the case of multicomponent films if one of the deposited components is highly volatile (or presents very different evaporation characteristics). For instance, molecular beam epitaxy (MBE) is very powerful and essential tool in the semiconductor industry. Sputtering and evaporation techniques are widely applied for deposition of metal, oxide, and nitride films in the microfabrication of devices. Pulsed laser deposition (PLD) is broadly used by researchers for fundamental studies of new materials. However, physical methods, such as PLD, evaporation, and magnetron sputtering, are not compatible for depositions on non-planar surfaces and cannot offer precision control of the film thicknesses as required by some industries.

The industrial requirements (quality and reproducibility) for designed coatings and especially on 3D surfaces are more likely to be attained by chemical deposition methods, such as (metal organic) chemical vapor deposition (MOCVD) and atomic layer deposition (ALD) [6, 7]. ALD provides a tool for conformal coating on very high aspect-ratio nanostructures with excellent uniformity. It has become a technique for both template-directed nanofabrications and engineering of surface properties. For example, ALD is used to deposit HfO2 layers with nanometer scale thickness in the complementary metal oxide semi-conductor (CMOS) fabrication. CVD systems ensured high productivity and demonstrated excellent film uniformity (up to 12 inch wafers) and repeatability with high throughput for a variety of different films including semiconductors (Si, III–V, etc.), dielectrics (e.g. SiO2 , AlN, etc.), rare-earth oxides, ceramic materials (e.g. TiN, ZrO2 , etc.), ferroelectrics (e.g. PbTiO<sup>3</sup> , LaNiO<sup>3</sup> , etc.), superconductors (e.g. YBa2 Cu<sup>3</sup> O<sup>7</sup>−<sup>x</sup> ), chalcogenides, and noble metals. Despite industrial success of CVD in the manufacture of thin films and its flexibility, offered by chemistry, CVD is still an expensive method compared to the wet chemistry techniques.

prefers conservative approaches in terms of available subcontracting environment, which makes that process modification requires much higher performance, durability, and lower cost than

There are two basic technologies, used for the synthesis of functional coatings: dry methods (depositions in vacuum conditions) and wet chemistry. The liquid-phase techniques, such as ink-jet printing, aerosol/spray deposition, sol-gel method, spin-coating, or dip-coating, are widely used in the coating technology due to their scalability and low cost [1, 2]. The usage of liquids/solvents in the wet techniques may induce degradation of the properties of the bottom layers and substrates and may introduce the impurities/defects in the heterostructure, which may be a critical limitation to device performance in the applications such as electronics [2]. Moreover, the drying of the solvent can also lead to the inhomogeneous distribution of the solute—"coffee stain" effect [3]. It is very challenging to grow the homogeneous coatings on 3D substrates with high aspect features by means of wet techniques due to dewetting and surface tension issues on the sidewalls and edges [2]. There is a considerable effort to be made to solve the strong drawbacks and limitations to the types of substrates of the wet techniques. Moreover, these methods usually provide poor reproducibility and non-uniform compositional distributions in the coatings. Thus, dry methods are highly preferred for the controlled synthesis of functional coatings. However, due to the strong advantages (low cost and easy up-scaling), wet techniques are able to meet actual specifications and thus take lead

Different physical and chemical vapor deposition methods are used for the dry synthesis of the functional films at laboratory scale [4]. Physical vapor deposition techniques, such as pulsed laser deposition (PLD), magnetron sputtering, etc. offer in most cases a direct link between the composition of the gas phase and that of the coating [5], they may require less tuning and for this reason they are able to provide to the academia and the industry with "real samples" easier and faster than chemical vapor deposition techniques. However, these techniques will face issues in the stoichiometry adjustment in the case of multicomponent films if one of the deposited components is highly volatile (or presents very different evaporation characteristics). For instance, molecular beam epitaxy (MBE) is very powerful and essential tool in the semiconductor industry. Sputtering and evaporation techniques are widely applied for deposition of metal, oxide, and nitride films in the microfabrication of devices. Pulsed laser deposition (PLD) is broadly used by researchers for fundamental studies of new materials. However, physical methods, such as PLD, evaporation, and magnetron sputtering, are not compatible for depositions on non-planar surfaces and cannot offer precision control

The industrial requirements (quality and reproducibility) for designed coatings and especially on 3D surfaces are more likely to be attained by chemical deposition methods, such as (metal organic) chemical vapor deposition (MOCVD) and atomic layer deposition (ALD) [6, 7]. ALD provides a tool for conformal coating on very high aspect-ratio nanostructures with excellent uniformity. It has become a technique for both template-directed nanofabrications and engineer-

thickness in the complementary metal oxide semi-conductor (CMOS) fabrication. CVD systems

layers with nanometer scale

actually implemented solutions, the safety/environmental issues are considered, as well.

in "technology competition" with regard to dry techniques.

30 Chemical Vapor Deposition for Nanotechnology

of the film thicknesses as required by some industries.

ing of surface properties. For example, ALD is used to deposit HfO2

In the CVD process, gaseous or evaporated precursors are transported to the heated substrate where decompositions or other type of reactions take place. However, not all precursors are under gaseous form at atmospheric pressure and ambient temperature (see **Figure 1**). In fact, only about 10% of the industrially available precursors are gaseous (e.g., trimethylborane B(CH<sup>3</sup> )3 , trimethylsilane Si(CH<sup>3</sup> )3 , etc.). The vast majority of precursors are available in solid form (powders or crystals) (80%). About 45% of elements can be grown by using liquid precursors and the precursors of 5% elements are only liquid. Most available precursors exhibit safety hazards and must be carefully operated and stored. The main disadvantages of classical MOCVD systems are the difficulty to control the chemistry and the evaporation of the solid/ liquid precursors and therefore the composition and structure of coating (especially those of multicomponents coatings), which affect the reproducibility. It makes difficult the control of the film thickness and growth rate, which affects crystalline orientation, surface morphology, and film density. Nevertheless, chemistry of deposition process might be a potential advantage for precision manufacturing of films, if used in a controlled manner.

**Figure 1.** Physical states of commercially available precursors at ambient temperature for each element in the periodic table (gas in green, liquid in red, and solid in gold). www.sciencenotes.org

In the case of solid precursors, the sublimation rate is directly linked to its free surface, which in turn depends on the powder grain size and the amount of remaining precursor. Consequently, as sublimation occurs, its surface area in contact with the gas changes, thus changing the mass transport rate during the deposition. Due to the difficulty in precisely controlling the evaporation of a powder, depositions involving multiple solid precursors frequently were done by grinding optimized amounts of multiple precursors together to have a homogeneous single-source powder and rarely multiple evaporators were used. Various single-source powder precursors and techniques used to introduce powder in the evaporator were reviewed in Ref. [8]. In 1998, Gorbenko and Bosak [9] used a vibrating feeder to control the introduction of a mixture of β-diketonates precursors (see Section 2.1), namely La(thd)<sup>3</sup> and Ni(acim)2 or Ni(thd)<sup>3</sup> , for the deposition of LaNiO<sup>3</sup> thin films and succeeded in growing oriented stoichiometric layers. However, as many precursor powders are moisture sensitive, another method was later proposed in which pellets were covered by an inert material [10]. An alternative way was a packaging of a mixture of precursor powders in a quartz tube, which was then slowly introduced into the vaporizer. The large thermal gradient in the evaporator combined with the slow entrainment of the tube ensured that the growth rate would only be dictated by the density of the powder [11]. Although it worked well in laboratories, it proved to be difficult a scaling-up to industrial requirements.

were imagined, from two bubblers in series [14] to a self-metering reservoir [15]. Before 1990, the main research effort on the heretofore mentioned interesting materials were dedicated to physical deposition processes such as physical vapor deposition (PVD), as problems with

In the 1990s, the growing interest in the thin films of superconducting materials, ferroelec-

development of more reliable and versatile delivery systems adapted to a wider range of precursors. The first generation of the IIA group solid precursors were thermally unstable at their sublimation temperature. They had a strong tendency to oligomerize and aging issues [17]. Much effort was made in precursor chemistry and in development of new liquid injection systems. In this chapter, we overview liquid delivery systems developed to date emphasizing

The injection devices have greatly enhanced the vaporization efficiency, thus the high volatility is not the main issue as it was with previous delivery systems. From a point of view for large-scale CVD applications, choosing the best precursors for a given application can sometimes be troublesome. As a matter of fact, a suitable precursor should meet several key

**iv.** Congruent volatilization with a significant temperature gap between evaporation and

Additionally, it was shown that the molecular structure of the precursor had a significant impact on the growth dynamics and properties of the films, as well [18–20]. An *organometallic compound* should have a direct metal-carbon bond in their molecular structure, and it is admitted that precursors with a metal ion and a coordination complex (e.g. a metal ion is linked to a carbon chain through oxygen or nitrogen) are *metalorganic compounds*. Most of commercially available products belong to these five major classes of metalorganic compounds: metal β-diketonates, metal cyclopentadienyls, metal alkoxides, metal alkyls, and metal alkylamides

Metal β-diketonates are the most used and common precursors for MOCVD and especially

thermally and hydrolytically stable complex with metal ion. Although it is mostly an advantage, it implies that the deposition temperature should be high enough to reach the precursor

) chelating ligands of β-diketone usually form both

Tix O3

, BaTiO<sup>3</sup>

, SrTiO<sup>3</sup>

Direct Liquid Injection Chemical Vapor Deposition http://dx.doi.org/10.5772/intechopen.80244

, etc.) [16] leads to the

33

, PbZr1−<sup>x</sup>

vapor phase control and stoichiometry were not totally overcome in CVD.

Cu<sup>3</sup> O<sup>7</sup>−<sup>x</sup>

on the problems it solved and the challenges it rose.

trics, and high-κ dielectrics (YBa2

**2. Precursors for DLI-CVD**

**i.** High purity and high yield,

decomposition.

**ii.** Stability at ambient temperature,

**iii.** Easiness to handle and non-toxicity, and

for depositions of oxide films. The (O,O′-η<sup>2</sup>

depending on the organic complex associated to the metal.

**2.1. Choice of the precursors**

requirements:

The term "direct liquid injection (DLI)-CVD" refers to reactors, which use liquid delivery units to feed the deposition zone in reactants. It is much easier to regulate precisely a liquid flow rate than that of powders. In some systems, the liquid goes through an evaporation unit where it is vaporized, and in other cases, the precursors reach the surface of a heated substrate in a liquid state and then they are vaporized prior to decomposition. If the precursor is still in liquid phase when it reaches the substrate, the deposition method is called spray or aerosol pyrolysis [12] depending on the generation method of the liquid droplets. Sometimes spray/ aerosol pyrolysis is called atmospheric pressure CVD as usually the evaporation takes place in proximity of the substrate heated to high temperatures. The main advantage of DLI-CVD is that, in most cases, the solution is kept under pressure at room temperature, which makes it possible to use precursors with low thermal stability and low vapor pressure. Moreover, powder precursors may be dissolved in the solvent and used in DLI-CVD systems, as well.

A bubbler technology is well established as a liquid precursor delivery system since many years [13]. The principle is rather simple and robust: the liquid is stored inside a so-called bubbler, typically a stainless steel canister, and an inert carrier gas (usually Ar or N2 ) is introduced inside the liquid to bubble. The precursor vapor saturates the atmosphere in the bubbler and the vapor is then entrained to the heated substrate surface. The delivery of reactants depends on three parameters: the temperature of the bubbler, the carrier gas flow rate, and the pressure over the surface of the liquid. This system works particularly well with liquid precursors which vapor pressures are not too low and not too high. In the case of precursors with low vapor pressure, the only solution is heating the bubbler to increase the equilibrium vapor pressure inside the canister. Yet, this has some major drawbacks as it restricts the usage to precursors that are stable at high temperature, and implies to heat the whole delivery line so that condensation does not occur before reaching the substrate. To overcome the problems of nonsaturation of the carrier gas and fluctuation of the precursor flow rate, many different devices were imagined, from two bubblers in series [14] to a self-metering reservoir [15]. Before 1990, the main research effort on the heretofore mentioned interesting materials were dedicated to physical deposition processes such as physical vapor deposition (PVD), as problems with vapor phase control and stoichiometry were not totally overcome in CVD.

In the 1990s, the growing interest in the thin films of superconducting materials, ferroelectrics, and high-κ dielectrics (YBa2 Cu<sup>3</sup> O<sup>7</sup>−<sup>x</sup> , PbZr1−<sup>x</sup> Tix O3 , BaTiO<sup>3</sup> , SrTiO<sup>3</sup> , etc.) [16] leads to the development of more reliable and versatile delivery systems adapted to a wider range of precursors. The first generation of the IIA group solid precursors were thermally unstable at their sublimation temperature. They had a strong tendency to oligomerize and aging issues [17]. Much effort was made in precursor chemistry and in development of new liquid injection systems. In this chapter, we overview liquid delivery systems developed to date emphasizing on the problems it solved and the challenges it rose.

## **2. Precursors for DLI-CVD**

### **2.1. Choice of the precursors**

In the case of solid precursors, the sublimation rate is directly linked to its free surface, which in turn depends on the powder grain size and the amount of remaining precursor. Consequently, as sublimation occurs, its surface area in contact with the gas changes, thus changing the mass transport rate during the deposition. Due to the difficulty in precisely controlling the evaporation of a powder, depositions involving multiple solid precursors frequently were done by grinding optimized amounts of multiple precursors together to have a homogeneous single-source powder and rarely multiple evaporators were used. Various single-source powder precursors and techniques used to introduce powder in the evaporator were reviewed in Ref. [8]. In 1998, Gorbenko and Bosak [9] used a vibrating feeder to control the introduction of a mixture of β-diketonates precursors (see Section 2.1), namely La(thd)<sup>3</sup>

oriented stoichiometric layers. However, as many precursor powders are moisture sensitive, another method was later proposed in which pellets were covered by an inert material [10]. An alternative way was a packaging of a mixture of precursor powders in a quartz tube, which was then slowly introduced into the vaporizer. The large thermal gradient in the evaporator combined with the slow entrainment of the tube ensured that the growth rate would only be dictated by the density of the powder [11]. Although it worked well in laboratories, it proved

The term "direct liquid injection (DLI)-CVD" refers to reactors, which use liquid delivery units to feed the deposition zone in reactants. It is much easier to regulate precisely a liquid flow rate than that of powders. In some systems, the liquid goes through an evaporation unit where it is vaporized, and in other cases, the precursors reach the surface of a heated substrate in a liquid state and then they are vaporized prior to decomposition. If the precursor is still in liquid phase when it reaches the substrate, the deposition method is called spray or aerosol pyrolysis [12] depending on the generation method of the liquid droplets. Sometimes spray/ aerosol pyrolysis is called atmospheric pressure CVD as usually the evaporation takes place in proximity of the substrate heated to high temperatures. The main advantage of DLI-CVD is that, in most cases, the solution is kept under pressure at room temperature, which makes it possible to use precursors with low thermal stability and low vapor pressure. Moreover, powder precursors may be dissolved in the solvent and used in DLI-CVD systems, as well.

A bubbler technology is well established as a liquid precursor delivery system since many years [13]. The principle is rather simple and robust: the liquid is stored inside a so-called bub-

inside the liquid to bubble. The precursor vapor saturates the atmosphere in the bubbler and the vapor is then entrained to the heated substrate surface. The delivery of reactants depends on three parameters: the temperature of the bubbler, the carrier gas flow rate, and the pressure over the surface of the liquid. This system works particularly well with liquid precursors which vapor pressures are not too low and not too high. In the case of precursors with low vapor pressure, the only solution is heating the bubbler to increase the equilibrium vapor pressure inside the canister. Yet, this has some major drawbacks as it restricts the usage to precursors that are stable at high temperature, and implies to heat the whole delivery line so that condensation does not occur before reaching the substrate. To overcome the problems of nonsaturation of the carrier gas and fluctuation of the precursor flow rate, many different devices

bler, typically a stainless steel canister, and an inert carrier gas (usually Ar or N2

thin films and succeeded in growing

) is introduced

, for the deposition of LaNiO<sup>3</sup>

and Ni(acim)2

or Ni(thd)<sup>3</sup>

32 Chemical Vapor Deposition for Nanotechnology

to be difficult a scaling-up to industrial requirements.

The injection devices have greatly enhanced the vaporization efficiency, thus the high volatility is not the main issue as it was with previous delivery systems. From a point of view for large-scale CVD applications, choosing the best precursors for a given application can sometimes be troublesome. As a matter of fact, a suitable precursor should meet several key requirements:


Additionally, it was shown that the molecular structure of the precursor had a significant impact on the growth dynamics and properties of the films, as well [18–20]. An *organometallic compound* should have a direct metal-carbon bond in their molecular structure, and it is admitted that precursors with a metal ion and a coordination complex (e.g. a metal ion is linked to a carbon chain through oxygen or nitrogen) are *metalorganic compounds*. Most of commercially available products belong to these five major classes of metalorganic compounds: metal β-diketonates, metal cyclopentadienyls, metal alkoxides, metal alkyls, and metal alkylamides depending on the organic complex associated to the metal.

Metal β-diketonates are the most used and common precursors for MOCVD and especially for depositions of oxide films. The (O,O′-η<sup>2</sup> ) chelating ligands of β-diketone usually form both thermally and hydrolytically stable complex with metal ion. Although it is mostly an advantage, it implies that the deposition temperature should be high enough to reach the precursor decomposition temperature or that additional source of energy (plasma, UV light, etc.) is provided at low deposition temperatures. However, it can heavily increase the carbon contamination when working at low temperatures. Additionally, their low reactivity with water has proven to be a complication in ALD processes. Many derivatives of β-diketonates precursors and the abbreviations of their names are presented in **Figure 2**. The volatility of the precursor can be tuned by modifying R-groups of ligands. For instance, Schweitzer et al. have shown that the lead precursors with bulkier R-groups became more volatile (Pb(acac)2 < Pb(dhd)2 < Pb(thd)2 ) [21]. They explained this tendency by the increased shielding effect on the highly positive metal center and the reduced intermolecular oxygen-metal interactions in the solid state. Additionally, Fahlman et al. have reported that the increased fluorine substitution in the R-groups could improve significantly the volatility of zirconium β-diketonate precursors [22]. But on the other hand, fluorinated precursors often led to fluorine contamination in the deposited oxide layers. This contamination could be reduced by the introduction of excess H2 O vapor [23]. Another way to increase the volatility and/or the solubility in a solvent of the β-diketonate complex [24] is to add stoichiometric amount of a Lewis-base compound in the solution or directly on the complex in order to saturate the coordination sphere of the metal. The β-diketonates precursors have been successfully used to grow a large variety of films from pure metals, simple oxides, solid solutions, and composite materials (e.g., ruthenium and ruthenium oxide (RuO2 ) [25], lanthanum-modified lead zirconate-titanate (PLZT) [26], yttria-stabilized zirconia (YSZ) [27], lithium niobate [27, 28], etc.).

be attenuated, or even suppressed, by using bulky ligand (e.g. isopropoxide (O<sup>i</sup>

NMe2

(CH<sup>3</sup> )6

Bu)) [30]. For example, the volatility of zirconium precursor increases in the order

a monomer. Another possible way is to use the alkoxides functionalized with a donor like

oxide precursors can be hydrolyzed very easily [31] and some of them are highly toxic. These precursors too have been extensively used for the last two decades in the growth of oxide

Metal cyclopentadienyl (Cp) precursors show good thermal stability and volatility. However,

[40]. On the other hand, metal alkylamides usually have medium volatility and some of them exhibit good stability to hydrolysis. Most of alkylamides can be easily aminolyzed. Thus, they are reliable precursor sources for the growth of metal nitrides. Although they do not have any metal-oxygen bond, high purity oxide films were grown under big oxygen partial pressures

As briefly presented, different types of metalorganic precursors have their own strengths and weaknesses and the operators should know the deposition temperature before choosing their precursors. Most of β-diketonates will show good yield at high growth temperatures and they will be more resistant to hydrolysis but a synthesis will be inefficient at low temperatures. The alkoxides could be more adapted to depositions at low temperature where the main issue is the carbon contamination. The cyclopentadienyls have shown good efficiency in ALD processes. To overcome limitations of different families of precursors, a number of scientific teams have tailored new precursors combining the good stability of β-diketonates with the high volatility of alkoxides and have shown that the precursors consisting of different types of ligands showed better performance than those with single type of ligands. In fact, Jones et al. [19] showed

was more suitable precursor for the growth of Pb(Zr,Ti)O<sup>3</sup>

The theory describing the heat transfer between a gaseous medium and a droplet of liquid is well known and the basics of the evaporation process were given by Maxwell in 1877 [43]. His

**ii.** The vapor concentration at the surface of the droplet was equal to the saturation concen-

**i.** The droplet was spherical and without any relative motion with regards to the gas;

N film growth was described in detail by Jensen et al. [18].


[34], YSZ [35], etc.

, the first being a trimer, the second a dimer, and the third

) [39] and ZnO from diethylzinc (Zn(C2

] often referred to as (dmae)). However, metal alk-

Direct Liquid Injection Chemical Vapor Deposition http://dx.doi.org/10.5772/intechopen.80244

butoxide (Ot

Zr(OEt)<sup>4</sup> < Zr(O<sup>i</sup>

films like LiNbO<sup>3</sup>

their η<sup>5</sup>

Al2 O3

that Zr2

(O<sup>i</sup> Pr)<sup>6</sup> (thd)2

GaAs, ZnSe, and Alx

tration; and

Pr)<sup>4</sup> < Zr(Ot

from trimethylaluminum (Al2

by MOCVD [41] and ALD [42].

[32, 33], LaAlO<sup>3</sup>

temperatures than the commercially available Zr(thd)<sup>4</sup>

model for stationary state evaporation [44] assumed that:

Ga1−<sup>x</sup>

**2.2. Evaporation of liquid droplets**

dimethylaminoethoxide ([OCH2

Bu)<sup>4</sup>

CH2

Pr) or tert-

35

H5 )2

or DEZ)

at moderate

. The influence of precursor structure on

Metal alkoxide (M(OR)n) precursors are the second type of the most used precursors in MOCVD processes. They are usually more volatile than β-diketonates and provide less carbon contamination even at low oxygen partial pressure. Their volatility is linked to the electron density on the metallic center and the saturation level by ligands as it will condition the tendency to form oligomeric (M(OR)n) m clusters [29]. The oligomerization can however

**Figure 2.** Composition, names/abbreviations of β-diketonate ligands [31].

be attenuated, or even suppressed, by using bulky ligand (e.g. isopropoxide (O<sup>i</sup> Pr) or tertbutoxide (Ot Bu)) [30]. For example, the volatility of zirconium precursor increases in the order Zr(OEt)<sup>4</sup> < Zr(O<sup>i</sup> Pr)<sup>4</sup> < Zr(Ot Bu)<sup>4</sup> , the first being a trimer, the second a dimer, and the third a monomer. Another possible way is to use the alkoxides functionalized with a donor like dimethylaminoethoxide ([OCH2 CH2 NMe2 ] often referred to as (dmae)). However, metal alkoxide precursors can be hydrolyzed very easily [31] and some of them are highly toxic. These precursors too have been extensively used for the last two decades in the growth of oxide films like LiNbO<sup>3</sup> [32, 33], LaAlO<sup>3</sup> [34], YSZ [35], etc.

Metal cyclopentadienyl (Cp) precursors show good thermal stability and volatility. However, their η<sup>5</sup> -bonding mode makes them a considerable source of carbon contamination. They are reactive with water even at moderate temperature and this makes them suitable candidates for ALD processes [36, 37]. Metal alkyl precursors are usually very reactive, toxic, and/or pyrophoric, as well as being a source of heavy carbon contamination in oxide films [38]. Nevertheless, aluminum and zinc alkyls are successfully used in ALD processes to grow Al2 O3 from trimethylaluminum (Al2 (CH<sup>3</sup> )6 ) [39] and ZnO from diethylzinc (Zn(C2 H5 )2 or DEZ) [40]. On the other hand, metal alkylamides usually have medium volatility and some of them exhibit good stability to hydrolysis. Most of alkylamides can be easily aminolyzed. Thus, they are reliable precursor sources for the growth of metal nitrides. Although they do not have any metal-oxygen bond, high purity oxide films were grown under big oxygen partial pressures by MOCVD [41] and ALD [42].

As briefly presented, different types of metalorganic precursors have their own strengths and weaknesses and the operators should know the deposition temperature before choosing their precursors. Most of β-diketonates will show good yield at high growth temperatures and they will be more resistant to hydrolysis but a synthesis will be inefficient at low temperatures. The alkoxides could be more adapted to depositions at low temperature where the main issue is the carbon contamination. The cyclopentadienyls have shown good efficiency in ALD processes. To overcome limitations of different families of precursors, a number of scientific teams have tailored new precursors combining the good stability of β-diketonates with the high volatility of alkoxides and have shown that the precursors consisting of different types of ligands showed better performance than those with single type of ligands. In fact, Jones et al. [19] showed that Zr2 (O<sup>i</sup> Pr)<sup>6</sup> (thd)2 was more suitable precursor for the growth of Pb(Zr,Ti)O<sup>3</sup> at moderate temperatures than the commercially available Zr(thd)<sup>4</sup> . The influence of precursor structure on GaAs, ZnSe, and Alx Ga1−<sup>x</sup> N film growth was described in detail by Jensen et al. [18].

#### **2.2. Evaporation of liquid droplets**

decomposition temperature or that additional source of energy (plasma, UV light, etc.) is provided at low deposition temperatures. However, it can heavily increase the carbon contamination when working at low temperatures. Additionally, their low reactivity with water has proven to be a complication in ALD processes. Many derivatives of β-diketonates precursors and the abbreviations of their names are presented in **Figure 2**. The volatility of the precursor can be tuned by modifying R-groups of ligands. For instance, Schweitzer et al. have shown that the lead precursors with bulkier R-groups became more volatile (Pb(acac)2 < Pb(dhd)2

positive metal center and the reduced intermolecular oxygen-metal interactions in the solid state. Additionally, Fahlman et al. have reported that the increased fluorine substitution in the R-groups could improve significantly the volatility of zirconium β-diketonate precursors [22]. But on the other hand, fluorinated precursors often led to fluorine contamination in the deposited oxide layers. This contamination could be reduced by the introduction of excess

O vapor [23]. Another way to increase the volatility and/or the solubility in a solvent of the β-diketonate complex [24] is to add stoichiometric amount of a Lewis-base compound in the solution or directly on the complex in order to saturate the coordination sphere of the metal. The β-diketonates precursors have been successfully used to grow a large variety of films from pure metals, simple oxides, solid solutions, and composite materials (e.g., ruthenium

Metal alkoxide (M(OR)n) precursors are the second type of the most used precursors in MOCVD processes. They are usually more volatile than β-diketonates and provide less carbon contamination even at low oxygen partial pressure. Their volatility is linked to the electron density on the metallic center and the saturation level by ligands as it will condition

yttria-stabilized zirconia (YSZ) [27], lithium niobate [27, 28], etc.).

) [21]. They explained this tendency by the increased shielding effect on the highly

) [25], lanthanum-modified lead zirconate-titanate (PLZT) [26],

m clusters [29]. The oligomerization can however

< Pb(thd)2

and ruthenium oxide (RuO2

34 Chemical Vapor Deposition for Nanotechnology

the tendency to form oligomeric (M(OR)n)

**Figure 2.** Composition, names/abbreviations of β-diketonate ligands [31].

H2

The theory describing the heat transfer between a gaseous medium and a droplet of liquid is well known and the basics of the evaporation process were given by Maxwell in 1877 [43]. His model for stationary state evaporation [44] assumed that:


**iii.** The evaporation was an equilibrium process.

However, in the case of droplet evaporation with a low vapor pressure, this model is not sufficient. There are six different models, used to explain vaporizing process at low vapor pressure:

stainless steel "tee" [50]) with flowing a neutral gas, or more conventionally, a nebulizer. The principle of this method is based on the capillary effect. The flow inside the capillary is laminar, the liquid in contact with the walls is stationary and there is no radial flow. Thus, the pressure across the cross-section is constant. According to Hagen-Poiseuille law, the flow volume through the tube, *Q,* is governed by four parameters such as the viscosity of the precursor solution, *μ*; the radius of tube, *r*; the length of the cylindrical tube, *d*; and lastly the pressure difference between the top (usually sealing to the bottle which contains the precur-

The main advantage of this liquid delivery system is an ability to provide a steady flow of precursors to the vaporizer. However, the substantial drawback is that the flow volume itself can only be tuned by varying the viscosity of the liquid. This means changing the solvent or the concentration of the precursor in the solution, which in turn could affect the quality of the deposition.

in isopropanol (i-PrOH), led to an unstable flow rate through the capillary due to high viscosity. To tune the injected flow itself, syringes were used as the precursor vessel, and the process was sometimes referred to as lirect liquid evaporation-CVD (DLE-CVD) [51]. The syringe is placed into a syringe driver, or syringe pump, connected to the capillary tube. The liquid flow is controlled precisely by adjusting the rotational speed of a stepper motor. The multiple precursors can be introduced to the evaporator by means of either multiple syringes and their drivers, or by using their mixture in a single solution. However, this technology is well adapted only to the liquid precursors at ambient temperature and the usage of the precursor solutions may face several issues. Indeed, since the bottom of the capillary tube is in contact with a heated zone, the vapor pressure difference between the diluted solid precursor and the solvent may lead to the evaporation of the solvent with low vapor pressure prior to exiting the capillary. This in turn can introduce a partial/complete clogging of the tip with the precipitated solid precursor and disruption of the solution flow. The usage of heavy solvents may eliminate these issues but it can have a significant impact on the carbon contamination of the deposited layer. A possible solution to this problem was proposed by ASM International by an introduction a three-way valve in order to introduce a carrier gas together with the solution in the capillary system [16]. An example of such system using an advanced syringecapillary system to control the precursor injection rate, adapted to the synthesis of carbon nanotubes, is presented in **Figure 3** [52]. Many different thin films of metals and binary oxides

Borgharkar et al. [50] have even reported that increasing the concentration of Cu(hfac)2

thin were grown using this technique: Cu [50], transparent amorphous Al2

Some CVD processes rely on the nebulization, or the atomization of liquid precursors into a "mist", or an aerosol, which is then brought to the heated substrate by carrier gases. The ultrasonic atomization process takes roots in the Lord Rayleigh's "*The Theory of Sound"* [58], and Lang's works on the absorption of energy by a liquid [59]. When a liquid is placed on a flat surface vibrating along the surface normal, some of the energy is absorbed and transformed

[57], etc.

**3.2. Aerosol assisted-CVD and pulsed-pressure MOCVD**

TiO2

[55], La2

O3

[56], VO2

(1)

37

Direct Liquid Injection Chemical Vapor Deposition http://dx.doi.org/10.5772/intechopen.80244

, diluted

O3

[53], ZrO2

[54],

sor) and the bottom "pressure drop", Δ*P*:


The infinite-liquid-conductivity model is commonly used in modeling of industrial spray processes. In order to simplify the description, this model is based on several assumptions:


Detailed description of fluid dynamics and transport of droplets and sprays can be found in Ref. [46]. This model was used to describe the fuel droplet vaporization in combustion processes [47, 48]. Although the description of evaporation phenomenon is complex and many assumptions are needed, basically it can be easily understood, that smaller droplets are more easily evaporated. In a heated environment at low vacuum, the heat is transferred through the exchange between the gas and the droplet. Two parameters have to be taken into account: the specific heat capacity, Cp, of the gas and the free surface of the droplet. The surface/volume ratio can be increased by minimizing the radius and evaporation at a given flow rate can be ameliorated by introducing more but smaller droplets. This possibility was not really considered in the first liquid delivery devices, but very quickly the atomization has become one of the major concerns of the injector industry and the key parameter, used to discriminate liquid delivery devices.

## **3. Liquid delivery systems**

In this section, different liquid delivery devices used in DLI-CVD such as capillary tubes, syringes, aerosol delivery systems, and valves and their developments are reviewed in detail.

#### **3.1. Capillary tubes and syringes**

Capillary tubes are one of the simplest ways to introduce a flow of precursor with controlled rate into an evaporator. The liquid drops into a heated zone (either a quartz tube [49] or a stainless steel "tee" [50]) with flowing a neutral gas, or more conventionally, a nebulizer. The principle of this method is based on the capillary effect. The flow inside the capillary is laminar, the liquid in contact with the walls is stationary and there is no radial flow. Thus, the pressure across the cross-section is constant. According to Hagen-Poiseuille law, the flow volume through the tube, *Q,* is governed by four parameters such as the viscosity of the precursor solution, *μ*; the radius of tube, *r*; the length of the cylindrical tube, *d*; and lastly the pressure difference between the top (usually sealing to the bottle which contains the precursor) and the bottom "pressure drop", Δ*P*:

**iii.** The evaporation was an equilibrium process.

36 Chemical Vapor Deposition for Nanotechnology

of the droplet diameter, d, also called d2

**ii.** The infinite-liquid-conductivity model;

**iv.** The effective-conductivity model;

**vi.** The Navier-Stokes equation solution.

the droplet vaporization rate [45].

**v.** The vortex model; and

delivery devices.

**3. Liquid delivery systems**

**3.1. Capillary tubes and syringes**

pressure:

However, in the case of droplet evaporation with a low vapor pressure, this model is not sufficient. There are six different models, used to explain vaporizing process at low vapor

**i.** The constant-droplet-temperature model (decreasing linearly as a function of the square

law);

The infinite-liquid-conductivity model is commonly used in modeling of industrial spray processes. In order to simplify the description, this model is based on several assumptions:

**ii.** A negligible transient liquid heating although it would appear to be a governing factor of

Detailed description of fluid dynamics and transport of droplets and sprays can be found in Ref. [46]. This model was used to describe the fuel droplet vaporization in combustion processes [47, 48]. Although the description of evaporation phenomenon is complex and many assumptions are needed, basically it can be easily understood, that smaller droplets are more easily evaporated. In a heated environment at low vacuum, the heat is transferred through the exchange between the gas and the droplet. Two parameters have to be taken into account: the specific heat capacity, Cp, of the gas and the free surface of the droplet. The surface/volume ratio can be increased by minimizing the radius and evaporation at a given flow rate can be ameliorated by introducing more but smaller droplets. This possibility was not really considered in the first liquid delivery devices, but very quickly the atomization has become one of the major concerns of the injector industry and the key parameter, used to discriminate liquid

In this section, different liquid delivery devices used in DLI-CVD such as capillary tubes, syringes, aerosol delivery systems, and valves and their developments are reviewed in detail.

Capillary tubes are one of the simplest ways to introduce a flow of precursor with controlled rate into an evaporator. The liquid drops into a heated zone (either a quartz tube [49] or a

**iii.** The spherically symmetric transient droplet-heating (or conduction-limit) model;

**i.** A single Lewis number in the gas phase surrounding the droplet and

$$Q = \frac{\pi r^4}{8\mu} \ast \frac{\Delta P}{d} \tag{1}$$

The main advantage of this liquid delivery system is an ability to provide a steady flow of precursors to the vaporizer. However, the substantial drawback is that the flow volume itself can only be tuned by varying the viscosity of the liquid. This means changing the solvent or the concentration of the precursor in the solution, which in turn could affect the quality of the deposition. Borgharkar et al. [50] have even reported that increasing the concentration of Cu(hfac)2 , diluted in isopropanol (i-PrOH), led to an unstable flow rate through the capillary due to high viscosity.

To tune the injected flow itself, syringes were used as the precursor vessel, and the process was sometimes referred to as lirect liquid evaporation-CVD (DLE-CVD) [51]. The syringe is placed into a syringe driver, or syringe pump, connected to the capillary tube. The liquid flow is controlled precisely by adjusting the rotational speed of a stepper motor. The multiple precursors can be introduced to the evaporator by means of either multiple syringes and their drivers, or by using their mixture in a single solution. However, this technology is well adapted only to the liquid precursors at ambient temperature and the usage of the precursor solutions may face several issues. Indeed, since the bottom of the capillary tube is in contact with a heated zone, the vapor pressure difference between the diluted solid precursor and the solvent may lead to the evaporation of the solvent with low vapor pressure prior to exiting the capillary. This in turn can introduce a partial/complete clogging of the tip with the precipitated solid precursor and disruption of the solution flow. The usage of heavy solvents may eliminate these issues but it can have a significant impact on the carbon contamination of the deposited layer. A possible solution to this problem was proposed by ASM International by an introduction a three-way valve in order to introduce a carrier gas together with the solution in the capillary system [16]. An example of such system using an advanced syringecapillary system to control the precursor injection rate, adapted to the synthesis of carbon nanotubes, is presented in **Figure 3** [52]. Many different thin films of metals and binary oxides thin were grown using this technique: Cu [50], transparent amorphous Al2 O3 [53], ZrO2 [54], TiO2 [55], La2 O3 [56], VO2 [57], etc.

#### **3.2. Aerosol assisted-CVD and pulsed-pressure MOCVD**

Some CVD processes rely on the nebulization, or the atomization of liquid precursors into a "mist", or an aerosol, which is then brought to the heated substrate by carrier gases. The ultrasonic atomization process takes roots in the Lord Rayleigh's "*The Theory of Sound"* [58], and Lang's works on the absorption of energy by a liquid [59]. When a liquid is placed on a flat surface vibrating along the surface normal, some of the energy is absorbed and transformed

**Figure 3.** Schematic representation of a DLE-CVD reactor using a syringe pump associated with a 9″ needle. Reproduced with permission [52]. Copyright 2018, MDPI.

into transverse standing waves called capillary waves in the liquid. As the amplitude of the vibration increases, the difference in height between the crests and troughs increases to the point where it compromises the stability of the wave. At this stage, the waves collapse and tiny droplets of liquid are ejected from the crests. If the vibrational frequency is too high, a cavitation can occur and large portions of liquid are sent [60].

Size of droplets, produced by industrial atomizers, follows a narrow Gaussian distribution and the mean size value depends entirely on the wavelength of the capillary wave, which in turn depends on the frequency of the transducer and the properties of the liquid [58, 59]:

$$
\lambda\_L = \sqrt[3]{\left(\frac{8\pi^\*\gamma}{\rho^\*\mathfrak{t}^2}\right)}\text{ and}\tag{2}
$$

output. This transformation is accompanied by a heating of both the transducer and the precursor solution resulting in the shift in the resonance frequency and in the change of the liquid viscosity, respectively. This alters the flow rate of the liquid resulting in an unstable film growth. Thus, most deposition cycles to finish in a relatively short time (matter of minutes) [63]. Ultrasonic atomization in aerosol-assisted CVD (AACVD) has the benefit to allow a wider choice of precursors. In fact, a high volatility is not anymore a requirement and a

Different CVD systems based on aerosol delivery systems have been developed with customized vaporization zones. In the case of AACVD, also referred to as liquid source misted chemical deposition (LSMCD) [63] or liquid source chemical vapor deposition (LSCVD) [65, 66], the precursors or their solution are sent continuously to the reactor where they evaporate in the vicinity of the substrate surface due to high thermal gradient. Although McMillan et al. [67] have reported that in the case of LSCVD, an uniform film deposition could not be achieved on a rotating heated substrate unless a barrier plate was located in the vicinity of its surface, the homogeneous depositions were demonstrated by later developed AACVD systems. A possibility to grow thin films of a large variety of oxides and metals was studied by using an

atmospheric pressure (AP)-AACVD using standard Vicks humidifier with a continuous

In the case of pulsed-pressure CVD (PP-CVD) [69] or pulsed liquid injection with an ultrasonic nozzle [70], a small shot of liquid is pushed by an inert gas into the vaporizer through an ultrasonic nozzle (**Figure 4**), and quickly evaporates [71]. This produces a pressure pulse, and then the chamber is subsequently pumped back to the initial pressure in order to send another shot. High efficiency of deposition process has been achieved by means of PP-CVD [72]. Conventional nebulizer was employed in PP-MOCVD such as Sono-Tek ultrasonic nozzle. PP-MOCVD was used to grow sealing layers of yttria-stabilized zirconia on porous solid oxide fuel cell electrodes [35], tantalum oxide [73], epitaxial titanium dioxide [71, 74, 75], as well as lithium tantalate [76].

The liquid delivery systems in CVD, presented previously, were developed by implementing different industrially available valves. According to the operational mode, valves can be classified into ON/OFF valves and proportional valves. The proportional valve allows very precise control of the opening area via a piezoelectric material, which in return allows for an adjustable flow rate. The ON/OFF operating valve was typically used in the motor industry prior to the 2014 Euro 6 emission standards. In the 1980s, electromagnetic ON/OFF valves, in which a current is applied to the solenoid generating attraction force in order to pull the plunger below up, releasing the valve seat and letting the liquid flow out of the chamber through a very fine nozzle [77], are well known [78, 79] and used in many ON/OFF systems where the control of

, from polyoxotungstate anions

Direct Liquid Injection Chemical Vapor Deposition http://dx.doi.org/10.5772/intechopen.80244 39

[PW11O39] using a PIFCO ultrasonic humidifier [64] and

nanoparticles by means of a hybrid method combining the

thin films with embedded cerium dioxide CeO2

solubility in solvent becomes the main selection criteria [64].

ultrasonic delivery systems. As illustrative examples of AACVD could be:

**i.** The deposition of partially reduced tungsten oxide, WO<sup>3</sup>−<sup>x</sup>

N]<sup>4</sup> H3

O19] and [n-Bu<sup>4</sup>

hydrothermal flow synthesis (CHFS) [68].

**ii.** The growth of nanocomposite of VO2

and titanium dioxide TiO2

[n-Bu<sup>4</sup>

**3.3. Valves**

N]2 [W<sup>6</sup>

$$\mathbf{D}\_{\text{N},0.5} = \mathbf{0}, \mathbf{3}4^{\ast}\mathbf{\lambda}\_{\text{L}} \tag{3}$$

where *<sup>λ</sup> <sup>L</sup>* is a wavelength of the liquid, *γ* is a surface tension, *ρ* is the density, *f* is a frequency of the actuator, and *<sup>D</sup> <sup>N</sup>*,0.5 is the median diameter of droplets. Tsai et al. have demonstrated as expected that the drop size could be decreased by increasing the ultrasonic frequency [61]. However, it should be noted that the working frequency of some atomizers cannot be tuned. In fact, the working frequency of an ultrasonic spray nozzle is governed by its dimensions. The high-frequency nozzles work with small wavelength and are consequently smaller than low-frequency nozzles. The rate of liquid flow to atomizer can be calibrated and controlled accurately by means of liquid mass flow controller. Theoretically, a very large range of flow rates should be possible, but in fact, the ratio of maximum to minimum flow rates produced by the same atomizer is limited to 5 due to design constraints [62]. Usually, atomizer works with a piezoelectric transducer, which converts an electrical input into a mechanical output. This transformation is accompanied by a heating of both the transducer and the precursor solution resulting in the shift in the resonance frequency and in the change of the liquid viscosity, respectively. This alters the flow rate of the liquid resulting in an unstable film growth. Thus, most deposition cycles to finish in a relatively short time (matter of minutes) [63]. Ultrasonic atomization in aerosol-assisted CVD (AACVD) has the benefit to allow a wider choice of precursors. In fact, a high volatility is not anymore a requirement and a solubility in solvent becomes the main selection criteria [64].

Different CVD systems based on aerosol delivery systems have been developed with customized vaporization zones. In the case of AACVD, also referred to as liquid source misted chemical deposition (LSMCD) [63] or liquid source chemical vapor deposition (LSCVD) [65, 66], the precursors or their solution are sent continuously to the reactor where they evaporate in the vicinity of the substrate surface due to high thermal gradient. Although McMillan et al. [67] have reported that in the case of LSCVD, an uniform film deposition could not be achieved on a rotating heated substrate unless a barrier plate was located in the vicinity of its surface, the homogeneous depositions were demonstrated by later developed AACVD systems. A possibility to grow thin films of a large variety of oxides and metals was studied by using an ultrasonic delivery systems. As illustrative examples of AACVD could be:


In the case of pulsed-pressure CVD (PP-CVD) [69] or pulsed liquid injection with an ultrasonic nozzle [70], a small shot of liquid is pushed by an inert gas into the vaporizer through an ultrasonic nozzle (**Figure 4**), and quickly evaporates [71]. This produces a pressure pulse, and then the chamber is subsequently pumped back to the initial pressure in order to send another shot. High efficiency of deposition process has been achieved by means of PP-CVD [72]. Conventional nebulizer was employed in PP-MOCVD such as Sono-Tek ultrasonic nozzle. PP-MOCVD was used to grow sealing layers of yttria-stabilized zirconia on porous solid oxide fuel cell electrodes [35], tantalum oxide [73], epitaxial titanium dioxide [71, 74, 75], as well as lithium tantalate [76].

#### **3.3. Valves**

(2)

(3)

into transverse standing waves called capillary waves in the liquid. As the amplitude of the vibration increases, the difference in height between the crests and troughs increases to the point where it compromises the stability of the wave. At this stage, the waves collapse and tiny droplets of liquid are ejected from the crests. If the vibrational frequency is too high, a

**Figure 3.** Schematic representation of a DLE-CVD reactor using a syringe pump associated with a 9″ needle. Reproduced

Size of droplets, produced by industrial atomizers, follows a narrow Gaussian distribution and the mean size value depends entirely on the wavelength of the capillary wave, which in turn depends on the frequency of the transducer and the properties of the liquid [58, 59]:

is a wavelength of the liquid, *γ* is a surface tension, *ρ* is the density, *f* is a frequency

of the actuator, and *<sup>D</sup> <sup>N</sup>*,0.5 is the median diameter of droplets. Tsai et al. have demonstrated as expected that the drop size could be decreased by increasing the ultrasonic frequency [61]. However, it should be noted that the working frequency of some atomizers cannot be tuned. In fact, the working frequency of an ultrasonic spray nozzle is governed by its dimensions. The high-frequency nozzles work with small wavelength and are consequently smaller than low-frequency nozzles. The rate of liquid flow to atomizer can be calibrated and controlled accurately by means of liquid mass flow controller. Theoretically, a very large range of flow rates should be possible, but in fact, the ratio of maximum to minimum flow rates produced by the same atomizer is limited to 5 due to design constraints [62]. Usually, atomizer works with a piezoelectric transducer, which converts an electrical input into a mechanical

cavitation can occur and large portions of liquid are sent [60].

with permission [52]. Copyright 2018, MDPI.

38 Chemical Vapor Deposition for Nanotechnology

where *<sup>λ</sup> <sup>L</sup>*

The liquid delivery systems in CVD, presented previously, were developed by implementing different industrially available valves. According to the operational mode, valves can be classified into ON/OFF valves and proportional valves. The proportional valve allows very precise control of the opening area via a piezoelectric material, which in return allows for an adjustable flow rate. The ON/OFF operating valve was typically used in the motor industry prior to the 2014 Euro 6 emission standards. In the 1980s, electromagnetic ON/OFF valves, in which a current is applied to the solenoid generating attraction force in order to pull the plunger below up, releasing the valve seat and letting the liquid flow out of the chamber through a very fine nozzle [77], are well known [78, 79] and used in many ON/OFF systems where the control of

computer, which allows digital control of the injection frequency, number of injected droplets, opening time, and consequently thin film growth rate and thickness. As in the case of ultrasonic nozzles, there are usually no corrosion problems inside the injector with organic

Direct Liquid Injection Chemical Vapor Deposition http://dx.doi.org/10.5772/intechopen.80244 41

It was shown that the precursors with low vapor pressure can be efficiently used in PI-MOCVD. PI-MOCVD reactors equipped with multiple injectors are used for the growth of multi-element films with controlled composition/stoichiometry, multilayers, nanocomposites and complex nanostructures of superconductors [16], high-κ dielectrics [86, 87], ferroelectrics,

As the solvent can cause a carbon contamination in the films, Kaul and Seleznev [89] have proposed a method to remove the solvent of a solution prior to vaporizing the precursor. In this process, a fiberglass belt is wetted with a controlled amount of solution outside of the evaporation chamber. Then, the tape is brought into a warm zone of the reactor where only the solvent evaporates. The belt with solid precursor is then mechanically carried to a zone where flash evaporation of precursors occurs. Later, the same process has been applied with in situ wetting of the tape with sequential injection of micro-amounts of solution (illustrated in **Figure 5**) [90]. Although a solvent was introduced like in PI-MOCVD, only a mixture of precursor vapor and carrier gas reaches the substrate. This method may be particularly useful

precursors/solvents, but some applications may require specially coated injectors.

in the synthesis at low temperatures in order to solve carbon contamination issues.

An injection system was first commercialized under the JetPulse® name by Jipelec, a division of Qualiflow, and it was later acquired and further developed by AIXTRON® under the name TriJet® [91]. TriJet® evaporators are installed in the latest AIXTRON® ALD machines and have been used in MOCVD processes, called atomic vapor deposition (AVD). In fact, the reproducibility of the commercially available injectors was very poor. To face fluctuations in the partial pressure of the liquid, operators had either to dilute significantly the precursor solution or to lower the injection frequency. Additionally, since the nozzle of the injector is subjected to the heated environment, a premature evaporation of liquid solution trapped inside the injector may happen, which can significantly alter the injection flow rate of diluted

**Figure 5.** Schematic representation of a DLI-CVD furnace using solenoid electrovalves in combination with a moving

tape. Reproduced with permission [90]. Copyright 1998, Elsevier.

conductors [88], etc.

**Figure 4.** Schematic representation of a PP-CVD reactor using an ultrasonic nozzle. Reproduced with permission [75]. Copyright 2001, Elsevier.

an injected volume is crucial. In 1993, Sénateur et al. have proposed to use them in a pulsed injection (PI)-MOCVD process [80]. A strong advantage of PI-MOCVD systems is a clear interface between the pressurized environment at ambient temperature where the precursor is stored safely and the low-vacuum atmosphere of the heated evaporator. Thus, the problems related to vapor pressure differences between the solvent and its solute could be mostly overlooked in the case of PI-MOCVD. The injector works as a valve, regulating the flow of liquid introduced in the evaporation chamber and as a nozzle atomizing the flow into tiny droplets.

As explained earlier (see Section 2.2), the dimensions of droplets have a fundamental impact on the evaporation efficiency of the liquid solution. Therefore, there are several key parameters of conventional solenoid injectors defining a quality of atomization: (i) the nozzle geometry (size, number of orifices, and degree of conicity), (ii) the flow regime inside the injector, and (iii) properties of the solution/liquid such as viscosity and concentration. The nozzle geometries and flow regimes have been extensively studied in the fuel injection industry [81–85]. Most commercially available injectors work in the frequency range between 0.1 and 200 Hz with very small opening times. These characteristics offer a very large working range in terms of injected flow rate and ultimately, in terms of film growth rate. In such injection system, the injection flow rate depends on the concentration and viscosity of the precursor solution, the opening frequency and time of the nozzle, and the pressure difference between the stored solution and the evaporator. Very small opening time ensures the fast evaporation process and helps to avoid the dissociation or premature decomposition of precursors before evaporation. However, it should be noted that the injector may produce instable flow at opening time below 1 ms. It is important to note, that injectors are commanded by computer, which allows digital control of the injection frequency, number of injected droplets, opening time, and consequently thin film growth rate and thickness. As in the case of ultrasonic nozzles, there are usually no corrosion problems inside the injector with organic precursors/solvents, but some applications may require specially coated injectors.

It was shown that the precursors with low vapor pressure can be efficiently used in PI-MOCVD. PI-MOCVD reactors equipped with multiple injectors are used for the growth of multi-element films with controlled composition/stoichiometry, multilayers, nanocomposites and complex nanostructures of superconductors [16], high-κ dielectrics [86, 87], ferroelectrics, conductors [88], etc.

As the solvent can cause a carbon contamination in the films, Kaul and Seleznev [89] have proposed a method to remove the solvent of a solution prior to vaporizing the precursor. In this process, a fiberglass belt is wetted with a controlled amount of solution outside of the evaporation chamber. Then, the tape is brought into a warm zone of the reactor where only the solvent evaporates. The belt with solid precursor is then mechanically carried to a zone where flash evaporation of precursors occurs. Later, the same process has been applied with in situ wetting of the tape with sequential injection of micro-amounts of solution (illustrated in **Figure 5**) [90]. Although a solvent was introduced like in PI-MOCVD, only a mixture of precursor vapor and carrier gas reaches the substrate. This method may be particularly useful in the synthesis at low temperatures in order to solve carbon contamination issues.

An injection system was first commercialized under the JetPulse® name by Jipelec, a division of Qualiflow, and it was later acquired and further developed by AIXTRON® under the name TriJet® [91]. TriJet® evaporators are installed in the latest AIXTRON® ALD machines and have been used in MOCVD processes, called atomic vapor deposition (AVD). In fact, the reproducibility of the commercially available injectors was very poor. To face fluctuations in the partial pressure of the liquid, operators had either to dilute significantly the precursor solution or to lower the injection frequency. Additionally, since the nozzle of the injector is subjected to the heated environment, a premature evaporation of liquid solution trapped inside the injector may happen, which can significantly alter the injection flow rate of diluted

an injected volume is crucial. In 1993, Sénateur et al. have proposed to use them in a pulsed injection (PI)-MOCVD process [80]. A strong advantage of PI-MOCVD systems is a clear interface between the pressurized environment at ambient temperature where the precursor is stored safely and the low-vacuum atmosphere of the heated evaporator. Thus, the problems related to vapor pressure differences between the solvent and its solute could be mostly overlooked in the case of PI-MOCVD. The injector works as a valve, regulating the flow of liquid introduced in the evaporation chamber and as a nozzle atomizing the flow into tiny droplets. As explained earlier (see Section 2.2), the dimensions of droplets have a fundamental impact on the evaporation efficiency of the liquid solution. Therefore, there are several key parameters of conventional solenoid injectors defining a quality of atomization: (i) the nozzle geometry (size, number of orifices, and degree of conicity), (ii) the flow regime inside the injector, and (iii) properties of the solution/liquid such as viscosity and concentration. The nozzle geometries and flow regimes have been extensively studied in the fuel injection industry [81–85]. Most commercially available injectors work in the frequency range between 0.1 and 200 Hz with very small opening times. These characteristics offer a very large working range in terms of injected flow rate and ultimately, in terms of film growth rate. In such injection system, the injection flow rate depends on the concentration and viscosity of the precursor solution, the opening frequency and time of the nozzle, and the pressure difference between the stored solution and the evaporator. Very small opening time ensures the fast evaporation process and helps to avoid the dissociation or premature decomposition of precursors before evaporation. However, it should be noted that the injector may produce instable flow at opening time below 1 ms. It is important to note, that injectors are commanded by

**Figure 4.** Schematic representation of a PP-CVD reactor using an ultrasonic nozzle. Reproduced with permission [75].

Copyright 2001, Elsevier.

40 Chemical Vapor Deposition for Nanotechnology

**Figure 5.** Schematic representation of a DLI-CVD furnace using solenoid electrovalves in combination with a moving tape. Reproduced with permission [90]. Copyright 1998, Elsevier.

solids and clog partially/completely the injector [92]. To overcome these issues, Kemstream® came up with the idea to have a two-stage injection process (**Figure 6**). The liquid injector introduces a precursor solution into the "mixing chamber", where a controlled amount of carrier gas is mixed with the precursor solution. A few milliseconds later, the second injector opens and the mixture is injected and atomized into the evaporator. The operator must make sure that there is a short delay between the opening times of two injectors. The most important advantage of double injector is the formation of a homogeneous liquid-gas phase in the mixing chamber. This drastically diminishes the radius of the injected droplets, hence increasing the evaporation efficiency. This method offers a far better atomization of the precursor solution along with the possibility to use even lower vapor pressure precursors than in single-injection systems. Brooks® Instrument also proposes a multiple liquid inlet direct liquid vaporizer system (**Figure 6**), which injects the solution into a chamber where it is both atomized by sending a carrier gas perpendicularly to the stream and vaporized in contact with a hot gas [93]. Kemstream®'s injector (**Figure 7a**) can be found on Annealsys® CVD and ALD machines and process are called "direct liquid injection (DLI)-CVD and ALD" [94–96]. The Brooks® DLI Vaporizer Systems (**Figure 7b**) were successfully implemented in industrial CVD [97–99] and ALD [34, 100–105] processes, as well. The major part of the DLI-CVD industry is dedicated to the deposition of very well known III–V semiconductors, in which HORIBASTEC Liquid Vaporization Systems (**Figure 7c**) are broadly used [106]. It is also interesting to note that although the latest emission standards pushed the fuel industry to go toward piezoelectric injectors, the piezoelectrically driven injectors and piezo-valves are still very rare in DLI-CVD systems [107].

**4. Conclusions and perspectives**

liquid vaporization systems (c).

oxide and chalcogenide, and noble metals.

Liquid mass flow controllers appear as a viable option for controlling a rate of a liquid flow, but they cannot be used as a direct liquid delivery unit in CVD systems. The main problem lies in the fact that it delivers a continuous flow of precursors in the evaporator. The process then depends entirely on the ability of the evaporator to vaporize the precursor, which is inefficient without any other atomizing system. The usage of a heated plate would be very unreliable in time. In the case of nebulizers or transducers, the progressive heating of the element leads to a drift in the aerosol flow. On the other hand, the ultrasonic nozzle shows a good reliability in evaporation and a good capability to handle "difficult" precursors. Each nozzle works at a certain frequency dictated by its dimension and it limits the tenability of the feeding rate. To regulate the amount of precursor sent to the substrate, the operator can either

**Figure 7.** Kemstream DLI injector plugged into a vaporizing box (a), Brooks DLI vaporizer system (b), and HORIBASTEC

Direct Liquid Injection Chemical Vapor Deposition http://dx.doi.org/10.5772/intechopen.80244 43

dilute/concentrate the solution or limit/increase the flow that comes to the nozzle.

Injection systems based on solenoid valves have reached very good performance and technological maturity. Major advantages brought by injection delivery systems are versatility, the digital control of the growth rate and the thickness of the film, and the storage of precursors at ambient conditions ensuring their stability in time. Advanced new-generation injection systems guarantee a controllable and stable feeding rate. DLI-CVD permits to vary and to control easily the composition of complex material film and doping level from sample to sample. Thus, it provides an excellent film composition control (<1%), has low cost, high efficiency, offers high uniformity on large area (<1.5%, up to 12 inch wafers), good conformal step coverage, high flexibility in precursors selection (solid or liquid), atomic control of film thickness (up to 10 Å), and possibility to deposit complex materials and multilayers. DLI-CVD is widely applied in industry of thin films fabrication ensuring high productivity and demonstrating excellent film uniformity and repeatability with high throughput in the CMOS era for a variety of different films including high-k dielectrics, rare-earth oxides, perovskite

Although several solutions were proposed in the literature, the industrial DLI-CVD systems are still not well adapted to the depositions at low temperatures due to issues of carbon contamination originating from organic part of the precursors and the presence of the solvent. Thus,

**Figure 6.** Schematic representation of Kemstream® two-stage injection process.

**Figure 7.** Kemstream DLI injector plugged into a vaporizing box (a), Brooks DLI vaporizer system (b), and HORIBASTEC liquid vaporization systems (c).

## **4. Conclusions and perspectives**

**Figure 6.** Schematic representation of Kemstream® two-stage injection process.

still very rare in DLI-CVD systems [107].

42 Chemical Vapor Deposition for Nanotechnology

solids and clog partially/completely the injector [92]. To overcome these issues, Kemstream® came up with the idea to have a two-stage injection process (**Figure 6**). The liquid injector introduces a precursor solution into the "mixing chamber", where a controlled amount of carrier gas is mixed with the precursor solution. A few milliseconds later, the second injector opens and the mixture is injected and atomized into the evaporator. The operator must make sure that there is a short delay between the opening times of two injectors. The most important advantage of double injector is the formation of a homogeneous liquid-gas phase in the mixing chamber. This drastically diminishes the radius of the injected droplets, hence increasing the evaporation efficiency. This method offers a far better atomization of the precursor solution along with the possibility to use even lower vapor pressure precursors than in single-injection systems. Brooks® Instrument also proposes a multiple liquid inlet direct liquid vaporizer system (**Figure 6**), which injects the solution into a chamber where it is both atomized by sending a carrier gas perpendicularly to the stream and vaporized in contact with a hot gas [93]. Kemstream®'s injector (**Figure 7a**) can be found on Annealsys® CVD and ALD machines and process are called "direct liquid injection (DLI)-CVD and ALD" [94–96]. The Brooks® DLI Vaporizer Systems (**Figure 7b**) were successfully implemented in industrial CVD [97–99] and ALD [34, 100–105] processes, as well. The major part of the DLI-CVD industry is dedicated to the deposition of very well known III–V semiconductors, in which HORIBASTEC Liquid Vaporization Systems (**Figure 7c**) are broadly used [106]. It is also interesting to note that although the latest emission standards pushed the fuel industry to go toward piezoelectric injectors, the piezoelectrically driven injectors and piezo-valves are

Liquid mass flow controllers appear as a viable option for controlling a rate of a liquid flow, but they cannot be used as a direct liquid delivery unit in CVD systems. The main problem lies in the fact that it delivers a continuous flow of precursors in the evaporator. The process then depends entirely on the ability of the evaporator to vaporize the precursor, which is inefficient without any other atomizing system. The usage of a heated plate would be very unreliable in time. In the case of nebulizers or transducers, the progressive heating of the element leads to a drift in the aerosol flow. On the other hand, the ultrasonic nozzle shows a good reliability in evaporation and a good capability to handle "difficult" precursors. Each nozzle works at a certain frequency dictated by its dimension and it limits the tenability of the feeding rate. To regulate the amount of precursor sent to the substrate, the operator can either dilute/concentrate the solution or limit/increase the flow that comes to the nozzle.

Injection systems based on solenoid valves have reached very good performance and technological maturity. Major advantages brought by injection delivery systems are versatility, the digital control of the growth rate and the thickness of the film, and the storage of precursors at ambient conditions ensuring their stability in time. Advanced new-generation injection systems guarantee a controllable and stable feeding rate. DLI-CVD permits to vary and to control easily the composition of complex material film and doping level from sample to sample. Thus, it provides an excellent film composition control (<1%), has low cost, high efficiency, offers high uniformity on large area (<1.5%, up to 12 inch wafers), good conformal step coverage, high flexibility in precursors selection (solid or liquid), atomic control of film thickness (up to 10 Å), and possibility to deposit complex materials and multilayers. DLI-CVD is widely applied in industry of thin films fabrication ensuring high productivity and demonstrating excellent film uniformity and repeatability with high throughput in the CMOS era for a variety of different films including high-k dielectrics, rare-earth oxides, perovskite oxide and chalcogenide, and noble metals.

Although several solutions were proposed in the literature, the industrial DLI-CVD systems are still not well adapted to the depositions at low temperatures due to issues of carbon contamination originating from organic part of the precursors and the presence of the solvent. Thus, further developments continue on the low-temperature CVD systems assisted by additional energy source such as plasma, UV, etc. Many research teams work on the molecular engineering of the precursor structure including combination of multiple ligands in order to ameliorate the decomposition of precursors at low deposition temperatures. CVD system efficiency and application at industrial scale remains highly dependent on the available precursor quality and reliability.

[2] Ozaydin-Ince G, Coclite AM, Gleason KK. CVD of polymeric thin films: Applications in sensors, biotechnology, microelectronics/organic electronics, microfluidics, MEMS,

[3] Deegan RD, Bakajin O, Dupont TF, Huber G, Nagel SR, Witten TA. Capillary flow as the

[4] Jensen KF, Kern W, Vossen JL. Thin Film Processes II. San Diego, California: Academic

[5] Reichelt K, Jiang X. The preparation of thin films by physical vapour deposition meth-

[6] Choy KL. Chemical vapour deposition of coatings. Progress in Materials Science. 2003;

[7] Liu M, Li X, Karuturi SK, Tokb AIY, Fan HJ. Atomic layer deposition for nanofabrication

[8] Choy KL. Innovative Processing of Films and Nanocrystalline Powders. Singapore:

[10] Chuprakov IS, Martin JD, Dahmen KH. New flash-evaporation feeder for chemical

[11] Hiskes R et al. Single source MOCVD of epitaxial oxide thin films. Journal of Crystal

[12] Perednis D, Gauckler LJ. Thin film deposition using spray pyrolysis. Journal of

[13] Manasevit HM, Simpson WI. The use of metal-organics in the preparation of semiconductor materials I. Epitaxial gallium-V compounds. Journal of the Electrochemical

[15] Rodgers DB. Self-metering Reservoir. Rhinebeck, New York: Icon Dynamics LLC; 1997 [16] Sénateur JP et al. Pulsed injection MOCVD of functional electronic oxides. Advanced

[17] Sievers RE et al. Volatile barium β-diketonates for use as MOCVD precursors. Coordinatio

[18] Jensen KF et al. Metalorganic chemical vapor deposition: Examples of the influence of precursor structure on film properties. Le Journal de Physique IV. 1991;**02**(C2):C2-243-C2-252

[19] Jones AC et al. Metalorganic chemical vapour deposition (MOCVD) of zirconia and lead zirconate titanate using a novel zirconium precursor. Journal of the European Ceramic

vapor deposition. Le Journal de Physique IV. 1999;**9**(PR8):Pr8-901-Pr8-908

thin films on MgO by flash MOCVD. Journal

Direct Liquid Injection Chemical Vapor Deposition http://dx.doi.org/10.5772/intechopen.80244 45

Cu<sup>3</sup> O<sup>7</sup>−<sup>x</sup>

superconducting films.

composites and membranes. Reports on Progress in Physics. 2012;**75**:016501

cause of ring stains from dried liquid drops. Nature. 1997;**389**:827

Press Inc.; 1991. p. 284

World Scientific; 2002

Growth. 1993;**128**:781-787

Electroceramics. 2005;**14**:103-111

Society. 1969;**116**(12):1725-1732

[14] Dickinson PH et al. Chemical vapor deposition of YBa2

Materials for Optics and Electronics. 2000;**10**:155-161

Journal of Applied Physics. 1989;**66**(1):444-447

Chemistry Reviews. 1993;**128**:285-291

Society. 1999;**19**:1431-1434

**48**:57

ods. Thin Solid Films. 1990;**191**:91

and interface engineering. Nanoscale. 2012;**4**:1522

[9] Gorbenko OY, Bosak AA. Growth of LaNiO<sup>3</sup>

of Crystal Growth. 1998;**186**(1-2):181-188

As described above, direct liquid delivery CVD has taken a profit from the developments in other fields, for example, chromatography, industrial spray coating, and automotive industry. The valves, transducers, liquid mass flow controllers, capillaries, etc. developed for other applications were successfully adapted to the liquid precursor/solution delivery in the CVD reactors. Thus, the advances in liquid delivery systems mainly used in large public applications determine the further developments of CVD systems, as well. The automotive industry has upgraded its fuel injection technology by implementing injectors with piezoelectric nozzles to adapt to the new emission standards. The piezoelectric nozzles react much faster, meaning a faster opening and closure of the valve assuring a better control of the injected flow, and the opening time can be tuned much easier by changing the allocated tension. Probably, all these improvements will be soon implemented in the next generation DLI-CVD systems, which will serve beyond CMOS Era.

## **Acknowledgements**

The authors are grateful to the French National Research Agency for funding through the grants ANR LiLit (ANR-16-CE24-0022-011), Labex ACTION program (ANR-11- LABX-0001-01), and the EUR EIPHI program (contract no. ANR-17-EURE-0002). This work was partly supported by the French RENATECH network. The authors are also grateful to A. Almirall, S. Kuprenaite, and G. Clementi for the fruitful discussions and corrections.

## **Author details**

Vincent Astié1 \*, Cyril Millon1 , Jean-Manuel Decams2 and Ausrine Bartasyte1

\*Address all correspondence to: vincent.astie@femto-st.fr

1 FEMTO-ST Institute, University of Bourgogne Franche-Comté, CNRS UMR 6174, Besançon, France

2 Annealsys, Montpellier, France

## **References**

[1] Braun D, Cherdron H, Rehahn M, Ritter H, Voit B. Polymer Synthesis: Theory and Practice: Fundamentals, Methods, Experiments. 5th ed. Heidelberg: Springer-Verlag Berlin; 2013

[2] Ozaydin-Ince G, Coclite AM, Gleason KK. CVD of polymeric thin films: Applications in sensors, biotechnology, microelectronics/organic electronics, microfluidics, MEMS, composites and membranes. Reports on Progress in Physics. 2012;**75**:016501

further developments continue on the low-temperature CVD systems assisted by additional energy source such as plasma, UV, etc. Many research teams work on the molecular engineering of the precursor structure including combination of multiple ligands in order to ameliorate the decomposition of precursors at low deposition temperatures. CVD system efficiency and application at industrial scale remains highly dependent on the available precursor quality and

As described above, direct liquid delivery CVD has taken a profit from the developments in other fields, for example, chromatography, industrial spray coating, and automotive industry. The valves, transducers, liquid mass flow controllers, capillaries, etc. developed for other applications were successfully adapted to the liquid precursor/solution delivery in the CVD reactors. Thus, the advances in liquid delivery systems mainly used in large public applications determine the further developments of CVD systems, as well. The automotive industry has upgraded its fuel injection technology by implementing injectors with piezoelectric nozzles to adapt to the new emission standards. The piezoelectric nozzles react much faster, meaning a faster opening and closure of the valve assuring a better control of the injected flow, and the opening time can be tuned much easier by changing the allocated tension. Probably, all these improvements will be soon implemented in the next generation DLI-CVD systems, which will

The authors are grateful to the French National Research Agency for funding through the grants ANR LiLit (ANR-16-CE24-0022-011), Labex ACTION program (ANR-11- LABX-0001-01), and the EUR EIPHI program (contract no. ANR-17-EURE-0002). This work was partly supported by the French RENATECH network. The authors are also grateful to A. Almirall, S. Kuprenaite, and G. Clementi for the fruitful discussions and corrections.

and Ausrine Bartasyte1

, Jean-Manuel Decams2

1 FEMTO-ST Institute, University of Bourgogne Franche-Comté, CNRS UMR 6174,

[1] Braun D, Cherdron H, Rehahn M, Ritter H, Voit B. Polymer Synthesis: Theory and Practice: Fundamentals, Methods, Experiments. 5th ed. Heidelberg: Springer-Verlag Berlin; 2013

reliability.

44 Chemical Vapor Deposition for Nanotechnology

serve beyond CMOS Era.

**Acknowledgements**

**Author details**

\*, Cyril Millon1

2 Annealsys, Montpellier, France

\*Address all correspondence to: vincent.astie@femto-st.fr

Vincent Astié1

Besançon, France

**References**


[20] Loboda MJ. Low temperature PECVD growth and characterization of a-SiC: H films deposited from silacyclobutane and silane/methane precursor gases. In: Amorphous and Crystalline Silicon Carbide IV. Heidelberg: Springer, Berlin; 1992. pp. 271-280

[37] Niinistö J et al. Controlled growth of HfO2

[39] Goldstein DN, McCormick JA, George SM. Al2

2271-2275

309-314

(H2

O)98-y(EtOH)y

1991;**139**:394-400

thin films by atomic layer deposition from

Direct Liquid Injection Chemical Vapor Deposition http://dx.doi.org/10.5772/intechopen.80244 47

atomic layer deposition with trimeth-

cyclopentadienyl-type precursor and water. Journal of Materials Chemistry. 2005;**15**(23):

ylaluminum and ozone studied by in situ transmission FTIR spectroscopy and quadrupole mass spectrometry. The Journal of Physical Chemistry C. 2008;**112**:19530-11939

[40] Lim SJ, Kwon S, Kim H. ZnO thin films prepared by atomic layer deposition and rf sputtering as an active layer for thin film transistor. Thin Solid Films. 2008;**516**(7):1523-1528

[41] Williams PA et al. Growth of hafnium dioxide thin films by liquid-injection MOCVD using alkylamide and hydroxylamide precursors. Chemical Vapor Deposition. 2003;**9**(6):

[42] Hausmann DM et al. Atomic layer deposition of hafnium and zirconium oxides using

[43] Maxwell JC. Collected Scientific Papers. Taken from Fuchs NA. Evaporation and Droplet Growth in Gaseous Media. Cambridge, New York: Pergamon Press; 1959;**11**:1877

[44] Duguid HA. A study of the evaporation rates of small freely falling water droplets.

[45] Sirignano WA. Fuel droplet vaporization and spray combustion theory. Progress in

[46] Sirignano WA. Fluid Dynamics and Transport of Droplets and Sprays. Irvine: Cambridge

[49] Sinnott SB et al. Model of carbon nanotube growth through chemical vapor deposition.

[50] Borgharkar NS et al. Solution delivery of Cu(hfac)2 for alcohol-assisted chemical vapor deposition of copper. Journal of the Electrochemical Society. 1999;**146**(3):1041-1045

[51] Li K. Direct Liquid Evaporation Chemical Vapor Deposition (DLE-CVD) of Nickel, Manganese and Copper-Based Thin Films for Interconnects in Three-Dimensional

[52] Esquenazi GL, Brinson B, Barron AR. Catalytic growth of carbon nanotubes by direct

]. C-Journal of Carbon Research. 2018;**4**(1):17

PMo12O40⊂H<sup>4</sup>

Mo72Fe30(O2

CMe)15O254

metal amide precursors. Chemistry of Materials. 2002;**14**(10):4350-4358

[47] Kuo KK. Principles of Combustion. New York: John Wiley and Sons; 1986

[48] Williams FA. Combustion Theory, Benjamin Cummins. CA: Menlo Park; 1985

Microelectronic Systems. Cambridge: Harvard University; 2016. p. 127

liquid injection CVD using the nanocluster [Hx

Master Theses 5295, Rolla: University of Missouri; 1969

Energy and Combustion Science. 1983;**9**(4):291-322

University Press, University of California; 1999

Chemical Physics Letters. 1999;**315**:25-30

O3

[38] Frenck H et al. Low temperature remote plasma-enhanced deposition of thin metal oxide films by decomposition of metal alkoxides. Materials Science and Engineering: A.


[37] Niinistö J et al. Controlled growth of HfO2 thin films by atomic layer deposition from cyclopentadienyl-type precursor and water. Journal of Materials Chemistry. 2005;**15**(23): 2271-2275

[20] Loboda MJ. Low temperature PECVD growth and characterization of a-SiC: H films deposited from silacyclobutane and silane/methane precursor gases. In: Amorphous and Crystalline Silicon Carbide IV. Heidelberg: Springer, Berlin; 1992. pp. 271-280

[21] Schweitzer GK, Pullen BP, Fang Y-H. The volatilities of some lead β-diketonates. Analytica

[22] Fahlman BD, Barron AR. Substituent effects on the volatility of metal β-diketonates.

[24] Jodin L et al. Method for producing lithium-based layers by cvd. 2013. Google Patents

[25] Lee D-J, Kang S-W, Rhee S-W. Chemical vapor deposition of ruthenium oxide thin films from Ru(tmhd)3 using direct liquid injection. Thin Solid Films. 2002;**413**:237-242

[26] Tao W, Desu SB, Li TK. Direct liquid injection MOCVD of high quality PLZT films.

tic and optical applications. Advanced Materials Interfaces. 2017;**4**(8):1600998

[29] Jones AC. Molecular design of improved precursors for the MOCVD of electroceramic

[30] Bradley DC, Mehrotra RC, Gaur DP. Alkoxides, Metal. New York: Academic Press; 1978 [31] El Eter M. Synthèse, caractérisation et évaluation de nouveaux précurseurs azotés pour

[32] Akiyama Y et al. Epitaxial growth of lithium niobate film using metalorganic chemical

[33] Tanaka A et al. Preparation of lithium niobate films by metalorganic chemical vapor deposition with a lithium alkoxide source. Journal of Crystal Growth. 1995;**148**:324-326

[34] Gaskell JM et al. Liquid injection ALD and MOCVD of lanthanum aluminate using a bimetallic alkoxide precursor. Journal of Materials Chemistry. 2006;**16**(39):3854-3860

[35] Krumdieck SP et al. Solid yttria-stabilized zirconia films by pulsed chemical vapor deposition from metal-organic precursors. Journal of the American Ceramic Society. 2002;

[36] Niinistö J et al. Advanced cyclopentadienyl precursors for atomic layer deposition of

thin films. Journal of Materials Chemistry. 2008;**18**(28):3385-3390

substrate by liquid source chemical vapor deposition using tetrahydrofuran solution of

thin films with yttria stabilized zirconia buffer layer on metal

and LiTaO<sup>3</sup>

(M = Hf, Zr) par MOCVD à injection liquide.

Cu<sup>3</sup>

O<sup>7</sup>−δ. Chemical

thin films for acous-

Advanced Materials for Optics and Electronics. 2000;**10**:223-232

[23] Watson IM. Metal-organic CVD of the high-Tc superconductor YBa2

β-diketonates. Japanese Journal of Applied Physics. 1995;**34**:2293

oxides. Journal of Materials Chemistry. 2002;**12**(9):2576-2590

vapor deposition. Thin Solid Films. 2007;**515**(12):4975-4979

Chimica Acta. 1968;**43**:332-334

46 Chemical Vapor Deposition for Nanotechnology

Vapor Deposition. 1997;**3**(1):9-26

Materials Letters. 1995;**23**:177-180

Cu<sup>3</sup> Ox

dépôt de films d'oxydes métalliques MO2

Lyon: Université Claude Bernard-Lyon I; 2008

[28] Bartasyte A et al. Toward high-quality epitaxial LiNbO<sup>3</sup>

[27] Matsuno S et al. YBa2

**85**(11):2873-2875

ZrO2


[53] Gordon RG, Kramer K, Liu X. Chemical vapor deposition and properties of amorphous aluminum oxide films. MRS Proceedings. 1996;**446**:383

[71] Krumdieck SP, Raj R.Experimental characterization and modeling of pulsed MOCVD with ultrasonic atomization of liquid precursor. Chemical Vapor Deposition. 2001;**7**(2):85-90 [72] Krumdieck SP, Raj R. Conversion efficiency of alkoxide precursor to oxide films grown by an ultrasonic-assisted, pulsed liquid injection, metalorganic chemical vapor deposition (pulsed-CVD) process. Journal of the American Ceramic Society. 1999;**82**(6):1605-1607

[73] Porporati A, Roitti S, Sbaizero O. Metallorganic chemical vapor deposition of Ta2

[74] Versteeg VA, Avedisian CT, Raj R. Metalorganic chemical vapor deposition by pulsed liquid injection using an ultrasonic nozzle: Titanium dioxide on sapphire from titanium(IV)

[75] Krumdieck S, Raj R. Growth rate and morphology for ceramic films by pulsed-

[77] Bosch Premium Fuel Injectors Trade Brochure. Robert Bosch LLC, Farmington Hills,

[78] Paule K. Fuel Injection System for Internal Combustion Engines with an Electromagnetic

[79] Kennedy HE, Brooks W. Method and apparatus for the electromagnetic control of the fuel injection valves of an internal combustion engine. Patent no. DE655355C licenced to

[80] Sénateur JP et al. Procédé et Dispositif d'Introduction de Précurseur dans une Enceinte de Dépôt Chimique en Phase Vapeur. Patent no. 93/08838 licenced to J.I.P. Elec, 11 che-

[81] Reitz RD.Atomization and Other Breakup Regimes of a Liquid Jet. Princeton, New Jersey:

[82] Tennant PA et al. Simulation of flow through single orifice nozzles. In ICLASS-90,

[83] Hiroyasu H, Arai M, Shimizu M. Break-up length of a liquid jet and internal flow in a

[84] Schmidt DP, Corradini ML. The internal flow of diesel fuel injector nozzles: A review.

[85] Payri R et al. Diesel nozzle geometry influence on spray liquid-phase fuel penetration in

[86] Dubourdieu C et al. Pulsed liquid-injection MOCVD of high-K oxides for advanced semiconductor technologies. Materials Science and Engineering: B. 2005;**118**(1-3):105-111

thin film by pulsed metalorganic chemical vapor deposi-

Direct Liquid Injection Chemical Vapor Deposition http://dx.doi.org/10.5772/intechopen.80244

isopropoxide. Journal of the American Ceramic Society. 1995;**78**(10):2763-2768

tion from a single precursor. Applied Physics Letters. 1993;**63**(23):3146-3148

films. Journal of the European Ceramic Society. 2003;**23**(2):247-251

MOCVD. Surface and Coatings Technology. 2001;**141**(1):7-14

[76] Xie H, Raj R. Epitaxial LiTaO<sup>3</sup>

BROOKS WALKER. 1930

Princeton University; 1978

Hartford, Connecticut. 1990

Michigan. 17032145 RevA. 2017

Injector. Stuttgart: Robert Bosch GmbH; 1957

min du Vieux Chêne, 38240 Meylan, France. 1993

nozzle. In ICLASS-91, Gaithersburg. 1991:275-292

evaporative conditions. Fuel. 2008;**87**(7):1165-1176

International Journal of Engine Research. 2005;**2**(1):1-22

O5

49


[71] Krumdieck SP, Raj R.Experimental characterization and modeling of pulsed MOCVD with ultrasonic atomization of liquid precursor. Chemical Vapor Deposition. 2001;**7**(2):85-90

[53] Gordon RG, Kramer K, Liu X. Chemical vapor deposition and properties of amorphous

[54] Jones AC et al. MOCVD of zirconia thin films by direct liquid injection using a new class

cursor Ti(mpd)(dmae)2. Le Journal de Physique IV. 2001;**11**(PR3):Pr3-531-Pr3-537

O3

[57] Vernardou D, Pemble ME, Sheel DW. Vanadium oxides prepared by liquid injection MOCVD using vanadyl acetylacetonate. Surface and Coatings Technology. 2004;**188-189**:

[59] Lang RJ.Ultrasonic atomization of liquids. The Journal of the Acoustical Society of America.

[60] Berger HL. Ultrasonic Liquid Atomization: Theory and Application. Hyde Park, New

[61] Tsai S, Childs P, Luu P. Correlation between droplet diameter and viscosity. AICHE Journal.

[62] Engie R. Maximizing the use of platinum catalyst by ultrasonic spray application. In: Proceedings of ASME 2011 5th International Conference on Energy Sustainability & 9th Fuell Cell Science, Engineering and Technology Conference; Wasington, DC, USA. 2011

[63] Huffman M. Liquid source misted chemical deposition (LSMCD)—A critical review.

[64] Cross WB, Parkin IP.Aerosol assisted chemical vapour deposition of tungsten oxide films from polyoxotungstate precursors: Active photocatalysts. Chemical Communications.

[68] Warwick MEA et al. Hybrid chemical vapour and nanoceramic aerosol assisted deposition for multifunctional nanocomposite thin films. Thin Solid Films. 2011;**519**(18):5942-5948

[69] Krumdieck SP. Pulsed-pressure MOCVD science. Materials and Technology. ECS

[70] Versteeg VA, Avedisian CT, Raj R. Method and Apparatus for CVD using Liquid Delivery System with an Ultrasonic Nozzle. USA: Cornell Research Foundation, Inc.; 1995

chemical vapor deposition. Journal of the Electrochemical Society. 2002;**149**(6):C345-C348

films using the pre-

films by direct liquid injection metallorganic

of zirconium precursor. Chemical Vapor Deposition. 1998;**4**(2):46-49

[55] Awaluddin A et al. Direct liquid injection MOCVD growth of TiO2

[58] Rayleigh JWS. The Theory of Sound. Vol. 2. London: MacMillan; 1896

[65] McMillan LD, Paz de Araujo CA. U.S. Patent number 5.119.760. 1992 [66] McMillan LD, Paz de Araujo CA. U.S. Patent number 5.138.520. 1992

[67] McMillan LD et al. Liquid source CVD. Integrated Ferroelectrics. 1991;**2**:351-359

aluminum oxide films. MRS Proceedings. 1996;**446**:383

[56] Kang S-W, Rhee S-W. Deposition of La2

48 Chemical Vapor Deposition for Nanotechnology

York: Partridge Hill Publishers; 1998

Integrated Ferroelectrics. 1995;**10**(1-4):39-53

Transactions. 2009;**25**(8):1209-1219

250-254

1962;**34**(1):6-8

1996;**42**(120):3340

2003;**14**:1696-1697


[87] Dubourdieu C et al. Growth by liquid-injection MOCVD and properties of HfO2 films for microelectronic applications. Chemical Vapor Deposition. 2006;**12**(2-3):187-192

[103] Chalker PR et al. Liquid injection atomic layer deposition of silver nanoparticles.

[104] Karim Z et al. Advances in ALD equipment for sub-40nm memory capacitor dielectrics: Precursor delivery. Materials and Processes. ECS Transactions. 2008;**16**(4):125-134 [105] Kim SK, Hoffmann-Eifert S, Waser R. Growth of noble metal Ru thin films by liquid injection atomic layer deposition. The Journal of Physical Chemistry C. 2009;**113**:11329-11335

[106] PGV-CE23A, Liquid Source Vaporization System Datasheet. Kyoto: Horiba Stec, Co.,

cal vapor deposition. Microporous and Mesoporous Materials. 2012;**163**:229-236

membranes prepared by liquid-injection chemi-

Direct Liquid Injection Chemical Vapor Deposition http://dx.doi.org/10.5772/intechopen.80244 51

Nanotechnology. 2010;**21**(40):405602

[107] Hemmer E et al. Nanostructured ZrO2

Ltd.; 2018


[103] Chalker PR et al. Liquid injection atomic layer deposition of silver nanoparticles. Nanotechnology. 2010;**21**(40):405602

[87] Dubourdieu C et al. Growth by liquid-injection MOCVD and properties of HfO2 films for microelectronic applications. Chemical Vapor Deposition. 2006;**12**(2-3):187-192

[89] Kaul AR, Seleznev BV. New principle of feeding for flash evaporation MOCVD devices.

[91] Senzaki Y et al. Atomic level solutions® for advanced microelectronic applications. In: Solid-State and Integrated-Circuit Technology, ICSICT 2008. IEEE. 2008:1308-1311 [92] Guillon H et al. Device for Introducing, Injecting or Spraying a Mixture of a Carrier Gas and Liquid Compounds and Method for Implementing Said Device. Montpellier:

[93] DS-VAP-DLI Systems-eng (0611) Data Sheet: Liquid Vaporizer Systems, Brooks Instrument,

[96] Boisselier G, Maury F, Schuster F. SiC coatings grown by liquid injection chemical vapor deposition using single source metal-organic precursors. Surface and Coatings Technology.

[97] Li N et al. Growth of atomically smooth epitaxial nickel ferrite films by direct liquid

[98] Archana PS et al. Photocatalytic water oxidation at bismuth vanadate thin film electrodes grown by direct liquid injection chemical vapor deposition method. International

[99] Yengantiwar A et al. Direct liquid injection chemical vapor deposition of molybdenumdoped bismuth vanadate photoelectrodes for efficient solar water splitting. The Journal

[100] Gaskell JM et al. Deposition of lanthanum zirconium oxide high-k films by liquid injec-

hafnium-cyclopentadienyl precursors. Chemical Vapor Deposition. 2007;**13**(11):609-617

ALD using ansa-metallocene zirconium and hafnium precursors. Journal of Materials

tion ALD and MOCVD. Chemical Vapor Deposition. 2007;**13**(12):684-690

and HfO2

[94] Avril L, Decams JM, Imhoff L. Pulsed direct liquid injection ALD of TiO2

titanium tetraisopropoxide precursor. Physics Procedia. 2013;**46**:33-39

injection CVD. Chemical Vapor Deposition. 2011;**17**(7-9):261-269

Journal of Hydrogen Energy. 2017;**42**(12):8475-8485

of Physical Chemistry C. 2017;**121**(11):5914-5924

[101] O'Kane R et al. Growth of HfO2

[102] Black K et al. Deposition of ZrO2

Chemistry. 2008;**18**:4561-4571

[90] Pignard S et al. Chemical vapor deposition and structural study of La0.8MnO3-<sup>δ</sup>

thin films by liquid injection atomic layer deposi-

thin

films using

composite films grown by direct liquid

by liquid injection MOCVD and ALD using new

thin films by liquid injection MOCVD and

[88] Hušeková K et al. Growth of RuO2

50 Chemical Vapor Deposition for Nanotechnology

KEMSTREAM; 2016

2013;**215**:152-160

LLC, Hatfield, Pennsylvania. 2011

[95] Avril L et al. Alumina particle reinforced TiO2

injection MOCVD. Vacuum. 2014;**107**:259-263

tion. Thin Solid Films. 2010;**518**(16):4701-4704

Journal De Physique. IV. 1993;**3**(3):375-378

films. Thin Solid Films. 1998;**347**:161-166


**Section 2**

**CVD of Thin Films**

**Section 2**

## **CVD of Thin Films**

**Chapter 3**

**Provisional chapter**

N4

and thus

**DLC Layers Created Using CVD Techniques and Their**

**DLC Layers Created Using CVD Techniques and Their** 

One way to obtain new materials with different properties is to modify existing ones to improve their inadequate properties. Due to the fact that many useful properties of materials, including resistance to wear and corrosion, coefficient of friction and biocompatibility, depend on the state of the surface, modern methods of surface engineering are particularly useful. They include the deposition of layers with a matching chemical composition and structure. In terms of applications, the most suitable seem to be amorphous or nanocrystalline layers containing carbon, nitrogen, silicon and hydrogen. They com-

have a strong resistance to oxidation at high temperatures, high modulus of elasticity, low coefficient of friction and wear resistance. However, the silicon carbonitride compound is not thermodynamically stable under normal conditions and therefore must be obtained as a result of unconventional synthesis. One of these methods is chemical vapor deposition (CVD), including the most widely used plasma-assisted chemical vapor deposition (PACVD) techniques. The materials obtained, thanks to these techniques, have found and are still finding wider and wider application in many branches of industry and medicine.

For several dozen years, scientists have been involved in obtaining diamond-like carbon (DLC) layers by various methods. Depending on the predominance of phenomena or the type of physical, chemical, or physicochemical interaction on the core or the substrate, it can be divided into six groups of methods: mechanical, thermomechanical, thermal, electrochemical, chemical, and physical [1]. Among them, plasma-assisted chemical vapor deposition (PACVD)

bine the advantageous properties of silicon carbide SiC and silicon nitride Si<sup>3</sup>

**Keywords:** CVD, plasma, DLC layers, polymers, Ti alloy, Al alloy

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

DOI: 10.5772/intechopen.79526

**Application**

**Application**

Marta Januś

Marta Januś

**Abstract**

**1. Introduction**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.79526

#### **DLC Layers Created Using CVD Techniques and Their Application DLC Layers Created Using CVD Techniques and Their Application**

DOI: 10.5772/intechopen.79526

#### Marta Januś Marta Januś

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.79526

#### **Abstract**

One way to obtain new materials with different properties is to modify existing ones to improve their inadequate properties. Due to the fact that many useful properties of materials, including resistance to wear and corrosion, coefficient of friction and biocompatibility, depend on the state of the surface, modern methods of surface engineering are particularly useful. They include the deposition of layers with a matching chemical composition and structure. In terms of applications, the most suitable seem to be amorphous or nanocrystalline layers containing carbon, nitrogen, silicon and hydrogen. They combine the advantageous properties of silicon carbide SiC and silicon nitride Si<sup>3</sup> N4 and thus have a strong resistance to oxidation at high temperatures, high modulus of elasticity, low coefficient of friction and wear resistance. However, the silicon carbonitride compound is not thermodynamically stable under normal conditions and therefore must be obtained as a result of unconventional synthesis. One of these methods is chemical vapor deposition (CVD), including the most widely used plasma-assisted chemical vapor deposition (PACVD) techniques. The materials obtained, thanks to these techniques, have found and are still finding wider and wider application in many branches of industry and medicine.

**Keywords:** CVD, plasma, DLC layers, polymers, Ti alloy, Al alloy

## **1. Introduction**

For several dozen years, scientists have been involved in obtaining diamond-like carbon (DLC) layers by various methods. Depending on the predominance of phenomena or the type of physical, chemical, or physicochemical interaction on the core or the substrate, it can be divided into six groups of methods: mechanical, thermomechanical, thermal, electrochemical, chemical, and physical [1]. Among them, plasma-assisted chemical vapor deposition (PACVD)

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

techniques are most commonly used. These methods, thanks to the activation of chemical processes in the gas phase, which is a low-temperature plasma, allow for obtaining layers at relatively low temperatures [2, 3].

Materials with this type of applied layers have found wide application in many branches of the industry (electronics, optics, aviation, shipbuilding, construction) and in medicine.

DLC Layers Created Using CVD Techniques and Their Application

http://dx.doi.org/10.5772/intechopen.79526

57

In CVD processes, the growth of the coating is limited by the rate of chemical reactions between the adsorbed gas molecules on the surface of the substrate. At low temperatures, the growth of the layers is limited by chemical reactions and at high temperatures by diffusion of

The later methods (LP CVD) are divided by the way in which the process is activated:

A more detailed discussion of these problems will be provided in this chapter.

The partition of CVD methods (**Figure 2**) can be carried out in terms of:

• Gas substrate type: halide (H) and organometallic (MO)

• Pressure: atmospheric (AP) and depressed (LP)

**2. CVD techniques**

active products to the surface.

○ Glow discharge (DC) ○ Radio frequency (RF)

○ Microwaves (MW)

**Figure 2.** The partition of CVD methods [1].

• Plasma (PA):

The history of amorphous carbon nitride (a-C:N:H) layers originated in the 1990s of the last century, when Liu and Cohen [4], using numerical methods, suggested the stability of β-C<sup>3</sup> N4 crystal (**Figure 1**). According to them, this crystal would have a structure analogous to β-Si<sup>3</sup> N4 , characterized by short bonds and high covalence (about 93%). According to the predictions of these authors, C<sup>3</sup> N4 should have a modulus of stiffness and hardness even greater than diamond. Attractive, predicted properties of this material have become the cause of intense research on the synthesis of this crystal. In most experiments, however, amorphous materials were obtained with maximum nitrogen consisting of up to 45%. Only in a small number of cases, evidence for the existence of C<sup>3</sup> N4 in crystalline form has been found. In works [5, 6], small crystals in an amorphous matrix were observed under a transmission electron microscope (TEM).

Failed attempts to synthesize crystalline C<sup>3</sup> N4 lead to an increased interest in amorphous carbon nitride in the form of layers. These layers, as it turned out, are characterized by high wear resistance and a low coefficient of friction and thus can become competitive to diamond-like carbon (DLC) layers, which are well known and used in various branches of technology and medicine. This fact makes the a-C:N:H layers the focus of many research centers in the world.

Pioneer, though unsuccessful, research in crystalline C<sup>3</sup> N4 production also resulted in the discovery of the new SiCx Ny silicon carbonitride compound, which combines the properties of silicon nitride, silicon carbide, and carbon nitride. Depending on the synthesis conditions and the precursors used, this compound may exist in both crystalline and amorphous forms [8, 9], and its chemical composition may change.

For example, at low temperatures, using precursors containing hydrogen in their composition, amorphous layers are formed in which this element occurs in their structure in a bonded form (SiCx Ny (H)) [10, 11].

It is understood that the properties of these layers depend on their chemical composition and structure.

**Figure 1.** Computer model C3N4 [7].

Materials with this type of applied layers have found wide application in many branches of the industry (electronics, optics, aviation, shipbuilding, construction) and in medicine.

A more detailed discussion of these problems will be provided in this chapter.

## **2. CVD techniques**

techniques are most commonly used. These methods, thanks to the activation of chemical processes in the gas phase, which is a low-temperature plasma, allow for obtaining layers at

The history of amorphous carbon nitride (a-C:N:H) layers originated in the 1990s of the last century, when Liu and Cohen [4], using numerical methods, suggested the stability of

greater than diamond. Attractive, predicted properties of this material have become the cause of intense research on the synthesis of this crystal. In most experiments, however, amorphous materials were obtained with maximum nitrogen consisting of up to 45%. Only in a small

works [5, 6], small crystals in an amorphous matrix were observed under a transmission

bon nitride in the form of layers. These layers, as it turned out, are characterized by high wear resistance and a low coefficient of friction and thus can become competitive to diamond-like carbon (DLC) layers, which are well known and used in various branches of technology and medicine. This fact makes the a-C:N:H layers the focus of many research centers in the world.

of silicon nitride, silicon carbide, and carbon nitride. Depending on the synthesis conditions and the precursors used, this compound may exist in both crystalline and amorphous forms

For example, at low temperatures, using precursors containing hydrogen in their composition, amorphous layers are formed in which this element occurs in their structure in a bonded

It is understood that the properties of these layers depend on their chemical composition and

N4

N4

number of cases, evidence for the existence of C<sup>3</sup>

Pioneer, though unsuccessful, research in crystalline C<sup>3</sup>

Ny

[8, 9], and its chemical composition may change.

(H)) [10, 11].

Failed attempts to synthesize crystalline C<sup>3</sup>

crystal (**Figure 1**). According to them, this crystal would have a structure analogous

, characterized by short bonds and high covalence (about 93%). According to the

N4

N4

silicon carbonitride compound, which combines the properties

should have a modulus of stiffness and hardness even

in crystalline form has been found. In

production also resulted in the

lead to an increased interest in amorphous car-

relatively low temperatures [2, 3].

56 Chemical Vapor Deposition for Nanotechnology

predictions of these authors, C<sup>3</sup>

electron microscope (TEM).

discovery of the new SiCx

Ny

**Figure 1.** Computer model C3N4 [7].

form (SiCx

structure.

β-C<sup>3</sup> N4

to β-Si<sup>3</sup>

N4

In CVD processes, the growth of the coating is limited by the rate of chemical reactions between the adsorbed gas molecules on the surface of the substrate. At low temperatures, the growth of the layers is limited by chemical reactions and at high temperatures by diffusion of active products to the surface.

The partition of CVD methods (**Figure 2**) can be carried out in terms of:


The later methods (LP CVD) are divided by the way in which the process is activated:

	- Glow discharge (DC)
	- Radio frequency (RF)
	- Microwaves (MW)

**Figure 2.** The partition of CVD methods [1].

	- Thermoemission (HF)
	- Bunch (EA)

The main difference between conventional and plasma-activated CVD methods lies in the fact that, in the case of PACVD, the molecules are in a higher state of internal energy. Electric discharges of 2.45 GHz (microwaves) and 13.56 MHz (radio waves) are among the most useful methods of producing low-temperature plasma [12, 13].

**2.1. The role of plasma**

technology was also quickly found.

types of surface substrates.

in the system.

surface (active sites).

port toward the core of the material.

**2.2. Synergism in coating technology**

The commonly known definition of plasma is, according to Langmuir [15], partially or almost completely ionized gas, which is also the fourth state of matter (97% of the universe [16]). Initially, it was an object of interest only among physicists. However, application in materials

DLC Layers Created Using CVD Techniques and Their Application

http://dx.doi.org/10.5772/intechopen.79526

59

Research into the improvement of physical properties, as well as physicochemical properties, such as wear and corrosion resistance of thin films and coatings, has somehow contributed to the use of plasma energy for obtaining materials in the form of layers and coatings on various

The reactor chamber is a kind of unloading pipe in which (under reduced pressure (about 10–80 Pa), the influence of electromagnetic waves is present) gas ionizing energy is supplied with a strictly defined and controlled flow rate [17]. The results of this process are also:

• Increasing the temperature of the substrate, ions and neutral molecules with high kinetic energy falling on its surface can be additionally supported by the use of heating elements

• Transport and adsorption processes of atoms and ions—under conditions of temperature gradient and concentration of components on the surface of the substrate—and their trans-

• Coating growth—heterogeneous reactions in thermodynamically privileged places on the

The concept of synergism is used in various fields of science. As a technique, it is usually referred to as the interaction of more than one factor for the final effect obtained (improvement of a given physicochemical property or obtaining a completely new functionality).

This definition is also reflected in surface technology. However, it is assumed here every time that the obtained effects are due to the influence of many factors [1]. In this case the term synergism refers here to the usable properties of the modified surface. This action is achieved by:

• Using transitional layers (so-called interlayers), this has a significant impact on adhesion of

Therefore, we can say that it is necessary to design technology in many experimental works that ensures optimal material parameters that are maximally beneficial for the given applica-

tion (both the properties of the coatings obtained and their adhesion to the surface).

• Choosing the right method for treating the substrate surface.

• Selecting chemical and phase composition of coatings.

the obtained coatings to the surface.

• Reactions between dissociated molecules that may be neutral or ionized.

**Figure 3a** shows the reactions occurring in the CVD process involving plasma. The presence of particles with an increased state of internal energy and ion bombardment on the surface of the substrate increases the surface diffusion rate and decreases the activation energy of chemical reactions leading to the formation of a coating. Layering takes place at a much lower temperature.

This raises the question of what can affect the design of coating technologies. This can be considered from two sides:


For processes taking place under reduced pressure, aside from the choice of the method, the individual processing phases are of particular importance. An important role is also played by the type of intermediate layers and final obtained coatings and thus the technological parameters of each stage.

Temperature is one of the process parameters. If CVD processes are generated by plasma, the treatment can already be carried out at room temperature. This is related to the activation of chemical reactions in the gas phase by electrons accelerated in an electromagnetic field.

In that case of coating deposition, with time, subsequent stages of layer growth occur, which consequently form a continuous structure on the surface (**Figure 3b**).

In addition to the substrate temperature and the power of the plasma generator, the technological parameters related to the gas precursors themselves, i.e., their type and partial pressure, also have an influence on the structure and properties of the obtained coatings. Controlling these parameters also affects the microstructure of the resulting layers [14], which can be:


#### **2.1. The role of plasma**

• Electrons (EA)

○ Bunch (EA) • Photons (PhA)

temperature.

considered from two sides:

surface of the workpiece.

parameters of each stage.

can be:

• Amorphous • Fine grained

• Polycrystalline

• Compliance with the economic aspect.

○ Thermoemission (HF)

58 Chemical Vapor Deposition for Nanotechnology

methods of producing low-temperature plasma [12, 13].

The main difference between conventional and plasma-activated CVD methods lies in the fact that, in the case of PACVD, the molecules are in a higher state of internal energy. Electric discharges of 2.45 GHz (microwaves) and 13.56 MHz (radio waves) are among the most useful

**Figure 3a** shows the reactions occurring in the CVD process involving plasma. The presence of particles with an increased state of internal energy and ion bombardment on the surface of the substrate increases the surface diffusion rate and decreases the activation energy of chemical reactions leading to the formation of a coating. Layering takes place at a much lower

This raises the question of what can affect the design of coating technologies. This can be

• The need to meet the requirements in relation to the physicochemical parameters of the

For processes taking place under reduced pressure, aside from the choice of the method, the individual processing phases are of particular importance. An important role is also played by the type of intermediate layers and final obtained coatings and thus the technological

Temperature is one of the process parameters. If CVD processes are generated by plasma, the treatment can already be carried out at room temperature. This is related to the activation of chemical reactions in the gas phase by electrons accelerated in an electromagnetic field.

In that case of coating deposition, with time, subsequent stages of layer growth occur, which

In addition to the substrate temperature and the power of the plasma generator, the technological parameters related to the gas precursors themselves, i.e., their type and partial pressure, also have an influence on the structure and properties of the obtained coatings. Controlling these parameters also affects the microstructure of the resulting layers [14], which

consequently form a continuous structure on the surface (**Figure 3b**).

The commonly known definition of plasma is, according to Langmuir [15], partially or almost completely ionized gas, which is also the fourth state of matter (97% of the universe [16]). Initially, it was an object of interest only among physicists. However, application in materials technology was also quickly found.

Research into the improvement of physical properties, as well as physicochemical properties, such as wear and corrosion resistance of thin films and coatings, has somehow contributed to the use of plasma energy for obtaining materials in the form of layers and coatings on various types of surface substrates.

The reactor chamber is a kind of unloading pipe in which (under reduced pressure (about 10–80 Pa), the influence of electromagnetic waves is present) gas ionizing energy is supplied with a strictly defined and controlled flow rate [17]. The results of this process are also:


#### **2.2. Synergism in coating technology**

The concept of synergism is used in various fields of science. As a technique, it is usually referred to as the interaction of more than one factor for the final effect obtained (improvement of a given physicochemical property or obtaining a completely new functionality).

This definition is also reflected in surface technology. However, it is assumed here every time that the obtained effects are due to the influence of many factors [1]. In this case the term synergism refers here to the usable properties of the modified surface. This action is achieved by:


Therefore, we can say that it is necessary to design technology in many experimental works that ensures optimal material parameters that are maximally beneficial for the given application (both the properties of the coatings obtained and their adhesion to the surface).


**3. Techniques for obtaining DLC layers**

ence of hydrogen [20] (**Figure 4a**).

(**Figure 4b**).

methods such as:

• Arc discharge method

laser.

**3.1. Overview of techniques for receiving thin layers**

this division is the type of precursor used in the process.

• MW CVD (microwave chemical vapor deposition)

• RF CVD (radio-frequency chemical vapor deposition)

• DC CVD (direct-current chemical vapor deposition)

more detail, that is, obtaining DLC layers using PA CVD methods.

to continuous and intensive modernization.

Techniques for obtaining DLC layers can be divided into two groups [19]. The criterion for

DLC Layers Created Using CVD Techniques and Their Application

http://dx.doi.org/10.5772/intechopen.79526

61

• Atomizing a carbon electrode with a single-ion beam (sputtering beam)—spraying a carbon disk with a single argon ion beam with energy from 0.5 to a few keV; the carbon atoms removed from the electrode after getting bombarded with gas ions are ionized themselves and deposited on the appropriate surface, forming an amorphous layer without the pres-

• Atomization of the carbon electrode with a double-ion beam (dual-ion beam sputtering)—introducing of an additional source of hydrogen or hydrocarbon ions with 1 keV energy; their task is to bombard the growing carbon layer, after which carbon ions react with hydrogen or hydrocarbon to form a hard layer with a quasi-amorphous structure

The second technique includes methods in which saturated or unsaturated hydrocarbon gas is used as a precursor (acetylene, benzene, butane, cyclohexane, ethane, ethylene, hexane, isopropane, methane, pentane, propane, and propylene). This group of methods consists primarily of plasma-assisted chemical vapor deposition (PACVD) technologies. These include

Currently, the methods included in the second group receive the most interest and are subject

There is also a method that cannot be assigned to the abovementioned groups in any way. It is a method of laser graphite ablation involving the evaporation of a graphite target using a

Due to the circumstances (CVD methods described above), I will discuss this second group in

The first group brings together methods in which the precursor is solid carbon:

**Figure 3.** (a) Scheme of reactions occurring in the PACVD process. (b) Scheme of processes occurring in the plasma reactor with the stages of coating deposition. (according to Kyziol's work [18]).

## **3. Techniques for obtaining DLC layers**

### **3.1. Overview of techniques for receiving thin layers**

Techniques for obtaining DLC layers can be divided into two groups [19]. The criterion for this division is the type of precursor used in the process.

The first group brings together methods in which the precursor is solid carbon:


The second technique includes methods in which saturated or unsaturated hydrocarbon gas is used as a precursor (acetylene, benzene, butane, cyclohexane, ethane, ethylene, hexane, isopropane, methane, pentane, propane, and propylene). This group of methods consists primarily of plasma-assisted chemical vapor deposition (PACVD) technologies. These include methods such as:


**Figure 3.** (a) Scheme of reactions occurring in the PACVD process. (b) Scheme of processes occurring in the plasma

reactor with the stages of coating deposition. (according to Kyziol's work [18]).

60 Chemical Vapor Deposition for Nanotechnology

Currently, the methods included in the second group receive the most interest and are subject to continuous and intensive modernization.

There is also a method that cannot be assigned to the abovementioned groups in any way. It is a method of laser graphite ablation involving the evaporation of a graphite target using a laser.

Due to the circumstances (CVD methods described above), I will discuss this second group in more detail, that is, obtaining DLC layers using PA CVD methods.

**Figure 4.** Scheme of systems for deposition by ion spraying: (a) single ion beam, (b) dual ion beam [21].

#### **3.2. PA CVD and DLC layers**

In the twenty-first century, most research pertains to the technology of DLC coatings, including the modification of these other atoms (in medical applications N, Si, Ti, O, F, and Ag) [22–30]. For their synthesis, the most common method is chemical vapor deposition.

Dwivedi et al. [50] confirmed that nitrogen, as an additive (N-DLC), causes changes in the morphology of coatings both on a nanometric and micrometric scale, which favors the bind-

**Table 1.** Selected properties of carbonaceous materials, including DLC and N-DLC layers (based on[18, 31–37]).

 **(%) H (%) Density (g/cm3**

Diamond 100 0 3.5 100

a-C:H (soft) 60 40–50 1.2–1.6 <10 a-C:H (hard) 40 30–40 1.6–2.2 10–20 ta-C:H 70 30 2.4 50 N-DLC 14–21 About 36 1.9–2.4 22–28

Graphite 0 0 2.3

silicon atoms was noticed by Kim et al. [51], even with the Si contribution not exceeding 2% at. In addition to the abovementioned influence, Si-DLC coatings also exhibit a more favorable

in Si content from zero to 19.73% at [48]) [54] in comparison to, among others, Si/N-DLC coatings, high growth rate in the production process [52], good oxidation resistance, and low value of the coefficient of friction [53]. Thanks to these properties, these coatings have a wide

Therefore, it can be argued that for the design of technology based on chemical reactions, in short CVD, the key step is the optimization of the structure of the obtained layers and the related functional properties of the materials. The goal of this approach to the discussed issue is to ensure the best possible adjustment of the surface treatment parameters in terms of the

Proper selection of the technological process parameters for coating production is one of the key factors that improves their synergism on various types of substrates. In the case of chemical plasma vapor deposition (PA CVD (plasma-assisted chemical vapor deposition)) processes, this is particularly important. The keys to the construction and physicochemical

range of applications in the automotive, aerospace, and shipbuilding industries.

**3.3. The influence of PA CVD process parameters on the structure of N-DLC** 

hybridization clusters. The improvement of anticorrosive properties by doping with

hybridization (e.g., a decrease in hardness from 23.1 to 13.7 GPa, with an increase

moieties for the contribution of amorphous struc-

**) Hardness (GPa)**

http://dx.doi.org/10.5772/intechopen.79526

63

DLC Layers Created Using CVD Techniques and Their Application

ing of sp<sup>2</sup>

**Type of material sp3**

ture to sp<sup>3</sup>

**coatings**

hardness—hindering the formation of sp<sup>2</sup>

use of a given structural element.

properties of a given surface are:

• Pressure in the reaction chamber

• Temperature in the reactor and at the surface itself

• Duration of the process

The discussed layers, due to their number of interesting properties (**Table 1**), are used in many branches of the industry.

In "Introduction," I already mentioned pioneer studies concerning these layers. Up to now, however, carbon nitride has not been obtained in a crystalline form with stoichiometric or a very similar atomic ratio, and a series of works carried out in this area led to obtaining carbon layers subsidized with nitrogen atoms (N-DLC) [38–40], silicon (Si-DLC) [25, 30, 41–43], or more complex, in terms of chemical composition, SiCN:H structures [44, 45].

DLC coatings are largely amorphous, in which carbon is present in the sp<sup>2</sup> and sp<sup>3</sup> hybridizations. The clusters of atoms have a structure typical for graphite, somehow immersed in an amorphous matrix, which consist of carbon atoms [46]. The general classification of the discussed coatings is as follows [31, 47]:


In both cases, they can be doped with both metals and nonmetals. Many scientists have been investigating layers with various types of atoms [48–54].

The addition of Ti, W or Si, and N improves adhesion to the substrate, and according to Galvan et al. [49], they decrease the level of their own stresses in the DLC structure. In turn,


**Table 1.** Selected properties of carbonaceous materials, including DLC and N-DLC layers (based on[18, 31–37]).

Dwivedi et al. [50] confirmed that nitrogen, as an additive (N-DLC), causes changes in the morphology of coatings both on a nanometric and micrometric scale, which favors the binding of sp<sup>2</sup> hybridization clusters. The improvement of anticorrosive properties by doping with silicon atoms was noticed by Kim et al. [51], even with the Si contribution not exceeding 2% at. In addition to the abovementioned influence, Si-DLC coatings also exhibit a more favorable hardness—hindering the formation of sp<sup>2</sup> moieties for the contribution of amorphous structure to sp<sup>3</sup> hybridization (e.g., a decrease in hardness from 23.1 to 13.7 GPa, with an increase in Si content from zero to 19.73% at [48]) [54] in comparison to, among others, Si/N-DLC coatings, high growth rate in the production process [52], good oxidation resistance, and low value of the coefficient of friction [53]. Thanks to these properties, these coatings have a wide range of applications in the automotive, aerospace, and shipbuilding industries.

Therefore, it can be argued that for the design of technology based on chemical reactions, in short CVD, the key step is the optimization of the structure of the obtained layers and the related functional properties of the materials. The goal of this approach to the discussed issue is to ensure the best possible adjustment of the surface treatment parameters in terms of the use of a given structural element.

### **3.3. The influence of PA CVD process parameters on the structure of N-DLC coatings**

Proper selection of the technological process parameters for coating production is one of the key factors that improves their synergism on various types of substrates. In the case of chemical plasma vapor deposition (PA CVD (plasma-assisted chemical vapor deposition)) processes, this is particularly important. The keys to the construction and physicochemical properties of a given surface are:

• Duration of the process

**3.2. PA CVD and DLC layers**

62 Chemical Vapor Deposition for Nanotechnology

many branches of the industry.

discussed coatings is as follows [31, 47]:

investigating layers with various types of atoms [48–54].

• Hydrogenated structures

• Nonhydrogenated structures

In the twenty-first century, most research pertains to the technology of DLC coatings, including the modification of these other atoms (in medical applications N, Si, Ti, O, F, and Ag)

The discussed layers, due to their number of interesting properties (**Table 1**), are used in

In "Introduction," I already mentioned pioneer studies concerning these layers. Up to now, however, carbon nitride has not been obtained in a crystalline form with stoichiometric or a very similar atomic ratio, and a series of works carried out in this area led to obtaining carbon layers subsidized with nitrogen atoms (N-DLC) [38–40], silicon (Si-DLC) [25, 30, 41–43], or

izations. The clusters of atoms have a structure typical for graphite, somehow immersed in an amorphous matrix, which consist of carbon atoms [46]. The general classification of the

In both cases, they can be doped with both metals and nonmetals. Many scientists have been

The addition of Ti, W or Si, and N improves adhesion to the substrate, and according to Galvan et al. [49], they decrease the level of their own stresses in the DLC structure. In turn,

and sp<sup>3</sup>

hybrid-

[22–30]. For their synthesis, the most common method is chemical vapor deposition.

**Figure 4.** Scheme of systems for deposition by ion spraying: (a) single ion beam, (b) dual ion beam [21].

more complex, in terms of chemical composition, SiCN:H structures [44, 45]. DLC coatings are largely amorphous, in which carbon is present in the sp<sup>2</sup>


In this respect, the technology is extremely difficult to develop. The processes occurring during deposition largely take place away from thermodynamic equilibrium. Summing up the design and development of this technology most often requires a number of experimental studies.

The research conducted by our team on the impact of gas mixture composition, process temperature, plasma generator power, and reactor pressure on the growth rate of N-DLC structures was a valuable guideline for its design. With regard to nitrogen-rich DLC layers, it can be concluded that the growth rate of these structures decreases with increasing temperature and as the proportion of N<sup>2</sup> in the gas mixture increases. This can be counteracted by increasing the power of the plasma generator and the pressure of the gaseous reactants in the working chamber. It is worth noting that for each type of layers, including N-DLC, limit values of individual parameters are defined, at which the aforementioned layers are thermodynamically stable [55].

## **4. DLC layers on different substrates**

Adhering strictly to the guidelines presented in this chapter, in designing technology for modification of all kinds of materials, the parameters for individual stages of machining were selected so as to minimize the possibility of ionic etching of the resulting coatings to ensure maximum speed of the deposition process.

**Figure 5** schematically represents the structure of the layers on Ti6Al4V.

The following subsections present examples of DLC layer deposition processes on several types of substrates, and the parameters of which are presented in **Table 2**.

> Our team conducted a lot of researches on DLC layers deposited on substrates [56]. The results for the Ti Grade 2 (after plasma processes) obtained from the biological and corrosion tests suggest that titanium with the plasmochemically modified surface did not cause severe

**Technological parameters**

/25; SiH<sup>4</sup>

/150; SiH<sup>4</sup>

/130;

/8; CH<sup>4</sup>

/85; CH<sup>4</sup>

/25; CH<sup>4</sup>

/104;

**Gas/flow (sccm) PRF/MW (W) Ts**

 **(o C) p (Tr)**

/3 PMW 400 600 0.3 30

/3 PMW 400 600 0.3 30

/30 PRF 100 350 0.7 120

DLC Layers Created Using CVD Techniques and Their Application

http://dx.doi.org/10.5772/intechopen.79526

/8 PRF 50 400 0.4 8

/10 PRF 80 24 0.4 30

, substrate temperature; p, pressure in the reactor; t, time of

PMW 400 600 0.3 30

PRF 80 24 0.4 30

**t (min)**

65

Similarly, beneficial results of the transfer of stresses between the substrate and the coating after using the surface modification process with nitrogen or silicon ions were obtained in the work on the modification of aluminum alloys [43, 67, 68]. For example, Lifang et al. [69] clearly indicate that obtaining an intermediate layer, at the Al-Cu-Mg-DLC alloy boundary, decisively influences, inter alia, the hardness of the surface exposed to working conditions. The various intermediate layers obtained by these authors, under different conditions of plasma ion implantation (PBII (plasma-based ion implantation)), lead to changes in the degree of surface hardening (in the range 17–22 GPa) and the extent of the alloy's strengthening (800–1350 nm).

cytotoxicity against *MG63* cells and improves corrosion properties.

**Figure 5.** The scheme of DLC layer structure.

Al-Zn alloy N+

process deposition.

**Substrate Type of plasmochemical process**

Ti Grade2 SiCH Ar/225; CH<sup>4</sup>

*Polyetherketone* CNH Ar/75; N<sup>2</sup>

PRF/MW, radio-frequency or microwave generator power; T<sup>s</sup>

SiNH Ar/225; NH<sup>3</sup>

SiCNH Ar/225; NH<sup>3</sup>

Si-DLC Ar/80; SiH<sup>4</sup>

*SiCNH* Ar/75; N<sup>2</sup>

**Table 2.** Details for plasma modification including carbon-based coating deposition [56].

ion N2

CH<sup>4</sup>

SiH<sup>4</sup> /3

/20; SiH<sup>4</sup> /3

/90; H<sup>2</sup>

#### **4.1. Titanium and aluminum alloys**

The diffusion processes are important for the treatment technology performed on these Tiand Al-based alloys, which consequently leads to the formation of phases that strengthen the surface layer. In the case when their surface is nitrided, this process is usually carried out in N2 [57, 58], N2 -H<sup>2</sup> [59, 60], and NH<sup>3</sup> -Ar atmospheres [61, 62]. While the role of nitrogen is obvious, the effect of hydrogen has been explained in various ways. For example, Matsumoto [63] believes that H<sup>2</sup> reduces the reaction rate, while Renevier et al. [64] assume that the addition of H<sup>2</sup> to N2 is not important. On the other hand, Hudis [65] demonstrates that the addition of hydrogen ensures a more effective cleaning of the surface, which promotes the nitriding process. In addition, studies conducted by Negm [66] have confirmed that the replacement of hydrogen with argon or another inert gas reduces the thickness of diffusion layers and the hardness of the treated surface.

**Figure 5.** The scheme of DLC layer structure.

• Power of the plasma generator

64 Chemical Vapor Deposition for Nanotechnology

• Condition and type of surface

studies.

• Flow rate and type of gas precursor

perature and as the proportion of N<sup>2</sup>

**4. DLC layers on different substrates**

maximum speed of the deposition process.

**4.1. Titanium and aluminum alloys**


hardness of the treated surface.

[59, 60], and NH<sup>3</sup>

N2

of H<sup>2</sup>

[57, 58], N2

believes that H<sup>2</sup>

to N2

thermodynamically stable [55].

In this respect, the technology is extremely difficult to develop. The processes occurring during deposition largely take place away from thermodynamic equilibrium. Summing up the design and development of this technology most often requires a number of experimental

The research conducted by our team on the impact of gas mixture composition, process temperature, plasma generator power, and reactor pressure on the growth rate of N-DLC structures was a valuable guideline for its design. With regard to nitrogen-rich DLC layers, it can be concluded that the growth rate of these structures decreases with increasing tem-

by increasing the power of the plasma generator and the pressure of the gaseous reactants in the working chamber. It is worth noting that for each type of layers, including N-DLC, limit values of individual parameters are defined, at which the aforementioned layers are

Adhering strictly to the guidelines presented in this chapter, in designing technology for modification of all kinds of materials, the parameters for individual stages of machining were selected so as to minimize the possibility of ionic etching of the resulting coatings to ensure

The following subsections present examples of DLC layer deposition processes on several

The diffusion processes are important for the treatment technology performed on these Tiand Al-based alloys, which consequently leads to the formation of phases that strengthen the surface layer. In the case when their surface is nitrided, this process is usually carried out in

ous, the effect of hydrogen has been explained in various ways. For example, Matsumoto [63]

of hydrogen ensures a more effective cleaning of the surface, which promotes the nitriding process. In addition, studies conducted by Negm [66] have confirmed that the replacement of hydrogen with argon or another inert gas reduces the thickness of diffusion layers and the

reduces the reaction rate, while Renevier et al. [64] assume that the addition

is not important. On the other hand, Hudis [65] demonstrates that the addition

**Figure 5** schematically represents the structure of the layers on Ti6Al4V.

types of substrates, and the parameters of which are presented in **Table 2**.

in the gas mixture increases. This can be counteracted



PRF/MW, radio-frequency or microwave generator power; T<sup>s</sup> , substrate temperature; p, pressure in the reactor; t, time of process deposition.

**Table 2.** Details for plasma modification including carbon-based coating deposition [56].

Our team conducted a lot of researches on DLC layers deposited on substrates [56]. The results for the Ti Grade 2 (after plasma processes) obtained from the biological and corrosion tests suggest that titanium with the plasmochemically modified surface did not cause severe cytotoxicity against *MG63* cells and improves corrosion properties.

Similarly, beneficial results of the transfer of stresses between the substrate and the coating after using the surface modification process with nitrogen or silicon ions were obtained in the work on the modification of aluminum alloys [43, 67, 68]. For example, Lifang et al. [69] clearly indicate that obtaining an intermediate layer, at the Al-Cu-Mg-DLC alloy boundary, decisively influences, inter alia, the hardness of the surface exposed to working conditions. The various intermediate layers obtained by these authors, under different conditions of plasma ion implantation (PBII (plasma-based ion implantation)), lead to changes in the degree of surface hardening (in the range 17–22 GPa) and the extent of the alloy's strengthening (800–1350 nm). However, with respect to the structure of the coatings themselves, for example, Wang et al. [70] and Ikeyama et al. [71] confirmed that the addition of Si to the DLC structure improves the adhesion of the coating to the substrate and reduces the level of internal stresses in its structure. In turn, subsidizing the DLC structure with nitrogen atoms results in a decrease in the coefficient of friction [72].

In the case of our team's research on modification of the Al-Zn alloy (after modification of nitrogen ions and deposition of SiCH layers), an improvement in hardness (up to 8 GPa), Young's modulus (up to 6 GPa), and surface smoothness (surface roughness parameter) were observed.

**Figure 6** presents photos of modified metallic materials.

**4.3. Austenitic-ferritic steel**

**Figure 7.** Image of PEEK with DLC layers.

lus of the investigated SiCx

**5. Application**

In this respect, especially in the case of DLC coatings obtained on metallic substrates (e.g., steel), this process is preceded by interlayer deposition. A transitional layer often containing silicon [43] significantly improves the adhesion of the specific coating to metal alloys and at the same time prevents an unfavorable phenomenon, which in this case is the precipitation of iron-carbon phases. In the work of Stypuła, et al. [77] it was found that nitriding austenitic-ferritic steel (duplex) using radio-frequency plasma, under low-pressure and low-temperature conditions (p = 0.7Tr, T = 573 K) improves the passivation parameters and corrosion resistance of the LOOH21N7MC alloy. This increase in corrosion resistance is associated with high nitrogen content (~21%), with the oxide-nitride structure of the nitrided outer layer, as well as a high

Brylewski et al. [78] used PACVD to obtain a silicon carbonitride layer with a defined desired chemical composition on Crofer 22APU steel, both without and after prior nitriding. It was determined that nitriding has a positive influence on the microhardness and elasticity modu-

the samples indicate that prior nitriding in plasmochemical conditions results in improved

studies it can be concluded that the deposition of the silicon carbonitride layer preceded by nitriding can be successfully used to obtain materials with potential applications as intercon-

Thanks to their properties, such modified materials have found a fairly wide application in several branches of the industry, construction, and medicine. The first group includes, among others, car industry, aviation, shipbuilding, electronics. The second one is mainly photovoltaics, various types of constructions, machines, and parts of construction machines. In contrast, the

(H) layer. High-temperature oxidation studies performed on

Ny

DLC Layers Created Using CVD Techniques and Their Application

http://dx.doi.org/10.5772/intechopen.79526

67

(H) layer. From these

degree of surface enrichment in the silicon in the form of a nitride.

corrosion resistance of the Crofer 22APU steel with the deposited SiCx

nects in planar-type intermediate-temperature solid oxide fuel cells.

Ny

**Figure 6.** Images of different substrates with DLC layers: (a) Ti Grade 2, (b) Ti6Al4V, (c) Ti-Al, and (d) Al-Zn.

#### **4.2. Polymers**

During the last decade, PEEK has generated much interest thanks to its thermoplastic properties, chemical resistance, high-temperature stability, and biocompatibility [73, 74]. It has been considered as a suitable material replacing metals in biomedical implants and as a matrix of carbon fiber composites [75, 76]. However, it also has disadvantages, namely, hydrophobicity and low surface free energy, resulting in poor adhesive properties. In order to improve the properties of PEEK, we modified the samples with ion etching and deposited layers a-CN:H/ a-SiCN:H by using RFCVD [56].

**Figure 7** presents photo of modified PEEK.

**Figure 7.** Image of PEEK with DLC layers.

#### **4.3. Austenitic-ferritic steel**

However, with respect to the structure of the coatings themselves, for example, Wang et al. [70] and Ikeyama et al. [71] confirmed that the addition of Si to the DLC structure improves the adhesion of the coating to the substrate and reduces the level of internal stresses in its structure. In turn, subsidizing the DLC structure with nitrogen atoms results in a decrease in

In the case of our team's research on modification of the Al-Zn alloy (after modification of nitrogen ions and deposition of SiCH layers), an improvement in hardness (up to 8 GPa), Young's modulus (up to 6 GPa), and surface smoothness (surface roughness parameter) were

During the last decade, PEEK has generated much interest thanks to its thermoplastic properties, chemical resistance, high-temperature stability, and biocompatibility [73, 74]. It has been considered as a suitable material replacing metals in biomedical implants and as a matrix of carbon fiber composites [75, 76]. However, it also has disadvantages, namely, hydrophobicity and low surface free energy, resulting in poor adhesive properties. In order to improve the properties of PEEK, we modified the samples with ion etching and deposited layers a-CN:H/

**Figure 6.** Images of different substrates with DLC layers: (a) Ti Grade 2, (b) Ti6Al4V, (c) Ti-Al, and (d) Al-Zn.

the coefficient of friction [72].

66 Chemical Vapor Deposition for Nanotechnology

**Figure 6** presents photos of modified metallic materials.

observed.

**4.2. Polymers**

a-SiCN:H by using RFCVD [56].

**Figure 7** presents photo of modified PEEK.

In this respect, especially in the case of DLC coatings obtained on metallic substrates (e.g., steel), this process is preceded by interlayer deposition. A transitional layer often containing silicon [43] significantly improves the adhesion of the specific coating to metal alloys and at the same time prevents an unfavorable phenomenon, which in this case is the precipitation of iron-carbon phases.

In the work of Stypuła, et al. [77] it was found that nitriding austenitic-ferritic steel (duplex) using radio-frequency plasma, under low-pressure and low-temperature conditions (p = 0.7Tr, T = 573 K) improves the passivation parameters and corrosion resistance of the LOOH21N7MC alloy. This increase in corrosion resistance is associated with high nitrogen content (~21%), with the oxide-nitride structure of the nitrided outer layer, as well as a high degree of surface enrichment in the silicon in the form of a nitride.

Brylewski et al. [78] used PACVD to obtain a silicon carbonitride layer with a defined desired chemical composition on Crofer 22APU steel, both without and after prior nitriding. It was determined that nitriding has a positive influence on the microhardness and elasticity modulus of the investigated SiCx Ny (H) layer. High-temperature oxidation studies performed on the samples indicate that prior nitriding in plasmochemical conditions results in improved corrosion resistance of the Crofer 22APU steel with the deposited SiCx Ny (H) layer. From these studies it can be concluded that the deposition of the silicon carbonitride layer preceded by nitriding can be successfully used to obtain materials with potential applications as interconnects in planar-type intermediate-temperature solid oxide fuel cells.

## **5. Application**

Thanks to their properties, such modified materials have found a fairly wide application in several branches of the industry, construction, and medicine. The first group includes, among others, car industry, aviation, shipbuilding, electronics. The second one is mainly photovoltaics, various types of constructions, machines, and parts of construction machines. In contrast, the

[5] Matsumoto O, Kotaki T, Sikano H. Synthesis of carbon nitride in plasma arc. Journal of the

[7] Xianhui B, Zheng N, Li Y, Feng P. Templated assembly of sulfide nanoclusters into

[8] Chen LC, Luang JY. Crystalline silicon carbon nitride: A wide band gap semiconductor.

[9] Chen LC, Wei SL. Crystalline SiCN: A hard material rivals to cubic BN. Thin Solid Films.

[10] Błaszczyk-Lezak I, Wróbel AM, Bieliński DM. Remote hydrogen microwave plasma chemical vapor deposition of silicon carbonitride films from a (dimethylamino) dimethylsilane precursor: Compositional and structural dependencies of film properties.

[11] Fainer NI, Maximovski EA, Rumyantsev YM, Kosinova ML, Kuznetsov FA. Composition and structure of silicon carbonitride layers grown on Si(100)/(Fe, N, Co) substrates.

[12] Norm ISO 5832/3-1978 (E). Implants for surgery—Metallic materials wrought titanium

[13] Norm ASTM F136-98. Standard specification for wrought titanium-6aluminum-4vana-

[16] Merche D, Vandencasteele N, Reniers F. Atmospheric plasmas for thin film deposition:

[17] Lebrun JP. Applications of low-temperature surface hardening of stainless steels.

[18] Kyziol K. Funkcjonalne powłoki otrzymywane w warunkach plazmochemicznych [Functional coatings obtained under plasmochemical conditions]. Ceramics. 2016;**123** [19] Spear KE, Dismukes JP. Synthetic Diamond: Emerging CVD Science and Technology.

[20] Paiva OC, Santos JD, Reis RL. An electrochemical study on anodised Ti-6Al-4V aiming at improving its biomedical performance. In: Proceedings of the Symposium on

[21] Anderson L-P. A review of recent work on hard i-C films. Thin Solid Films. 1981;**86**:193

Thermochemical Surface Engineering of Steels. Elsevier; 2015. pp. 633-647

Nuclear Instruments and Methods in Physics Research A. 2001;**470**:193-197

[14] Somiya S, Goto T. Handbook of Advanced Ceramics. Elsevier; 2013

A critical review. Thin Solid Films. 2012;**520**:4219-4236

Compatibility of Biomedical Implants. Vol. 94-15. p. 145

[15] Langmuir I. Oscillations inionied gases. Physical Review. 1928;**14**:627-637

type framework. Journal of the American Chemical Society. 2003;**125**(20):

N4

http://dx.doi.org/10.5772/intechopen.79526

DLC Layers Created Using CVD Techniques and Their Application

. Physical Review.

69

Electrochemical Society. 1994;**141**:L16-L18

Applied Physics Letters. 1998;**72**(19):2463-2465

Diamond and Related Materials. 2006;**15**:1650-1658

6-aluminium 4-vanadium alloy. ISO; 1978

USA: John Wiley & Sons, Inc.; 1994

1994;**B49**:5034-5037

N4

1999;**355-356**:112-116

dium ELI. USA; 1998

cubic-C<sup>3</sup>

6024-6025

[6] Yu KM, Cohen ML, Haller EE. Observation of crystalline C<sup>3</sup>

**Figure 8.** Examples of applications for modified materials.

third group consists of a wide range of applications, from bone implants to medical instruments and veterinary. A separate group includes sports, especially golf, sailing, or canoeing.

**Figure 8** presents photos of examples of applications for modified materials.

## **Author details**

Marta Januś

Address all correspondence to: martaj@agh.edu.pl

AGH–University of Science and Technology, Faculty of Materials Science and Ceramics, Krakow, Poland

## **References**


third group consists of a wide range of applications, from bone implants to medical instruments

and veterinary. A separate group includes sports, especially golf, sailing, or canoeing.

AGH–University of Science and Technology, Faculty of Materials Science and Ceramics,

[1] Burakowski T. Rozważania o synergizmie w inżynierii powierzchni [Considerations on Synergism in Surface Engineering]. Radom: Wydawnictwo Politechniki Radomskiej;

[2] Musil J, Vlček J, Růžička M. Recent progress in plasma nitriding. Vacuum. 2000;**59**:940-951 [3] Chu PK, Chen JY, Wang LP, Huang N. Materials science and engineering reports: A

[4] Liu AY, Cohen ML. Structural properties and electronic structure of low-compressibility

N4

. Physical Review. 1990;**B41**:10727

and hypothetical beta-C<sup>3</sup>

**Figure 8** presents photos of examples of applications for modified materials.

Address all correspondence to: martaj@agh.edu.pl

**Figure 8.** Examples of applications for modified materials.

68 Chemical Vapor Deposition for Nanotechnology

review. Journal. 2002;**36**:143-206

N4

materials: Beta-Si<sup>3</sup>

**Author details**

Marta Januś

Krakow, Poland

**References**

2004


[22] Kluba A, Bociąga D, Dudek M. Hydrogenated amorphous carbon films deposited on 316L stainless steel. Diamond and Related Materials. 2010;**19**:533-536

[35] Weiler M, Sattel S, Jung K, Ehrhardt H, Veerasamy VS, Robertson J. Highly tetrahedral, diamond-like amorphous hydrogenated carbon prepared from a plasma beam source.

DLC Layers Created Using CVD Techniques and Their Application

http://dx.doi.org/10.5772/intechopen.79526

71

[36] Donnet C. Recent progress on the tribology of doped diamond-like and carbon alloy

[37] Murata Y, Choo CK, Ono H, Nagai Y, Tanaka K. Characterization of N-doped DLC thin films prepared by hydrocarbons pyrolysis method. Materials Today: Proceedings.

[38] Motta EF, Pereyra I. Amorphous hydrogenated carbon-nitride films prepared by RF-PECVD in methane-nitrogen atmospheres. Journal of Non-Crystalline Solids.

[39] Neuville S, Matthews A. A perspective on the optimisation of hard carbon and related

[40] Yang P, Huang N, Leng YX, Chen JY, Wang J, Sun H, Wan GJ, Zhao AS. Wettability and bloodcompatibility of a-C:N:H films deposited by PIII-D. Surface and Coatings

[41] Soum-Glaude A, Thomas L, Dollet A, Ségur P, Bordage MC. Argon/tetramethysilane PECVD: From process diagnostic and modeling to a-Si:C:H hard coating composition.

[42] Iseki T, Mori H, Hasegawa H, Tachikawa H, Nakanishi K. Structural analysis of Si-containing diamond-like carbon. Diamond and Related Materials. 2006;**15**:1004-1010

[43] Cemin F, Boeira CD, Figueroa CA. On the understanding of the silicon-containing adhesion interlayer in DLC deposited on steel. Tribology International. 2016;**94**:464-469 [44] Peter S, Bernütz S, Berg S, Richter F. FTIR analysis of a-SiCN:H films deposited by

[45] Kafrouni W, Rouessac V, Julbe A, Durand J. Synthesis of PECVD a-SiCxNy:H membranes as molecular sieves for small gas separation. Journal of Membrane Science.

[46] Spencer EG, Schmidt PH, Joy DC, Sansalone FJ. Ion beam deposited polycrystalline

[47] Lifshitz Y. Diamond-like carbon—Present status. Diamond and Related Materials.

[48] Bogdanowicz R. Advancements in diamond-like carbon coatings. Handbook of Nanoceramic and Nanocomposite Coatings and Materials. Elsevier; 2015. pp. 183-200

[49] Galvan D, Pei YT, De Hosson JTM. Influence of deposition parameters on the structure and mechanical properties of nanocomposite coatings. Surface and Coatings Technology.

diamond-like films. Applied Physics Letters. 1976;**29**:118-120

coatings for engineering applications. Thin Solid Films. 2007;**515**:6619-6653

coatings: A review. Surface and Coatings Technology. 1998;**100-101**:180-186

Applied Physics Letters. 1994;**64**:2797-2799

2016;**3**:197-202

2004;**338-340**:525-529

Technology. 2010;**204**:3039-3042

PECVD. Vacuum. 2013;**98**:81-87

2009;**329**:130-137

1999;**8**:1659-1676

2006;**201**:590-598

Diamond and Related Materials. 2007;**16**:1259-1263


[35] Weiler M, Sattel S, Jung K, Ehrhardt H, Veerasamy VS, Robertson J. Highly tetrahedral, diamond-like amorphous hydrogenated carbon prepared from a plasma beam source. Applied Physics Letters. 1994;**64**:2797-2799

[22] Kluba A, Bociąga D, Dudek M. Hydrogenated amorphous carbon films deposited on

[23] Tétard F, Djemia P, Angleraud B, Mubumbila N, Tessier PY. Surface and bulk characterizations of CNx thin films made by r.f. magnetron sputtering. Surface and Coatings

[24] Zhang P, Tay BK, Yu GQ, Lau SP, Fu YQ. Surface energy of metal containing amorphous carbon films deposited by filtered cathodic vacuum arc. Diamond and Related

[25] Bociąga D, Kamińska M, Sobczyk-Guzenda A, Jastrzębski K, Świątek L, Olejnik A. Surface properties and biological behaviour of Si-DLC coatings fabricated by a multitarget DC-RF magnetron sputtering method for medical applications. Diamond and

[26] Bociąga D, Komorowski P, Batory D, Szymański W, Olejnik A, Jastrzębski K, Jakubowski W. Silver-doped nanocomposite carbon coatings (Ag-DLC) for biomedical applications—Physiochemical and biological evaluation. Applied Surface Science.

[27] Bociąga D, Jakubowski W, Komorowski P, Sobczyk-Guzenda A, Jȩdrzejczak A, Batory D, Olejnik A. Surface characterization and biological evaluation of silverincorporated DLC coatings fabricated by hybrid RF PACVD/MS method. Materials Science and Engineer-

[28] Batory D, Jędrzejczak A, Kaczorowski W, Szymański W, Kołodziejczyk L, Cłapa M, Niedzielski P. Influence of the process parameters on the characteristics of silicon-incorporated a-C:H:SiOx coatings. Surface and Coatings Technology. 2015;**271**:112-118

[29] Batory D, Jędrzejczak A, Szymański W, Niedzielski P, Fijałkowski M, Louda P, Kotela I, Hromadka M, Musil J. Mechanical characterization of a-C:H:SiOx coatings synthesized using radio-frequency plasma-assisted chemical vapor deposition method. Thin Solid

[30] Szmidt J, Beck RB, Lisik Z, Mitura S, Sokołowska A. State of the silicon-DLC layer interface produced by plasma methods. Diamond and ReIated Materials. 1996;**5**:1204-1209

[31] Robertson J. Diamond-like amorphous carbon. Materials Science & Engineering R:

[32] Field JE, editor. The Properties of Natural and Synthetic Diamond. London: Academic

[34] Koidl P, Wild C, Dischler B, Wagner J, Ramsteiner M. Plasma deposition, properties and structure of amorphous hydrogenated carbon films. Materials Science Forum.

[33] Cahn RW. Physics of graphite. Journal of Nuclear Materials. 1983;**114**:116-116

316L stainless steel. Diamond and Related Materials. 2010;**19**:533-536

Technology. 2002;**151-152**:184-188

Materials. 2004;**13**:459-464

70 Chemical Vapor Deposition for Nanotechnology

2015;**355**:388-397

ing C. 2016;**63**:462-474

Films. 2015;**590**:299-305

Reports. 2002;**37**:129-281

Press; 1992

1990;**52-53**:41-70

Related Materials. 2016;**67**:41-50


[50] Dwivedi N, Kumar S, Malik HK, Rauthan CMS, Panwar OS. Influence of bonding environment on nano-mechanical properties of nitrogen containing hydrogenated amorphous carbon thin films. Materials Chemistry and Physics. 2011;**130**:775-785

[65] Hudis M. Study of ion nitriding. Journal of Applied Physics. 1973;**44**:1489-1496

[66] Negm NZ. A study on rf plasma nitriding at a constant power in different H<sup>2</sup>

titanium. Applied Surface Science. 2005;**240**:71-76

technique. Applied Surface Science. 2004;**236**:328-335

ior of DLC coated 2024 aluminum alloy. Wear. 2004;**257**:599-605

2004;**256**:840-845

Technology. 2013;**235**:326-332

and Atoms. Vol. 208. 2003. pp. 40-47

Biomaterials. 2007;**28**:4845-4869

Chapter 13

tures at different temperatures. Materials Science and Engineering B. 2006;**129**:207-210

[67] Liao JX, Xia LF, Sun MR, Liu WM, Xu T, Xue QJ. The tribological properties of a gradient layer prepared by plasma-based ion implantation on 2024 aluminum alloy. Wear.

[68] Liao JX, Xia LF, Sun MR, Liu WM, Xu T, Yang CR, Chen HW, Fu CL, Leng WJ. Structural characteristics of 2024 aluminum alloy plasma-based ion implanted with nitrogen then

[69] Lifang X, Zhaohui Y, Jiaxuan L. Effects of intermediate layers on the tribological behav-

[70] Wang J, Pu J, Zhang G, Wang L. Tailoring the structure and property of silicon-doped diamond-like carbon films by controlling the silicon content. Surface and Coatings

[71] Ikeyama M, Nakao S, Miyagawa Y, Miyagawa S. Effects of Si content in DLC films on their friction and wear properties. Surface and Coatings Technology. 2005;**191**:38-42

[72] Yan X, Xu T, Chen G, Yang S, Liu H. Study of structure, tribological properties and growth mechanism of DLC and nitrogen-doped DLC films deposited by electrochemical

[73] Oehr C. Plasma surface modification of polymers for biomedical use, Nuclear instruments and methods. In: Physics Research Section B: Beam Interactions with Materials

[74] Kurtz SM, Devine JN. PEEK biomaterials in trauma, orthopedic, and spinal implants.

[75] Briem D, Strametz S, Schroeder K, Meenen NM, Lehmann W, Linhart W, Ohl A, Rueger

[76] Kurtz SM. Applications of polyaryletheretherketone in spinal implants: Fusion and mations preservation. PEEK Biomaterials Handbook. William Andrew Publishing; 2011. pp. 201-220.

[77] Stypuła B, Stepuch-Galik A, Kowalski K, Januś M, Bernasik A. Korozja wysokostopowego staliwa azotowanego plazmowo [Corrosion of plasma nitrided cast stainless steel].

[78] Januś M, Kluska S, Bobruk M, Brylewski T, Jastrzębski W, Zimowski S, Grzesik Z. Effect of plasma-assisted CVD surface modification of Crofer 22APU steel on its selected physicochemical properties. Ochrona przed Korozją. 2015;**58**(11):418-422. ISSN: 0473-7733

JM. Journal of Materials Science: Materials in Medicine. 2005;**16**:671-677

Ochrona przed Korozją. 2005;**48**(11s/A):66-71. ISSN: 0473-7733

–N<sup>2</sup> mix73

DLC Layers Created Using CVD Techniques and Their Application

http://dx.doi.org/10.5772/intechopen.79526


[65] Hudis M. Study of ion nitriding. Journal of Applied Physics. 1973;**44**:1489-1496

[50] Dwivedi N, Kumar S, Malik HK, Rauthan CMS, Panwar OS. Influence of bonding environment on nano-mechanical properties of nitrogen containing hydrogenated amor-

[51] Kim H-G, Ahn S-H, Kim J-G, Park SJ, Lee K-R. Effect of Si-incorporation on wear–corrosion properties of diamond-like carbon films. Thin Solid Films. 2005;**482**:299-304

[52] Jiang L, Chen X, Wang X, Xu L, Stubhan F, Merkel K-H. a-SiCx:H films deposited by plasma-enhanced chemical vapor deposition at low temperature used for moisture and

[53] Damasceno JC, Camargo SS, Freire FL, Carius R. Deposition of Si-DLC films with high hardness, low stress and high deposition rates. Surface and Coatings Technology.

[54] Tang J, Wang Y. Characterization of DLC-Si films prepared by RF-PECVD. In: Opto-

[55] Kyzioł K, Jonas S, Tkacz-Śmiech K, Marszałek K. A role of parameters in RF PA CVD

[56] Januś M, Kluska S, Kyziol K, Jonas S. Plasma assisted chemical vapour deposition– Technical design of functional coatings. Polish Academy of Sciences, Committee of Metallurgy, Institute of Metallurgy and Materials Science, Archives of Metallurgy and

[57] Gredelj S, Gerson AR, Kumar S, McIntyre NS. Plasma nitriding and in situ characterisa-

[58] Quast M, Mayr P, Stock HR. Plasma monitoring of plasma-assisted nitriding of alu-

[59] Mubarak Ali M, Ganesh Sundara Raman S, Pathak SD, Gnanamoorthy R. Influence of plasma nitriding on fretting wear behaviour of Ti–6Al–4V. Tribology International.

[60] Vissutipitukul P, Aizawa T. Wear of plasma-nitrided aluminum alloys. Wear. 2005;**259**:

[61] Wang PW, Sui S, Wang W, Durrer W. Aluminum nitride and alumina composite film

[63] Matsumoto O. Nitriding reaction of metals under plasma conditions. In: Proceedings of the International Ion Engineering Congress ISIAT'83, Kyoto; 1983. pp. 1379-1390

[64] Renevier N, Czerwiec T, Collignon P, Michel H. Diagnostic of arc discharges for plasma nitriding by optical emission spectroscopy. Surface and Coatings Technology.

in D.C. Glow discharges at low pressures (less than 10 Torr). Materials Science and

, Ar-N2


mixtures

minium alloys. Surface and Coatings Technology. 1999;**120-121**:244-249

fabricated by DC plasma processes. Thin Solid Films. 1997;**295**:142-146

[62] Avni R, Spalvins T. Nitriding mechanisms in Ar-N<sup>2</sup>

phous carbon thin films. Materials Chemistry and Physics. 2011;**130**:775-785

corrosion resistant applications. Thin Solid Films. 1999;**352**:97-101

electronics and Microelectronics (ICOM). 2012. pp. 431-433

technology of a-C:N:H layers. Vacuum. 2008;**82**:998-1002

tion of aluminium. Applied Surface Science. 2002;**199**:234-247

2000;**133-134**:247-252

72 Chemical Vapor Deposition for Nanotechnology

Materials. 2015;**60**(2):909-914

Engineering. 1987;**95**:237-246

1998;**98**:1400-1405

2010;**43**:152-160

482-489


**Chapter 4**

**Provisional chapter**

**Controlled Post-treatment of Thick CVD-Diamond**

**Controlled Post-treatment of Thick CVD-Diamond** 

DOI: 10.5772/intechopen.79143

CVD-diamond coatings were posttreated by plasma oxidation to recycle an original WC (Co) mother tool substrate for recoating and reuse. A developed RF/DC plasma oxidation system was stated together with a hollow cathode device to intensify the oxygen ion and electron densities and with a quantitative plasma diagnosis equipment. Plasma oxidation ashing conditions were optimized by this quantitative diagnosis toward the perfect ashing of diamond films with less residuals and less tool edge damage. Geometric effect of tool teeth structure on this ashing process was discussed by in situ monitoring of plasmas. Engineering solution of this ashing process was proposed for industrial applications. **Keywords:** plasma oxidation, CVD-diamond coating, ashing, hollow cathode device,

CVD-diamond coatings as well as poly-crystalline diamonds have been widely utilized for the protection of dies and tools from wear and damage in cutting, drilling, and piercing operations in practice [1]. They are also expected to be working as a semi-conductive and thermal-conductive substrate for micro-electro-mechanical system (MEMS) and Bio-MEMS [2]. In parallel with high qualification in CVD-diamond coating technologies, how to make accurate shaping, micro−/nano-texturing, polishing and finishing of diamond films also

In the literature of patents to search for qualification in ashing the used diamond coatings, many studies have been developed to efficiently remove the diamond films from WC (Co) (Tungsten Carbide with Cobalt binder) and silicon substrates. Among them, three ashing processes were

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

**Coatings by High-Density Plasma Oxidation**

**Coatings by High-Density Plasma Oxidation**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.79143

quantitative plasma diagnosis

becomes an engineering issue [3].

Tatsuhiko Aizawa

Tatsuhiko Aizawa

**Abstract**

**1. Introduction**

#### **Controlled Post-treatment of Thick CVD-Diamond Coatings by High-Density Plasma Oxidation Controlled Post-treatment of Thick CVD-Diamond Coatings by High-Density Plasma Oxidation**

DOI: 10.5772/intechopen.79143

#### Tatsuhiko Aizawa Tatsuhiko Aizawa

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.79143

**Abstract**

CVD-diamond coatings were posttreated by plasma oxidation to recycle an original WC (Co) mother tool substrate for recoating and reuse. A developed RF/DC plasma oxidation system was stated together with a hollow cathode device to intensify the oxygen ion and electron densities and with a quantitative plasma diagnosis equipment. Plasma oxidation ashing conditions were optimized by this quantitative diagnosis toward the perfect ashing of diamond films with less residuals and less tool edge damage. Geometric effect of tool teeth structure on this ashing process was discussed by in situ monitoring of plasmas. Engineering solution of this ashing process was proposed for industrial applications.

**Keywords:** plasma oxidation, CVD-diamond coating, ashing, hollow cathode device, quantitative plasma diagnosis

## **1. Introduction**

CVD-diamond coatings as well as poly-crystalline diamonds have been widely utilized for the protection of dies and tools from wear and damage in cutting, drilling, and piercing operations in practice [1]. They are also expected to be working as a semi-conductive and thermal-conductive substrate for micro-electro-mechanical system (MEMS) and Bio-MEMS [2]. In parallel with high qualification in CVD-diamond coating technologies, how to make accurate shaping, micro−/nano-texturing, polishing and finishing of diamond films also becomes an engineering issue [3].

In the literature of patents to search for qualification in ashing the used diamond coatings, many studies have been developed to efficiently remove the diamond films from WC (Co) (Tungsten Carbide with Cobalt binder) and silicon substrates. Among them, three ashing processes were

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

noticed; the laser machining [4], the ion-beam and plasma discharging [5], and the reactive etching method [6]. The former two methods are difficult to be applied to tools with complex shaped teeth. Residuals of diamond were left on the treated tool surface. WC (Co) substrate suffered from severe thermal and chemical transients. Plasma oxidation method [7] has grown up as one of the most promising approaches to ashing. The CVD-diamond films are physically or chemically removed by ion bombardment and/or plasma-driven chemical reactions. These plasma ashing processes encounter some severe practical difficulties: (1) imperfect ashing of diamond films, (2) long leading time, (3) residual diamond films, especially behind the cutting teeth, (4) overoxidation of WC (Co) substrate, and (5) damage at cutting tooth edges. The used carbon-base coatings were removed from substrates by nontraditional plasma oxidation method for industrial application [8–11]. In this chapter, RF/DC plasma oxidation system with hollow cathode devices is introduced as an effective ashing method. Plasma oxidation processing for posttreatment of diamond films is stated with comments on its fundamental features and application to industries. Quantitative plasma diagnosis is utilized to characterize the oxygen plasmas and to describe the ashing process. Two types of CVD-diamond-coated cutting tools are employed as a specimen to demonstrate the effectiveness of present plasma oxidation ashing.

## **2. Plasma oxidation processing**

The glow discharging process is utilized to ignite the oxygen plasmas by the application of radio-frequency (RF) power and direct current (DC) bias. This oxygen plasma density is effectively intensified by using the hollow cathode devices. Emissive light optical spectroscopy (EOS) as well as the Langmuir probe are utilized to quantitatively characterize the oxygen plasma state and to describe the reaction process during oxidation. CVD-diamond-coated tools are prepared for ashing treatments.

**2.2. Plasma oxidation diagnosis**

activated species in plasmas.

RF- and DC-power supplies, ⑤ evacuation units, and ⑥ carrier gas supply.

EOS (Hamamatsu Photonics, Co., Ltd., Japan) is first utilized for diagnosis on the oxygen plasmas, as shown in **Figure 2**. The emissive light from plasmas is detected through a silica window of chamber to send the signals to the analyzer via the optical fibers. This spectroscopic analysis is also useful to make on-line monitoring on plasma state in ashing. Each activated species in the plasma is identified by its corresponding peak at the specified wave length in the measured spectrum. **Figure 2** also illustrates the Langmuir probe, equipped to the present ashing system. Both the ion/electron density and electron temperature are measured for optimization of ashing process. With this information of density and temperature

**Figure 2.** Quantitative plasma diagnosis to measure the electron density and temperature as well as the spectrum of

**Figure 1.** High density RF-DC oxygen plasma etching system. ① vacuum chamber, ② RF-generator, ③ control panel, ④

Controlled Post-treatment of Thick CVD-Diamond Coatings by High-Density Plasma Oxidation

http://dx.doi.org/10.5772/intechopen.79143

77

#### **2.1. RF/DC plasma oxidation system**

Different from the conventional DC- or RF-plasma generators, where plasmas are ignited and generated in the frequency of 13.56 MHz or its multiples, the present RF/DC oxygen plasmas are ignited at 2 MHz without use of matching box, as shown in **Figure 1**.

This frequency is automatically adjusted to keep matching between input and out powers around 2 MHz [7, 12–13]. This automatic and prompt response in power matching stabilizes the temporally varying plasma state. As reported in [7], a longer response time in the order of 1–10 s is needed for the mechanical matching box to adjust the RF-power. The present oxygen plasma state for ashing process is controlled within 1 ms in response; there is no time delay in power control. The vacuum chamber is electrically neutral without a self-biased current. Both the RF-voltage and DC-bias are controlled independently from each other to be from 0 to 250 V, and, from 0 to 600 V, respectively. This RF-plasma is ignited and controlled by using a dipole electrode, while the DC bias is directly applied to the specimens. In the following ashing experiments, the specimens were located on the cathode table before evacuation down to the base pressure. Then, a carrier gas was introduced into the chamber to attain the specified pressure. Both oxygen and argon gases were available in this system besides the nitrogen gas for bent.

Controlled Post-treatment of Thick CVD-Diamond Coatings by High-Density Plasma Oxidation http://dx.doi.org/10.5772/intechopen.79143 77

**Figure 1.** High density RF-DC oxygen plasma etching system. ① vacuum chamber, ② RF-generator, ③ control panel, ④ RF- and DC-power supplies, ⑤ evacuation units, and ⑥ carrier gas supply.

#### **2.2. Plasma oxidation diagnosis**

noticed; the laser machining [4], the ion-beam and plasma discharging [5], and the reactive etching method [6]. The former two methods are difficult to be applied to tools with complex shaped teeth. Residuals of diamond were left on the treated tool surface. WC (Co) substrate suffered from severe thermal and chemical transients. Plasma oxidation method [7] has grown up as one of the most promising approaches to ashing. The CVD-diamond films are physically or chemically removed by ion bombardment and/or plasma-driven chemical reactions. These plasma ashing processes encounter some severe practical difficulties: (1) imperfect ashing of diamond films, (2) long leading time, (3) residual diamond films, especially behind the cutting teeth, (4) overoxidation of WC (Co) substrate, and (5) damage at cutting tooth edges. The used carbon-base coatings were removed from substrates by nontraditional plasma oxidation method for industrial application [8–11]. In this chapter, RF/DC plasma oxidation system with hollow cathode devices is introduced as an effective ashing method. Plasma oxidation processing for posttreatment of diamond films is stated with comments on its fundamental features and application to industries. Quantitative plasma diagnosis is utilized to characterize the oxygen plasmas and to describe the ashing process. Two types of CVD-diamond-coated cutting tools are employed as a specimen to demonstrate the effectiveness of present plasma oxidation ashing.

The glow discharging process is utilized to ignite the oxygen plasmas by the application of radio-frequency (RF) power and direct current (DC) bias. This oxygen plasma density is effectively intensified by using the hollow cathode devices. Emissive light optical spectroscopy (EOS) as well as the Langmuir probe are utilized to quantitatively characterize the oxygen plasma state and to describe the reaction process during oxidation. CVD-diamond-coated

Different from the conventional DC- or RF-plasma generators, where plasmas are ignited and generated in the frequency of 13.56 MHz or its multiples, the present RF/DC oxygen plasmas

This frequency is automatically adjusted to keep matching between input and out powers around 2 MHz [7, 12–13]. This automatic and prompt response in power matching stabilizes the temporally varying plasma state. As reported in [7], a longer response time in the order of 1–10 s is needed for the mechanical matching box to adjust the RF-power. The present oxygen plasma state for ashing process is controlled within 1 ms in response; there is no time delay in power control. The vacuum chamber is electrically neutral without a self-biased current. Both the RF-voltage and DC-bias are controlled independently from each other to be from 0 to 250 V, and, from 0 to 600 V, respectively. This RF-plasma is ignited and controlled by using a dipole electrode, while the DC bias is directly applied to the specimens. In the following ashing experiments, the specimens were located on the cathode table before evacuation down to the base pressure. Then, a carrier gas was introduced into the chamber to attain the specified pressure. Both oxygen and argon gases were available in this system besides the nitrogen gas for bent.

are ignited at 2 MHz without use of matching box, as shown in **Figure 1**.

**2. Plasma oxidation processing**

76 Chemical Vapor Deposition for Nanotechnology

tools are prepared for ashing treatments.

**2.1. RF/DC plasma oxidation system**

EOS (Hamamatsu Photonics, Co., Ltd., Japan) is first utilized for diagnosis on the oxygen plasmas, as shown in **Figure 2**. The emissive light from plasmas is detected through a silica window of chamber to send the signals to the analyzer via the optical fibers. This spectroscopic analysis is also useful to make on-line monitoring on plasma state in ashing. Each activated species in the plasma is identified by its corresponding peak at the specified wave length in the measured spectrum. **Figure 2** also illustrates the Langmuir probe, equipped to the present ashing system. Both the ion/electron density and electron temperature are measured for optimization of ashing process. With this information of density and temperature

**Figure 2.** Quantitative plasma diagnosis to measure the electron density and temperature as well as the spectrum of activated species in plasmas.

**Figure 3.** An experimental set-up for plasma diagnosis by EOS.

both for electrons, ions and radicals, the plasma state in processing are precisely defined and described to investigate the effect of plasma processing parameters on the ashing behavior.

Let us first explain the experimental set-up for EOS and measure the typical spectra in the function of oxygen gas pressure [14]. **Figure 3** shows an experimental set-up to make EOS. The whole window was covered by a pure aluminum sheet to be free from noises. The measuring point can be varied by focusing; the plasma state at the focused point in the chamber is in situ measured during oxidation process.

When RF voltage was 250 V and DC-bias, −500 V, the emissive light optical spectra were measured in the function of gas pressure. As shown in **Figure 4**, the oxygen molecular ion O2 + as well as the oxygen ion (OII or O<sup>+</sup> ) has the highest peak at 774 and 470 nm, respectively.

The intensity of oxygen ions (OI or O\*; OII or O<sup>+</sup> ; OIII or O2+) and O2 + reduced with increasing the gas pressure in **Figure 4**. This is because the reaction cross-section significantly reduces for electron detachment from oxygen molecules.

The Langmuir probe is employed to measure the electron and ion densities as well as the electron temperature. The experimental set-up for I-V curve measurement in the RF/DC plasmas is depicted in **Figure 5**. The accuracy in measurement strongly depends on the tip geometry and resistivity; e.g., the tip radius is 350 μm, the tip holder, 3 mm, the probe tip length, 1 cm, and, the tip resistance, 36 Ohm, as shown in **Figure 5(b)**.

The measured I-V curve is nearly constant but abruptly changes at the vicinity of plasma potential. This I-V curve is deconvoluted to deduce each contribution of ions and electrons to measured current density. Each I-V curve for ion and electron is differentiated to calculate its first and second derivatives across the plasma potential; the electron and ion density as well as the electron temperature are deduced from this direct calculation. **Figure 6** depicts the variation of measured ion density with the oxygen gas pressure when the DC bias was −600 V. Reduction of ion density

was only measured in the range of 150–175 Pa; ion density monotonically increases with the pressure when DC bias was more than −500 V. Ionization is activated with increasing the pressure because of collision of electrons with oxygen molecules. Finally, the present RF/DC plasma characteristics is compared with the ICP (Induction-Coupled Plasma) where a cylindrical tube with diameter 40 mm and a length of 600 mm was used to ignite the plasmas. At the pressure of 75 Pa, the electron density (Ne) reaches to 4.8 × 10<sup>16</sup> m−3 with the electron temperature (Te) of 3.9 eV in

**Figure 5.** Experimental set-up for measurement of ion and electron densities as well as electron temperature at the

specified position in the RF/DC plasmas by using the Langmuir probe.

**Figure 4.** Measured emissive light optical spectra in the function of gas pressure for a standard RF/DC plasma under

Controlled Post-treatment of Thick CVD-Diamond Coatings by High-Density Plasma Oxidation

http://dx.doi.org/10.5772/intechopen.79143

79

[250 V; −500 V].

Controlled Post-treatment of Thick CVD-Diamond Coatings by High-Density Plasma Oxidation http://dx.doi.org/10.5772/intechopen.79143 79

**Figure 4.** Measured emissive light optical spectra in the function of gas pressure for a standard RF/DC plasma under [250 V; −500 V].

both for electrons, ions and radicals, the plasma state in processing are precisely defined and described to investigate the effect of plasma processing parameters on the ashing behavior. Let us first explain the experimental set-up for EOS and measure the typical spectra in the function of oxygen gas pressure [14]. **Figure 3** shows an experimental set-up to make EOS. The whole window was covered by a pure aluminum sheet to be free from noises. The measuring point can be varied by focusing; the plasma state at the focused point in the chamber is in situ

When RF voltage was 250 V and DC-bias, −500 V, the emissive light optical spectra were measured in the function of gas pressure. As shown in **Figure 4**, the oxygen molecular ion

the gas pressure in **Figure 4**. This is because the reaction cross-section significantly reduces for

The Langmuir probe is employed to measure the electron and ion densities as well as the electron temperature. The experimental set-up for I-V curve measurement in the RF/DC plasmas is depicted in **Figure 5**. The accuracy in measurement strongly depends on the tip geometry and resistivity; e.g., the tip radius is 350 μm, the tip holder, 3 mm, the probe tip length, 1 cm,

The measured I-V curve is nearly constant but abruptly changes at the vicinity of plasma potential. This I-V curve is deconvoluted to deduce each contribution of ions and electrons to measured current density. Each I-V curve for ion and electron is differentiated to calculate its first and second derivatives across the plasma potential; the electron and ion density as well as the electron temperature are deduced from this direct calculation. **Figure 6** depicts the variation of measured ion density with the oxygen gas pressure when the DC bias was −600 V. Reduction of ion density

) has the highest peak at 774 and 470 nm, respectively.

+

reduced with increasing

; OIII or O2+) and O2

measured during oxidation process.

78 Chemical Vapor Deposition for Nanotechnology

as well as the oxygen ion (OII or O<sup>+</sup>

The intensity of oxygen ions (OI or O\*; OII or O<sup>+</sup>

**Figure 3.** An experimental set-up for plasma diagnosis by EOS.

and, the tip resistance, 36 Ohm, as shown in **Figure 5(b)**.

electron detachment from oxygen molecules.

O2 +

**Figure 5.** Experimental set-up for measurement of ion and electron densities as well as electron temperature at the specified position in the RF/DC plasmas by using the Langmuir probe.

was only measured in the range of 150–175 Pa; ion density monotonically increases with the pressure when DC bias was more than −500 V. Ionization is activated with increasing the pressure because of collision of electrons with oxygen molecules. Finally, the present RF/DC plasma characteristics is compared with the ICP (Induction-Coupled Plasma) where a cylindrical tube with diameter 40 mm and a length of 600 mm was used to ignite the plasmas. At the pressure of 75 Pa, the electron density (Ne) reaches to 4.8 × 10<sup>16</sup> m−3 with the electron temperature (Te) of 3.9 eV in

**Figure 6.** Variation of the measured ion density with the gas pressure by the Langmuir probe.

the present system. When using ICP, Ne = 1.0 × 10<sup>16</sup> m−3 and Te = 5.0 eV in [15]. Higher electron density by five times can be attained even at the center of large chamber in **Figures 1** and **5(b)**.

**3. Plasma oxidation ashing of used diamond films**

process, and, this ashing must be done in short duration.

**3.1. Background**

Plasma oxidation ashing process is employed to make perfect removal of used diamond films, coated onto the WC (Co) cutting tools, with minimum damage to tool teeth and to recycle the mother WC (Co) substrate for recoating and reuse. As an engineering issue, no residuals of diamond films are left on the WC (Co) substrate, less damage is induced during ashing

**Figure 7.** Comparison of the ion and electron density variation with pressure with and without use of the hollow cathode device. (a) RF-DC plasmas in **Figure 1** without use of the hollow cathode, and, (b) with use of the hollow cathode.

Controlled Post-treatment of Thick CVD-Diamond Coatings by High-Density Plasma Oxidation

http://dx.doi.org/10.5772/intechopen.79143

81

CVD-diamond-coated WC (Co) cutting, drilling, and end-milling tools have been widely utilized for dry machining of the carbon fiber–reinforced plastic (CFRP) or Carbon fiber–reinforced thermoplastic (CFRTP) components and parts in the airplanes and automobiles [20, 21]. These materials are used for main cabin and wings; a hundred thousand holes must be machined or drilled into these CFRP/CFRTP structural members for each airplane in dry by using the diamond-coated tools. Since the number of airplanes is expected to be doubled in next 8 years, the above tooling cost percentage significantly increases in the total production cost. Event at present, the carbon fibers in CFRP/CFRTP have high strength and stiffness enough to make fatal damage and defect to the diamond coating of machining tools. Since an equivalent number of holes to be machined are estimated to be 100,000 per an airplane, the production cost, especially the tooling cost, increases with demanded numbers of planes or cars. Those new materials are too hard to machine or drill without damages to CVD-diamond-coated WC (Co) tools; the used tool is exchanged with new one in every machining by 10 m, or in every drilling

by 10 holes [22]. How to recycle the WC (Co) substrate is essential to reduce the cost.

These used or damaged diamond coatings must be once removed to recycle the WC (Co) tool substrate with less damages to its tooth geometry before recoating and to reuse this in cutting operations. RF/DC plasma oxidation process was demonstrated to be useful for perfect

#### **2.3. High-density plasma oxidation**

Various tools and devices are available to intensity ion and electron densities in the plasmas. A magnetism works as a lens to concentrate the ions in the magnetic field [16, 17]; ion density increases with increasing the magnetic field strength. Electric insulation is also effective to separate the plasmas into the specified spaces [18]. In this study on the plasma oxidation, a hollow cathode device [19] is selected and redesigned to confine the oxygen plasmas in the hollow and to intensity their ion and electron densities. The quantitative plasma diagnosis in 2.2 is also utilized to describe how much the oxygen ion and electron densities are enhanced by the hollow cathode device.

Variation of the electron and oxygen ion densities with pressure is compared in **Figure 7** without and with use of the hollow cathode. At the pressure of 75 Pa, N<sup>i</sup> = 7.8 × 10<sup>16</sup> m−3 in **Figure 7(a)**; this N<sup>i</sup> is enhanced to be 5 × 10<sup>17</sup> m−3 when using the hollow cathode device in **Figure 7(b)**. In the following, this hollow cathode device is redesigned to make effective ashing of diamond films coated on the cutting and end-milling tools.

#### **2.4. Observation and measurement**

High resolution optical microscopy as well as scanning electron microscopy (SEM) are utilized to observe the ashed surfaces of diamond-coated specimens. Raman spectroscopy is also used to detect a characteristic peak to the diamond.

Controlled Post-treatment of Thick CVD-Diamond Coatings by High-Density Plasma Oxidation http://dx.doi.org/10.5772/intechopen.79143 81

**Figure 7.** Comparison of the ion and electron density variation with pressure with and without use of the hollow cathode device. (a) RF-DC plasmas in **Figure 1** without use of the hollow cathode, and, (b) with use of the hollow cathode.

## **3. Plasma oxidation ashing of used diamond films**

Plasma oxidation ashing process is employed to make perfect removal of used diamond films, coated onto the WC (Co) cutting tools, with minimum damage to tool teeth and to recycle the mother WC (Co) substrate for recoating and reuse. As an engineering issue, no residuals of diamond films are left on the WC (Co) substrate, less damage is induced during ashing process, and, this ashing must be done in short duration.

#### **3.1. Background**

the present system. When using ICP, Ne = 1.0 × 10<sup>16</sup> m−3 and Te = 5.0 eV in [15]. Higher electron density by five times can be attained even at the center of large chamber in **Figures 1** and **5(b)**.

**Figure 6.** Variation of the measured ion density with the gas pressure by the Langmuir probe.

Various tools and devices are available to intensity ion and electron densities in the plasmas. A magnetism works as a lens to concentrate the ions in the magnetic field [16, 17]; ion density increases with increasing the magnetic field strength. Electric insulation is also effective to separate the plasmas into the specified spaces [18]. In this study on the plasma oxidation, a hollow cathode device [19] is selected and redesigned to confine the oxygen plasmas in the hollow and to intensity their ion and electron densities. The quantitative plasma diagnosis in 2.2 is also utilized to describe how much the oxygen ion and electron densities are enhanced

Variation of the electron and oxygen ion densities with pressure is compared in **Figure 7** without and with use of the hollow cathode. At the pressure of 75 Pa, N<sup>i</sup> = 7.8 × 10<sup>16</sup> m−3 in

**Figure 7(b)**. In the following, this hollow cathode device is redesigned to make effective ash-

High resolution optical microscopy as well as scanning electron microscopy (SEM) are utilized to observe the ashed surfaces of diamond-coated specimens. Raman spectroscopy is also

ing of diamond films coated on the cutting and end-milling tools.

is enhanced to be 5 × 10<sup>17</sup> m−3 when using the hollow cathode device in

**2.3. High-density plasma oxidation**

80 Chemical Vapor Deposition for Nanotechnology

by the hollow cathode device.

**2.4. Observation and measurement**

used to detect a characteristic peak to the diamond.

**Figure 7(a)**; this N<sup>i</sup>

CVD-diamond-coated WC (Co) cutting, drilling, and end-milling tools have been widely utilized for dry machining of the carbon fiber–reinforced plastic (CFRP) or Carbon fiber–reinforced thermoplastic (CFRTP) components and parts in the airplanes and automobiles [20, 21]. These materials are used for main cabin and wings; a hundred thousand holes must be machined or drilled into these CFRP/CFRTP structural members for each airplane in dry by using the diamond-coated tools. Since the number of airplanes is expected to be doubled in next 8 years, the above tooling cost percentage significantly increases in the total production cost. Event at present, the carbon fibers in CFRP/CFRTP have high strength and stiffness enough to make fatal damage and defect to the diamond coating of machining tools. Since an equivalent number of holes to be machined are estimated to be 100,000 per an airplane, the production cost, especially the tooling cost, increases with demanded numbers of planes or cars. Those new materials are too hard to machine or drill without damages to CVD-diamond-coated WC (Co) tools; the used tool is exchanged with new one in every machining by 10 m, or in every drilling by 10 holes [22]. How to recycle the WC (Co) substrate is essential to reduce the cost.

These used or damaged diamond coatings must be once removed to recycle the WC (Co) tool substrate with less damages to its tooth geometry before recoating and to reuse this in cutting operations. RF/DC plasma oxidation process was demonstrated to be useful for perfect removal of used tetragonal amorphous carbon coating or t-a:C coatings and for recycle of WC (Co) cutting substrate [23–25]. The recoated WC (Co) tools were proven to have the same capability in cutting performance as the virgin tools [24, 25]. This plasma oxidation process is further advanced to remove the used diamond coatings for recycle and reuse and to reduce the production cost in machining of CFRP/CFRTP members [26–28].

#### **3.2. Preparation of diamond-coated tools**

The diamond-coated WC (Co) end-milling tools were prepared for ashing experiments. As shown in **Figure 8**, two types of tools were employed to investigate the effect of tooth geometry on the plasma ashing process. The tool with the length of 70 mm, the diameter of 10 mm, and the edge skew angle of 30° was used as a standard specimen to search for an optimum ashing condition. A small-sized tool with the length of 70 mm, the diameter of 6 mm, and the edge skew angle of 10° was also used to investigate whether the optimal ashing condition is affected by down-sizing. The diamond film thickness was common in both tools by 15 μm.

#### **3.3. Experimental set-up**

An end-milling tool has skewed tooth geometry; diamond films are coated even on the curved surfaces backward the teeth. A special hollow cathode is designed to preserve the confined plasma in the hollow against the rotating tools. **Figure 9** depicts the developed hollow cathode device for ashing.

are variable from 100 to 250 V and from −400 to −800 V, respectively. The plasma pressure (p) is marginable to be determined; in the following, 25 < p < 100 Pa. The plasma oxidation conditions are optimized by the quantitative plasma diagnosis to be 100 V for RF-voltage, −500 V

**Figure 9.** Special hollow cathode device for ashing the diamond films on the WC (Co) end-milling tool substrate. (a)

Controlled Post-treatment of Thick CVD-Diamond Coatings by High-Density Plasma Oxidation

http://dx.doi.org/10.5772/intechopen.79143

83

A standard specimen is first employed to investigate the effect of plasma processing parameters on the ashing behavior. Under the optimal conditions, the small-scaled tool specimen is also used to describe the geometric effect of tool teeth on the ashing process. Difference in

Both RF-voltage and pressure are varied to describe the plasma ashing process. In the first experiment, RF-voltage was set to be 250 V, DC-bias, −650 V and oxygen gas pressure, 30 Pa.

Since cobalt in WC (Co) is mainly oxidized, the tool surface is colored in green in **Figure 10(a)**. No residual diamond debris are left on the tool surface in **Figure 10(b)**. Due to this excess in oxidation, the shrinkage of tool edge reaches to 7.7 μm in **Figure 10(c)**. In order to reduce the oxidation rate, the above processing parameters are tuned to be (250 V; −500 V; 45 Pa; 3.6 ks).

The surface is colored in reddish yellow in **Figure 11(a)**; this implies that a part of tungsten in WC (Co) is mainly oxidized even under this condition. Almost all the diamond films are removed on the tool surface in **Figure 11(b)**. This over-oxidation results in a significant shrinkage of edge by 9.7 μm in **Figure 11(c)**. To be discussed in later, the activated oxygen

**Figure 12(a)** depicts the ashed tool surface by lowering the RF-voltage down to 100 V. No oxide layers of cobalt and tungsten are seen on the WC (Co) substrate surface. The original diamond

for DC-bias, 45 Pa for pressure and 3.6 ks for duration time.

Illustration of hollow cathode device, and, (b) experimental set-up for ashing.

ashing process conditions is explained by the in situ plasma diagnosis.

**Figure 11** depicts the surface condition and SEM image of ashed specimen.

atom density must be directly controlled by lowering the RF-voltage.

**3.4. Experimental results**

*3.4.1. Standard end-milling tools*

**Figure 10** shows the ashed specimen for 3.6 ks.

The generated RF-plasma is confined in this hollow so that higher ion and electron densities are attained for ashing the diamond films. As shown in **Figure 9(a)**, a diamond-coated tool is placed in the center of hollow tube; the same DC-bias is applied to both the hollow and the tool. Although the rotating speed has influence on the plasma configuration, it is selected to be 6 rpm in the following experiments.

A standard experimental set up is summarized as follows. After placing the specimen in **Figure 9(b)**, the chamber is evacuated down to the base pressure, less than 5 × 10−3 Pa. The pure oxygen gas with its purity of 99.99% is introduced as a carrier gas. RF- and DC- bias voltages

**Figure 8.** Two diamond-coated end-milling tools as a specimen for plasma ashing process.

Controlled Post-treatment of Thick CVD-Diamond Coatings by High-Density Plasma Oxidation http://dx.doi.org/10.5772/intechopen.79143 83

**Figure 9.** Special hollow cathode device for ashing the diamond films on the WC (Co) end-milling tool substrate. (a) Illustration of hollow cathode device, and, (b) experimental set-up for ashing.

are variable from 100 to 250 V and from −400 to −800 V, respectively. The plasma pressure (p) is marginable to be determined; in the following, 25 < p < 100 Pa. The plasma oxidation conditions are optimized by the quantitative plasma diagnosis to be 100 V for RF-voltage, −500 V for DC-bias, 45 Pa for pressure and 3.6 ks for duration time.

#### **3.4. Experimental results**

removal of used tetragonal amorphous carbon coating or t-a:C coatings and for recycle of WC (Co) cutting substrate [23–25]. The recoated WC (Co) tools were proven to have the same capability in cutting performance as the virgin tools [24, 25]. This plasma oxidation process is further advanced to remove the used diamond coatings for recycle and reuse and to reduce

The diamond-coated WC (Co) end-milling tools were prepared for ashing experiments. As shown in **Figure 8**, two types of tools were employed to investigate the effect of tooth geometry on the plasma ashing process. The tool with the length of 70 mm, the diameter of 10 mm, and the edge skew angle of 30° was used as a standard specimen to search for an optimum ashing condition. A small-sized tool with the length of 70 mm, the diameter of 6 mm, and the edge skew angle of 10° was also used to investigate whether the optimal ashing condition is affected by down-sizing. The diamond film thickness was common in both tools by 15 μm.

An end-milling tool has skewed tooth geometry; diamond films are coated even on the curved surfaces backward the teeth. A special hollow cathode is designed to preserve the confined plasma in the hollow against the rotating tools. **Figure 9** depicts the developed hollow cath-

The generated RF-plasma is confined in this hollow so that higher ion and electron densities are attained for ashing the diamond films. As shown in **Figure 9(a)**, a diamond-coated tool is placed in the center of hollow tube; the same DC-bias is applied to both the hollow and the tool. Although the rotating speed has influence on the plasma configuration, it is selected to

A standard experimental set up is summarized as follows. After placing the specimen in **Figure 9(b)**, the chamber is evacuated down to the base pressure, less than 5 × 10−3 Pa. The pure oxygen gas with its purity of 99.99% is introduced as a carrier gas. RF- and DC- bias voltages

**Figure 8.** Two diamond-coated end-milling tools as a specimen for plasma ashing process.

the production cost in machining of CFRP/CFRTP members [26–28].

**3.2. Preparation of diamond-coated tools**

82 Chemical Vapor Deposition for Nanotechnology

**3.3. Experimental set-up**

ode device for ashing.

be 6 rpm in the following experiments.

A standard specimen is first employed to investigate the effect of plasma processing parameters on the ashing behavior. Under the optimal conditions, the small-scaled tool specimen is also used to describe the geometric effect of tool teeth on the ashing process. Difference in ashing process conditions is explained by the in situ plasma diagnosis.

#### *3.4.1. Standard end-milling tools*

Both RF-voltage and pressure are varied to describe the plasma ashing process. In the first experiment, RF-voltage was set to be 250 V, DC-bias, −650 V and oxygen gas pressure, 30 Pa. **Figure 10** shows the ashed specimen for 3.6 ks.

Since cobalt in WC (Co) is mainly oxidized, the tool surface is colored in green in **Figure 10(a)**. No residual diamond debris are left on the tool surface in **Figure 10(b)**. Due to this excess in oxidation, the shrinkage of tool edge reaches to 7.7 μm in **Figure 10(c)**. In order to reduce the oxidation rate, the above processing parameters are tuned to be (250 V; −500 V; 45 Pa; 3.6 ks). **Figure 11** depicts the surface condition and SEM image of ashed specimen.

The surface is colored in reddish yellow in **Figure 11(a)**; this implies that a part of tungsten in WC (Co) is mainly oxidized even under this condition. Almost all the diamond films are removed on the tool surface in **Figure 11(b)**. This over-oxidation results in a significant shrinkage of edge by 9.7 μm in **Figure 11(c)**. To be discussed in later, the activated oxygen atom density must be directly controlled by lowering the RF-voltage.

**Figure 12(a)** depicts the ashed tool surface by lowering the RF-voltage down to 100 V. No oxide layers of cobalt and tungsten are seen on the WC (Co) substrate surface. The original diamond

**Figure 10.** Ashing of CVD-diamond coating on the WC (Co) standard specimen under (250 V; −650 V; 30 Pa; 3.6 ks). (a) Surface of ashed specimen before cleansing, (b) Ashed tool edge, (c) SEM image of tool edge.

**Figure 11.** Ashing of CVD-diamond coating on the WC (Co) standard specimen under (250 V; −-500 V; 45 Pa; 3.6 ks). (a) Surface of ashed specimen before cleansing, (b) Ashed tool edge, (c) SEM image of tool edge.

micro-pores are accommodated as a nucleation site for diamonds by selective removing the cobalt binders before CVD-coating. Hence, this microstructure in **Figure 13(b)** proves that starting WC (Co) substrate surface before coating is reproduced by the present ashing process. The Raman spectroscopy is also utilized to precisely check whether a residual debris of diamond is left on the ashed tool surface. **Figure 14** compares the Raman spectra before and after ashing. The presence of diamond film is identified by the Raman peak around 1330 cm−1 as shown in **Figure 14**. No peaks are detected at the vicinity of 1330 cm−1 in **Figure 14** after

**Figure 13.** Comparison of microstructure on the diamond-coated tool surface before and after ashing under (100 V;

**Figure 12.** Ashing of CVD-diamond coating on the WC (Co) standard specimen under (100 V; −500 V; 45 Pa; 3.6 ks). (a)

Controlled Post-treatment of Thick CVD-Diamond Coatings by High-Density Plasma Oxidation

http://dx.doi.org/10.5772/intechopen.79143

85

Surface of ashed specimen before cleansing, (b) Ashed tool edge, (c) SEM image of tool edge.

The small sized tools, as shown in **Figure 8**, were posttreated by plasma oxidation under the optimum condition in the above, or, (100 V; −500 V; 45 Pa; 3.6 ks). High resolution optical

ashing; no diamond residuals are left on the tool surface after ashing.

*3.4.2. Small-scaled end-milling tools*

−500 V; 45 Pa; 3.6 ks). (a) before ashing, and, (b) after ashing.

coatings are completely removed only after ashing for 3.6 ks, as shown in **Figure 12(b)**. The shrinkage of tool edge is limited by 1.1 μm in **Figure 12(c)**. Fine SEM observation and the Raman spectroscopy are utilized to precisely describe this ashing process. **Figure 13** compares the top view in SEM image for the WC (Co) tool surface before and after ashing under this optimal condition of (100 V; −-500 V; 45 Pa; 3.6 ks).

The original tetrahedral diamond crystals with significant roughness in **Figure 13(a)** are completely removed to leave a flat WC (Co) surface with micro-pores among WC grains. These Controlled Post-treatment of Thick CVD-Diamond Coatings by High-Density Plasma Oxidation http://dx.doi.org/10.5772/intechopen.79143 85

**Figure 12.** Ashing of CVD-diamond coating on the WC (Co) standard specimen under (100 V; −500 V; 45 Pa; 3.6 ks). (a) Surface of ashed specimen before cleansing, (b) Ashed tool edge, (c) SEM image of tool edge.

**Figure 13.** Comparison of microstructure on the diamond-coated tool surface before and after ashing under (100 V; −500 V; 45 Pa; 3.6 ks). (a) before ashing, and, (b) after ashing.

micro-pores are accommodated as a nucleation site for diamonds by selective removing the cobalt binders before CVD-coating. Hence, this microstructure in **Figure 13(b)** proves that starting WC (Co) substrate surface before coating is reproduced by the present ashing process. The Raman spectroscopy is also utilized to precisely check whether a residual debris of diamond is left on the ashed tool surface. **Figure 14** compares the Raman spectra before and after ashing. The presence of diamond film is identified by the Raman peak around 1330 cm−1 as shown in **Figure 14**. No peaks are detected at the vicinity of 1330 cm−1 in **Figure 14** after ashing; no diamond residuals are left on the tool surface after ashing.

#### *3.4.2. Small-scaled end-milling tools*

**Figure 11.** Ashing of CVD-diamond coating on the WC (Co) standard specimen under (250 V; −-500 V; 45 Pa; 3.6 ks). (a)

**Figure 10.** Ashing of CVD-diamond coating on the WC (Co) standard specimen under (250 V; −650 V; 30 Pa; 3.6 ks). (a)

coatings are completely removed only after ashing for 3.6 ks, as shown in **Figure 12(b)**. The shrinkage of tool edge is limited by 1.1 μm in **Figure 12(c)**. Fine SEM observation and the Raman spectroscopy are utilized to precisely describe this ashing process. **Figure 13** compares the top view in SEM image for the WC (Co) tool surface before and after ashing under this

The original tetrahedral diamond crystals with significant roughness in **Figure 13(a)** are completely removed to leave a flat WC (Co) surface with micro-pores among WC grains. These

Surface of ashed specimen before cleansing, (b) Ashed tool edge, (c) SEM image of tool edge.

Surface of ashed specimen before cleansing, (b) Ashed tool edge, (c) SEM image of tool edge.

84 Chemical Vapor Deposition for Nanotechnology

optimal condition of (100 V; −-500 V; 45 Pa; 3.6 ks).

The small sized tools, as shown in **Figure 8**, were posttreated by plasma oxidation under the optimum condition in the above, or, (100 V; −500 V; 45 Pa; 3.6 ks). High resolution optical

surface are almost completely ashed away; the diamond residuals are left as a dendritic col-

Controlled Post-treatment of Thick CVD-Diamond Coatings by High-Density Plasma Oxidation

http://dx.doi.org/10.5772/intechopen.79143

87

This difference in ashing behavior between two specimens comes from the optimality in oxygen plasma sheath distribution surrounding the tool surface. The plasma diagnosis is also

EOS works to monitor the temporal variation of oxygen plasma state during the ashing process. Since the carbon in diamond reacts with oxygen atom, a peak for resultant of CO by the reaction, C (in diamond) + O (from O\* and O<sup>+</sup> in palsmas) -> CO, is detected by EOS together with oxygen-related peaks. Hence, in this in situ EOS-diagnosis, oxygen peak at 470 nm and

**Figure 16** depicts a time evolution of measured O- and CO-peak intensities by this in situ EOS. O-peak intensity is constant during ashing process; a constant oxygen atom flux is supplied to hot spots of plasma reaction. CO-peak intensity is enhanced in the early stage in ashing; it exponentially decays from t = 1000 s to be lower level and nearly minimum for t = 4000 s. This implies that homogeneous ashing process retards by itself with a constant time exponent. Hence, this exponential decay of CO-peak intensity with time proves that the

This time transient of CO-peak intensity is employed to describe the difference in ashing behavior between two diamond-coated tools. When using the standard tool, two CO-peak intensities, monitored at λ = 287 and 312 nm, exponentially decrease with time as shown in **Figure 17(a)**. Just as seen in **Figure 16**, the diamond film is expected to be homogeneously ashed away from this in situ diagnosis. This homogeneous ashing has been proven by precise observation on the posttreated tool surfaces as shown in **Figures 12**–**14**. On the other

employed to describe this effect of the plasma state on the ashing behavior.

CO-peak at 288 nm are monitored to describe the plasma reaction for ashing.

**Figure 16.** Temporal evolution of O- and CO-peak intensities, measured by the in situ EOS.

umn with the average diameter of 5–10 μm.

diamond coating is homogeneously ashed away.

*3.4.3. In situ plasma diagnosis*

**Figure 14.** Comparison of the Raman spectra for the diamond-coated tool surface before and after ashing under (100 V; −500 V; 45 Pa; 3.6 ks).

**Figure 15.** Ashing of CVD-diamond coating on the WC (Co) small-scaled specimen under (250 V; −500 V; 45 Pa; 3.6 ks). (a) Surface of ashed specimen before cleansing, (b) Ashed tool edge, (c) SEM image of tool edge.

microscopy and SEM are also utilized to make close observation on the ashed surfaces of small sized tools. As shown in **Figure 15(a)**, the outlook color of ashed tooth surfaces is gray in the similar manner to **Figure 12(a)**. **Figure 15(b)** shows the top tooth surfaces of ashed tools. No diamond films are left on the surfaces. These prove that most of diamond films coated on the small sized tools are also homogeneously removed just in correspondence to **Figure 12(b)** for the standard tool. After close observation on the whole tooth surfaces by SEM, a cold spot with the residual diamonds was detected on the rake face or the gash of tools. A typical SEM image is shown in **Figure 15(c)**. The diamond films at the upper edge and on the left-hand surface are almost completely ashed away; the diamond residuals are left as a dendritic column with the average diameter of 5–10 μm.

This difference in ashing behavior between two specimens comes from the optimality in oxygen plasma sheath distribution surrounding the tool surface. The plasma diagnosis is also employed to describe this effect of the plasma state on the ashing behavior.

#### *3.4.3. In situ plasma diagnosis*

**Figure 15.** Ashing of CVD-diamond coating on the WC (Co) small-scaled specimen under (250 V; −500 V; 45 Pa; 3.6 ks).

**Figure 14.** Comparison of the Raman spectra for the diamond-coated tool surface before and after ashing under (100 V;

−500 V; 45 Pa; 3.6 ks).

86 Chemical Vapor Deposition for Nanotechnology

microscopy and SEM are also utilized to make close observation on the ashed surfaces of small sized tools. As shown in **Figure 15(a)**, the outlook color of ashed tooth surfaces is gray in the similar manner to **Figure 12(a)**. **Figure 15(b)** shows the top tooth surfaces of ashed tools. No diamond films are left on the surfaces. These prove that most of diamond films coated on the small sized tools are also homogeneously removed just in correspondence to **Figure 12(b)** for the standard tool. After close observation on the whole tooth surfaces by SEM, a cold spot with the residual diamonds was detected on the rake face or the gash of tools. A typical SEM image is shown in **Figure 15(c)**. The diamond films at the upper edge and on the left-hand

(a) Surface of ashed specimen before cleansing, (b) Ashed tool edge, (c) SEM image of tool edge.

EOS works to monitor the temporal variation of oxygen plasma state during the ashing process. Since the carbon in diamond reacts with oxygen atom, a peak for resultant of CO by the reaction, C (in diamond) + O (from O\* and O<sup>+</sup> in palsmas) -> CO, is detected by EOS together with oxygen-related peaks. Hence, in this in situ EOS-diagnosis, oxygen peak at 470 nm and CO-peak at 288 nm are monitored to describe the plasma reaction for ashing.

**Figure 16** depicts a time evolution of measured O- and CO-peak intensities by this in situ EOS. O-peak intensity is constant during ashing process; a constant oxygen atom flux is supplied to hot spots of plasma reaction. CO-peak intensity is enhanced in the early stage in ashing; it exponentially decays from t = 1000 s to be lower level and nearly minimum for t = 4000 s. This implies that homogeneous ashing process retards by itself with a constant time exponent. Hence, this exponential decay of CO-peak intensity with time proves that the diamond coating is homogeneously ashed away.

This time transient of CO-peak intensity is employed to describe the difference in ashing behavior between two diamond-coated tools. When using the standard tool, two CO-peak intensities, monitored at λ = 287 and 312 nm, exponentially decrease with time as shown in **Figure 17(a)**. Just as seen in **Figure 16**, the diamond film is expected to be homogeneously ashed away from this in situ diagnosis. This homogeneous ashing has been proven by precise observation on the posttreated tool surfaces as shown in **Figures 12**–**14**. On the other

**Figure 16.** Temporal evolution of O- and CO-peak intensities, measured by the in situ EOS.

the commercial operation time by 20–30 hours experienced in several tooling companies. The whole ashing process can be in situ controlled by monitoring the CO-peak intensities at the specified wave length through EOS. A homogeneous, perfect ashing is guaranteed by an exponential decay of CO-peak intensity with time in this quantitative diagnosis. The tool teeth-edge geometry has significant effect on the formation of plasma sheaths on the tool surfaces. Inhomogeneous ashing can be also detected by the in situ plasma diagnosis; plasma

Controlled Post-treatment of Thick CVD-Diamond Coatings by High-Density Plasma Oxidation

http://dx.doi.org/10.5772/intechopen.79143

89

How to design this ashing process in practice and how to apply this posttreatment to indus-

Theoretical plasma processing design is often needed to construct a new device to control a plasma configuration in chamber and to understand the experimental results [29]. Some studies have just started to build up this theoretical model. Among them, for an example, the Boltzmann equation was solved to obtain electron transport coefficients and rate coefficients for fluid models [30].

In the present study, a multi-disciplinary program is used to build up a theoretical model of plasma processing and to simulate the effect of pressure on the plasma density [11]. Although the basic data for activation process is still limited, one-dimensional model is proposed to describe the argon plasma state in the function of plasma parameters such as RF-voltage, DC-bias, and pressure. **Figure 18(a)** illustrates this one-dimensional model of argon plasma. Ionization of argon is expected to concentrate at the vicinity of tool surface in **Figure 9**. As depicted in **Figure 18(b)**, a hot spot with high electron temperature or the plasma sheath is formed only at the vicinity of cathode table. Under this geometric configuration, the electron density distribution is calculated at each specified pressure. **Figure 19** depicts a variation of electron density distribution with increasing the argon pressure. This enhancement

**Figure 18.** One dimensional model of argon plasma by using the multi-disciplinary equations. (a) Theoretical model of

argon plasma, and, (b) decay of calculated electron temperature.

oxidation conditions must be tailored for each tool geometry in practice.

**4. Discussion**

tries are discussed in this session.

**4.1. Plasma processing design**

**Figure 17.** Time transients of CO-peak intensities at λ = 287 and 312 nm by the EOS in situ monitoring during ashing process.

hand, when using the small-sized tool, this time transient changes by itself as depicted in **Figure 17(b)**. The up-and-down deviation of CO-peak intensity reveals that ashing reaction localizes in hot spot at one surface but in cold spot at other surfaces. This heterogeneous ashing process results in a residual diamond film at the cold spots such as a small part on the rake edges of tools as shown in **Figure 15(c)**.

#### **3.5. Summary**

A diamond film coated onto a single WC (Co) tool substrate in a standard size was successfully ashed away without residuals and with much less damage at the tool tooth-edges. Due to this perfect ashing of diamond films, an original WC (Co) substrate surface was reproduced to have a decobalted WC (Co) microstructure. This posttreated surface is easy to be diamondrecoated for reuse of tools. The duration time for this ashing is only 3.6 ks, much faster than the commercial operation time by 20–30 hours experienced in several tooling companies. The whole ashing process can be in situ controlled by monitoring the CO-peak intensities at the specified wave length through EOS. A homogeneous, perfect ashing is guaranteed by an exponential decay of CO-peak intensity with time in this quantitative diagnosis. The tool teeth-edge geometry has significant effect on the formation of plasma sheaths on the tool surfaces. Inhomogeneous ashing can be also detected by the in situ plasma diagnosis; plasma oxidation conditions must be tailored for each tool geometry in practice.

## **4. Discussion**

**Figure 17.** Time transients of CO-peak intensities at λ = 287 and 312 nm by the EOS in situ monitoring during ashing

hand, when using the small-sized tool, this time transient changes by itself as depicted in **Figure 17(b)**. The up-and-down deviation of CO-peak intensity reveals that ashing reaction localizes in hot spot at one surface but in cold spot at other surfaces. This heterogeneous ashing process results in a residual diamond film at the cold spots such as a small part on the rake

A diamond film coated onto a single WC (Co) tool substrate in a standard size was successfully ashed away without residuals and with much less damage at the tool tooth-edges. Due to this perfect ashing of diamond films, an original WC (Co) substrate surface was reproduced to have a decobalted WC (Co) microstructure. This posttreated surface is easy to be diamondrecoated for reuse of tools. The duration time for this ashing is only 3.6 ks, much faster than

process.

**3.5. Summary**

edges of tools as shown in **Figure 15(c)**.

88 Chemical Vapor Deposition for Nanotechnology

How to design this ashing process in practice and how to apply this posttreatment to industries are discussed in this session.

#### **4.1. Plasma processing design**

Theoretical plasma processing design is often needed to construct a new device to control a plasma configuration in chamber and to understand the experimental results [29]. Some studies have just started to build up this theoretical model. Among them, for an example, the Boltzmann equation was solved to obtain electron transport coefficients and rate coefficients for fluid models [30].

In the present study, a multi-disciplinary program is used to build up a theoretical model of plasma processing and to simulate the effect of pressure on the plasma density [11]. Although the basic data for activation process is still limited, one-dimensional model is proposed to describe the argon plasma state in the function of plasma parameters such as RF-voltage, DC-bias, and pressure. **Figure 18(a)** illustrates this one-dimensional model of argon plasma. Ionization of argon is expected to concentrate at the vicinity of tool surface in **Figure 9**. As depicted in **Figure 18(b)**, a hot spot with high electron temperature or the plasma sheath is formed only at the vicinity of cathode table. Under this geometric configuration, the electron density distribution is calculated at each specified pressure. **Figure 19** depicts a variation of electron density distribution with increasing the argon pressure. This enhancement

**Figure 18.** One dimensional model of argon plasma by using the multi-disciplinary equations. (a) Theoretical model of argon plasma, and, (b) decay of calculated electron temperature.

**4.3. Plasma oxidation processing for industries**

**5. Conclusion**

the diamond-coated tools, capped by the hollow cathode tubes.

process to deal with tens of used diamond-coated tools by a single shot.

A single diamond-coated WC (Co) tool can be ashed away without residuals and tool edge damage by using the present plasma ashing system in **Figure 9**. Most of tooling companies are demanded to deal with hundreds of used diamond-coated tools in a day per a processing machine. The above plasma oxidation ashing system must be modified to make several or tens of works simultaneously even by using a single machine. The present system in **Figures 1** and **9** with use of the hollow cathode device is extended in the design concept to make simultaneous plasma oxidation for two diamond-coated tools [31, 32]. **Figure 21** depicts a developed simultaneous ashing system. The chamber is automatically opened for experimental set up of

Controlled Post-treatment of Thick CVD-Diamond Coatings by High-Density Plasma Oxidation

http://dx.doi.org/10.5772/intechopen.79143

91

An inserted photo in **Figure 21** shows that a pair of diamond-coated tool capped with the hollow cathode is subjected to the plasma oxidation. Only the inside of two hollows shines white; outside of hollows are in black. This reveals that oxygen plasmas are simultaneously confined into two hollows with high density and that ion density is much very low outside the hollows. This generation of oxygen plasmas is not limited to two hollows but extended to multi-ashing

High density plasma oxidation ashing provides an effective processing to remove the used diamond films coated on the WC (Co) tool for recoating and reuse the mother tool substrate of WC

**Figure 21.** Simultaneous ashing system of two diamond-coated cutting tools toward industrial demonstration.

**Figure 19.** Enhancement of electron density distributions with increasing the argon pressure.

of electron density just near the cathode implies that electrons are activated in the plasma sheath. This theoretical model can be extended to describe the plasma oxidation process even by two dimensional model.

#### **4.2. Down-sizing of plasma oxidation processing**

Surface and interface area ratio to volume increases by down-sizing or miniaturing the dimensional size of products and elements. Surface or interface quality and functionality must be improved by surface- and interface -treatment and modification. The present plasma oxidation system has to be also down-sized to make plasma processing onto the surfaces and interfaces. **Figure 20(a)** illustrates a typical design of miniature plasma oxidation unit. RF-plasma is confined by using the RF-coils to form a plasma sheath around a small-sized tool above the insulator. The population of activated species in this plasma can be also controlled by the DC-bias. A shown in **Figure 20(b)**, a small-sized tool is subjected to high intensity plasmas in the designated space [18, 31].

**Figure 20.** Down-sizing of an ashing unit for plasma oxidation of miniature parts and tools.

#### **4.3. Plasma oxidation processing for industries**

A single diamond-coated WC (Co) tool can be ashed away without residuals and tool edge damage by using the present plasma ashing system in **Figure 9**. Most of tooling companies are demanded to deal with hundreds of used diamond-coated tools in a day per a processing machine. The above plasma oxidation ashing system must be modified to make several or tens of works simultaneously even by using a single machine. The present system in **Figures 1** and **9** with use of the hollow cathode device is extended in the design concept to make simultaneous plasma oxidation for two diamond-coated tools [31, 32]. **Figure 21** depicts a developed simultaneous ashing system. The chamber is automatically opened for experimental set up of the diamond-coated tools, capped by the hollow cathode tubes.

An inserted photo in **Figure 21** shows that a pair of diamond-coated tool capped with the hollow cathode is subjected to the plasma oxidation. Only the inside of two hollows shines white; outside of hollows are in black. This reveals that oxygen plasmas are simultaneously confined into two hollows with high density and that ion density is much very low outside the hollows. This generation of oxygen plasmas is not limited to two hollows but extended to multi-ashing process to deal with tens of used diamond-coated tools by a single shot.

**Figure 21.** Simultaneous ashing system of two diamond-coated cutting tools toward industrial demonstration.

## **5. Conclusion**

**Figure 19.** Enhancement of electron density distributions with increasing the argon pressure.

by two dimensional model.

90 Chemical Vapor Deposition for Nanotechnology

**4.2. Down-sizing of plasma oxidation processing**

of electron density just near the cathode implies that electrons are activated in the plasma sheath. This theoretical model can be extended to describe the plasma oxidation process even

Surface and interface area ratio to volume increases by down-sizing or miniaturing the dimensional size of products and elements. Surface or interface quality and functionality must be improved by surface- and interface -treatment and modification. The present plasma oxidation system has to be also down-sized to make plasma processing onto the surfaces and interfaces. **Figure 20(a)** illustrates a typical design of miniature plasma oxidation unit. RF-plasma is confined by using the RF-coils to form a plasma sheath around a small-sized tool above the insulator. The population of activated species in this plasma can be also controlled by the DC-bias. A shown in **Figure 20(b)**, a

small-sized tool is subjected to high intensity plasmas in the designated space [18, 31].

**Figure 20.** Down-sizing of an ashing unit for plasma oxidation of miniature parts and tools.

High density plasma oxidation ashing provides an effective processing to remove the used diamond films coated on the WC (Co) tool for recoating and reuse the mother tool substrate of WC (Co). Different from the conventional plasma ashing processes in commercial, the diamond films are removed away without residuals even on the backsides of tool teeth and with less loss of teeth edges. In particular, the shrinkage in the tooth edge is more reduced down to 1.1 μm under the optimum ashing conditions than 5 μm in the commercial processes. In addition, the original microstructure with decobalted WC grains is reproduced by the present method to improve the engineering compatibility for nucleation and growth of diamond layers during the recoating process.

**References**

Precision Engineering. 2009;**75**(8):953-957

Manufacturing Technology. 2009;**58**:647-662

materials. Research Reports SIT. 2011;**55**(2):13-22

thesis]. Shibaura Institute of Technology; 2016

etching. Research Reports SIT. 2012;**56**(29):66-73

sion spectroscopy. Vacuum. 2008;**82**:224-227

(Canada, Victoria); 2013. pp. 459-466

Toyama); pp. 233-236

Related Materials. 1999;**8**:934-940

[1] Hasegawa R. Cutting tools for aircraft and application. Journal of the Japan Society for

Controlled Post-treatment of Thick CVD-Diamond Coatings by High-Density Plasma Oxidation

http://dx.doi.org/10.5772/intechopen.79143

93

[2] Bhushan B. Nano-tribology and nano-mechanics of MEMS/NEMS and bioMEMS/ bioNEMS material and device. Microelectronic Engineering. 2007;**84**:387-412

[3] Kohn E, Gluche P, et al. Diamond MEMS, a new emerging technology. Diamond and

[4] Kurita T et al. Advanced material processing with nano- and femto- second pulsed laser.

[5] Allen DM et al. Ion beam, focused ion beam, and plasma discharge machining.

[6] Syrkin A et al. Reactive ion etching of 6H-SiC in an ECR plasma of CF4 -O2 mixture using both Ni and Al mask. Materials Science and Engineering. 1997;**B46**:374-378

[7] Aizawa T, Sugita Y. High-density plasma technology for etching and ashing of carbon

[8] Aizawa T, Mizushima K, Redationo TN, Yang M. Micro-imprinting onto DLC and CNT coatings via high density oxygen plasma etching. In: Proc. 8th ICOMM Conference

[9] Yamauchi K, Yunata EE, Aizawa T. High density oxygen plasma ashing of used CVD diamond coating for recycling of WC (Co) tools. In: Proc. 4th IFMM & IFBF (2015,

[10] Yunata EE, Aizawa T. Micro-texturing into DLC/diamond coated molds and dies via

[11] Yunata EE. Characterization and application of hollow cathode oxygen plasma [PhD.

[12] Aizawa T. Micro-patterning onto diamond like carbon coating via RF-DC oxygen plasma etching. In: Proc. 8th SEATUC Conference (Vietnam, Hanoi); 2011. pp. 425-428

[13] Aizawa T, Fukuda T. Micro-texturing onto carbon-based coatings via oxygen plasma

[14] Suenaga R, Yunata EE, Aizawa T. Quantitative plasma diagnosis on high density RF-DC plasmas for surface processing. In: Proc. 7th SEATUC Conference OS6 19; 2013. pp. 1-6

[15] Cvelbar U et al. Inductively coupled RF oxygen plasma characterization by optical emis-

[16] Aizawa T, Fukuda T. Oxygen plasma etching of diamond-like carbon coated mold-die

for micro-texturing. Surface and Coating Technology. 2013;**215**:364-368

high density oxygen plasma etching. Manufacturing Review. 2015;**2**:1-8

International Journal of Machine Tools and Manufacture. 2008;**48**:220-227

The emissive light optical spectroscopy as well as the Langmuir probe, equipped with the present system, work as a sensing medium to make quantitative diagnosis on the oxygen plasma state for optimization and to make in situ monitoring on the plasma oxidation reaction during the ashing process.

There might be several steps to extend this technology toward industrial applications. The carbonbase films including the diamond films are often utilized as a substrate of MEMS and miniature media; the small-scaled plasma ashing device is expected to work as a cutting and drilling tool to reshape that substrate and to modify its surface and interface. A miniatured oxygen plasma generator is just on the research to make demonstration tests. In practical operations of ashing processes in tooling industries, a stack of used diamond coated WC (Co) tools is processed by 100–1000 per a day. Considering that the tact time for present ashing is only 3.6 ks or 1 hour, tens of tools must be posttreated. The simultaneous plasma oxidation treatment is expected to be a candidate processing to make ashing of used diamond-coated tools in the industrial scale.

With aid of the theoretical model and simulation, the present plasma oxidation process can be extended to etching, polishing and finishing of various carbon base films, substrates and solid products by using the tailored devices to each application.

## **Acknowledgements**

The author would like to express his gratitude to Dr. E.E. Yunata (SIT), Mr. K. Yamauchi (SIT), Ms. K. Kaminaga (SIT), and the late Mr. Y. Sugita (YS-Electric Industry, Co. Ltd.) for their help in experiments. This study was financially supported in part by the Grand-in-Aid from MEXT and by the supporting industry project from METI, Japan, respectively.

## **Conflict of interest**

No conflict of interest was declared.

## **Author details**

Tatsuhiko Aizawa

Address all correspondence to: taizawa@sic.shibaura-it.ac.jp

Surface Engineering and Design Laboratory, Shibaura Institute of Technology, Tokyo, Japan

## **References**

(Co). Different from the conventional plasma ashing processes in commercial, the diamond films are removed away without residuals even on the backsides of tool teeth and with less loss of teeth edges. In particular, the shrinkage in the tooth edge is more reduced down to 1.1 μm under the optimum ashing conditions than 5 μm in the commercial processes. In addition, the original microstructure with decobalted WC grains is reproduced by the present method to improve the engineering compatibility for nucleation and growth of diamond layers during the recoating process. The emissive light optical spectroscopy as well as the Langmuir probe, equipped with the present system, work as a sensing medium to make quantitative diagnosis on the oxygen plasma state for optimization and to make in situ monitoring on the plasma oxidation reac-

There might be several steps to extend this technology toward industrial applications. The carbonbase films including the diamond films are often utilized as a substrate of MEMS and miniature media; the small-scaled plasma ashing device is expected to work as a cutting and drilling tool to reshape that substrate and to modify its surface and interface. A miniatured oxygen plasma generator is just on the research to make demonstration tests. In practical operations of ashing processes in tooling industries, a stack of used diamond coated WC (Co) tools is processed by 100–1000 per a day. Considering that the tact time for present ashing is only 3.6 ks or 1 hour, tens of tools must be posttreated. The simultaneous plasma oxidation treatment is expected to be a candidate processing to make ashing of used diamond-coated tools in the industrial scale.

With aid of the theoretical model and simulation, the present plasma oxidation process can be extended to etching, polishing and finishing of various carbon base films, substrates and solid

The author would like to express his gratitude to Dr. E.E. Yunata (SIT), Mr. K. Yamauchi (SIT), Ms. K. Kaminaga (SIT), and the late Mr. Y. Sugita (YS-Electric Industry, Co. Ltd.) for their help in experiments. This study was financially supported in part by the Grand-in-Aid from MEXT

Surface Engineering and Design Laboratory, Shibaura Institute of Technology, Tokyo, Japan

products by using the tailored devices to each application.

and by the supporting industry project from METI, Japan, respectively.

Address all correspondence to: taizawa@sic.shibaura-it.ac.jp

tion during the ashing process.

92 Chemical Vapor Deposition for Nanotechnology

**Acknowledgements**

**Conflict of interest**

**Author details**

Tatsuhiko Aizawa

No conflict of interest was declared.


[17] Aizawa T, Morita H, Kurozumi S. Synthesis of films, coating system, coated products, dies and tools. In: Japanese Patent 2011-208140. 2011

**Section 3**

**CVD of 2D Materials**


**Section 3**

**CVD of 2D Materials**

[17] Aizawa T, Morita H, Kurozumi S. Synthesis of films, coating system, coated products,

[18] Aizawa T, Sugita Y. Distributed nitriding systems for surface treatment of miniature

[19] Bardos L. Radio frequency hollow cathodes for the plasma processing technology.

[20] Kitano A. The CFRP which supports the light-weighting of the plane. Journal of Chemical

[21] Aizawa T, Sugita Y. High density oxygen plasma ashing for recycling and reuse of DLC-

[22] A. Gilpin, Composite Drill; Tool Process Change Nets 450 Percent More Holes per Bit Life. http://www.diamondtc.com/media/tooling\_production/index.html [21/4/2018] [23] Aizawa T, Masaki E, Sugita Y. Oxygen plasma ashing of used DLC coating for reuse of milling and cutting tools. In: Proc. Int. Conf. Mater; Advanced Process Technology; 2011.

[24] Aizawa T, Masaki E, Sugita Y. Complete ashing of used DLC coating for reuse of the

[25] Aizawa T, Masaki E, Moromoto E, Sugita Y. Recycling of DLC-coated tools for dry machining of aluminium alloys via oxygen plasma ashing. Mechanical Engineering

[26] Yamauchi K, Aizawa T. Optimization of diamond ashing process for recoating of CVD

[27] Yunata EE, Yamauchi K, Aizawa T. High density plasma ashing of CVD diamond coated end-milling tools. In: Pro. 4th ASMP2015 (Lombok, Indonesia) 2015: CD-ROM

[28] Yamauchi K, AizawaT. High density plasma ashing of used diamond coated short-shank tools without damage to WC (Co) teeth. In: Proc. 11th ICOMM 6; 2015. pp. 1-6

[29] Mihailova D, Grozeva M, et al. Theoretical and experimental studies of the plasma processes in hollow cathode discharge lasers. In: Proc. 28th ICPIG, 98; 2007. pp. 533-536 [30] Hagelaar GJM, Pitchford LC. Solving the Boltzmann equation to obtain electron transport coefficients and rate coefficients for fluid models. Plasma Sources Science and

[31] Aizawa T. Development of micro-manufacturing by controlled plasma technologies.

[32] Aizawa T. Future prospects and advanced steps of die and mold technologies. Journal of

Journal of Japan Society for Technology of Plasticity. 2017;**58**(12):1064-1068

Japan Society for Technology of Plasticity. 2018;**59**(1):19-23

diamond coated tools. In: Proc. 8th AWMFT; 2015. pp. J16/1-J16/6

dies and tools. In: Japanese Patent 2011-208140. 2011

Surface and Coating Technology. 1996;**46**:648-656

Education. 2011;**59**(4):226-229

94 Chemical Vapor Deposition for Nanotechnology

Research. 2014;**4**(1):52-62

Technology. 2005;**14**:722-733

pp. 15-20

functional products. In: Proc. 10th 4M/ICOMM; 2015. pp. 449-453

coated tools and dies. Research Reports SIT. 2014;**58**(2):1-10

end-milling tools. Manufacturing Letters. 2014;**2**:1-3

**Chapter 5**

**Provisional chapter**

**Large-Area Synthesis and Growth Mechanism of**

**Large-Area Synthesis and Growth Mechanism of** 

DOI: 10.5772/intechopen.79959

There has been continuous progress in the development of different synthesis methods to readily produce graphene at a lower cost. Compared with the other methods, chemical vapor deposition (CVD) is an effective and powerful method of producing graphene and has attracted increased attention during the last decade. In this way, we can obtain good uniformity with a multitude of domains, excellent quality, and large scale of the produced graphene. Meanwhile, it is also helping for large-area synthesis of single-crystal graphene. In the CVD method, precursors are typically absorbed on the surface followed by pyrolytic decomposition, which leads to the generation of absorption sites on the sur-

**Keywords:** chemical vapor deposition, graphene, large-scale synthesis, growth

Graphene (shown in **Figure 1**), as a versatile two-dimensional material [1], has attracted great interest in the research community owing to its high mechanical stiffness [2], great flexibility [3], and stable chemical properties [4–6]. Based on the properties of graphene and its nanoscale thickness, it has been selected as a material for various applications in electrical, chemical, mechanical, optical, and catalysis industries [7–11]. Hence, it is very critical to propose

Compared with other methods, such as mechanical exfoliation of graphite [12–14], liquid-phase exfoliation [15, 16], and reduction of graphene oxide (GO) [17, 18], chemical vapor deposition

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

**Graphene by Chemical Vapor Deposition**

Chen Wang, Kizhanipuram Vinodgopal and Gui-Ping Dai

**Graphene by Chemical Vapor Deposition**

Chen Wang, Kizhanipuram Vinodgopal and

Additional information is available at the end of the chapter

face and promotes the growth of continuous thin films.

methods for preparing high-quality graphene film.

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.79959

Gui-Ping Dai

**Abstract**

mechanism

**1. Introduction**

#### **Large-Area Synthesis and Growth Mechanism of Graphene by Chemical Vapor Deposition Large-Area Synthesis and Growth Mechanism of Graphene by Chemical Vapor Deposition**

DOI: 10.5772/intechopen.79959

Chen Wang, Kizhanipuram Vinodgopal and Gui-Ping Dai Chen Wang, Kizhanipuram Vinodgopal and Gui-Ping Dai Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.79959

#### **Abstract**

There has been continuous progress in the development of different synthesis methods to readily produce graphene at a lower cost. Compared with the other methods, chemical vapor deposition (CVD) is an effective and powerful method of producing graphene and has attracted increased attention during the last decade. In this way, we can obtain good uniformity with a multitude of domains, excellent quality, and large scale of the produced graphene. Meanwhile, it is also helping for large-area synthesis of single-crystal graphene. In the CVD method, precursors are typically absorbed on the surface followed by pyrolytic decomposition, which leads to the generation of absorption sites on the surface and promotes the growth of continuous thin films.

**Keywords:** chemical vapor deposition, graphene, large-scale synthesis, growth mechanism

### **1. Introduction**

Graphene (shown in **Figure 1**), as a versatile two-dimensional material [1], has attracted great interest in the research community owing to its high mechanical stiffness [2], great flexibility [3], and stable chemical properties [4–6]. Based on the properties of graphene and its nanoscale thickness, it has been selected as a material for various applications in electrical, chemical, mechanical, optical, and catalysis industries [7–11]. Hence, it is very critical to propose methods for preparing high-quality graphene film.

Compared with other methods, such as mechanical exfoliation of graphite [12–14], liquid-phase exfoliation [15, 16], and reduction of graphene oxide (GO) [17, 18], chemical vapor deposition

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

In addition, great success has been achieved in the fabrication of large-area, single-crystalline graphene. Different teams have published reports on the single crystals. We also summarize

Large-Area Synthesis and Growth Mechanism of Graphene by Chemical Vapor Deposition

http://dx.doi.org/10.5772/intechopen.79959

99

There are already many comprehensive articles published about increasing the size of graphene. Among these articles, a publication by Xu et al. [34] reports the fabrication of metersize single-crystal graphene. The authors developed an innovative method to achieve the ultrafast epitaxial growth of meter-sized single-crystal graphene on a commercially available

In this chapter, we talk about the preparation of high-quality graphene in recent years, effect of experimental parameters, challenges of preparation of high-quality graphene, and timeline of graphene production. Finally, development trend and application prospect of CVD gra-

Great effort has been given to the synthesis of graphene with controlled size, morphology, edge structures, and layer numbers [22, 35–40]. Studies into the dynamics of CVD graphene growth on copper foils have chiefly focused on monolayer films that develop under vacuum conditions. Up to now, the preparation methods of graphene are usually divided into two ways: physical and chemical (shown in **Figure 3**). With respect to physical method, mechanical exfoliation [12–14] and liquid-phase exfoliation [15, 16] are always used. With respect to chemical methods, they include CVD, reduction of graphene oxide (GO) [17, 18], epitaxial growth on various substrates.

The mechanical exfoliation method can provide the highest quality graphene sheets, which strips the graphene layers from the surface of graphite crystals by mechanical force. It is not suitable for mass production, however, because of the difficulty in controlling the number of

these experimental results.

Cu foil (shown in **Figure 2**).

**2. Preparation of graphene**

**2.1. Mechanical exfoliation**

layers and the size of sheets.

**Figure 3.** Classification of graphene synthesis techniques.

phene are described.

**Figure 1.** The schematic diagram of graphene.

(CVD) is regarded as the most promising way for large-scale graphene production at a large scale with low defects, good uniformity, and controlled number of graphene layers, which has attracted intense research attention during the last decades [19–22]. CVD involves the activation of gaseous reactants and the subsequent chemical reaction, followed by the formation of a stable solid deposit over a suitable substrate such as Ni [6, 23], Cu [5, 24, 25], Fe [26], Pt [27, 28], or their alloys [29, 30]. As early as 1969, Robertson et al. [31] discovered that chemical vapor deposition in the presence of methane produces a layer of graphite on some transition metal surfaces. However, even if the prepared graphene displays excellent performance, it cannot be applied to the related devices due to the limitations of deposition on the metal surface. The advent of poly(methyl methacrylate)-mediated nanotransfer printing technology [32] enabled the successful transfer of graphene from the growth substrate to any other substrate, and it also completely changed the status of chemical vapor deposition methods in the preparation method of graphene.

It is worth mentioning that, compared to exfoliated graphene obtained from natural graphite, the CVD graphene still shows a lower carrier mobility ranging from 100 to 1000 cm<sup>2</sup> /(V⋅s), due to the presence of growth defects and boundaries [33]. Therefore, a feasible route to improve graphene film quality lies in increasing the domain size that enables to avoid the side effect of graphene domain boundaries.

Now, large-area graphene synthesized have generally been single crystalline, and related research is aimed to control the domain size, defects, number of graphene layers, and so on. With regard to desired graphene, there are many effect parameters including hydrogen, oxygen, gas flow rate, and residence time.

**Figure 2.** Image of meter-size single-crystal graphene. Reproduced with permission of ref. [34].

In addition, great success has been achieved in the fabrication of large-area, single-crystalline graphene. Different teams have published reports on the single crystals. We also summarize these experimental results.

There are already many comprehensive articles published about increasing the size of graphene. Among these articles, a publication by Xu et al. [34] reports the fabrication of metersize single-crystal graphene. The authors developed an innovative method to achieve the ultrafast epitaxial growth of meter-sized single-crystal graphene on a commercially available Cu foil (shown in **Figure 2**).

In this chapter, we talk about the preparation of high-quality graphene in recent years, effect of experimental parameters, challenges of preparation of high-quality graphene, and timeline of graphene production. Finally, development trend and application prospect of CVD graphene are described.

## **2. Preparation of graphene**

(CVD) is regarded as the most promising way for large-scale graphene production at a large scale with low defects, good uniformity, and controlled number of graphene layers, which has attracted intense research attention during the last decades [19–22]. CVD involves the activation of gaseous reactants and the subsequent chemical reaction, followed by the formation of a stable solid deposit over a suitable substrate such as Ni [6, 23], Cu [5, 24, 25], Fe [26], Pt [27, 28], or their alloys [29, 30]. As early as 1969, Robertson et al. [31] discovered that chemical vapor deposition in the presence of methane produces a layer of graphite on some transition metal surfaces. However, even if the prepared graphene displays excellent performance, it cannot be applied to the related devices due to the limitations of deposition on the metal surface. The advent of poly(methyl methacrylate)-mediated nanotransfer printing technology [32] enabled the successful transfer of graphene from the growth substrate to any other substrate, and it also completely changed the

status of chemical vapor deposition methods in the preparation method of graphene.

the CVD graphene still shows a lower carrier mobility ranging from 100 to 1000 cm<sup>2</sup>

**Figure 2.** Image of meter-size single-crystal graphene. Reproduced with permission of ref. [34].

graphene domain boundaries.

**Figure 1.** The schematic diagram of graphene.

98 Chemical Vapor Deposition for Nanotechnology

oxygen, gas flow rate, and residence time.

It is worth mentioning that, compared to exfoliated graphene obtained from natural graphite,

to the presence of growth defects and boundaries [33]. Therefore, a feasible route to improve graphene film quality lies in increasing the domain size that enables to avoid the side effect of

Now, large-area graphene synthesized have generally been single crystalline, and related research is aimed to control the domain size, defects, number of graphene layers, and so on. With regard to desired graphene, there are many effect parameters including hydrogen,

/(V⋅s), due

Great effort has been given to the synthesis of graphene with controlled size, morphology, edge structures, and layer numbers [22, 35–40]. Studies into the dynamics of CVD graphene growth on copper foils have chiefly focused on monolayer films that develop under vacuum conditions. Up to now, the preparation methods of graphene are usually divided into two ways: physical and chemical (shown in **Figure 3**). With respect to physical method, mechanical exfoliation [12–14] and liquid-phase exfoliation [15, 16] are always used. With respect to chemical methods, they include CVD, reduction of graphene oxide (GO) [17, 18], epitaxial growth on various substrates.

## **2.1. Mechanical exfoliation**

The mechanical exfoliation method can provide the highest quality graphene sheets, which strips the graphene layers from the surface of graphite crystals by mechanical force. It is not suitable for mass production, however, because of the difficulty in controlling the number of layers and the size of sheets.

**Figure 3.** Classification of graphene synthesis techniques.

## **2.2. Liquid-phase exfoliation**

Liquid-phase exfoliation cannot produce graphene films with high electronic quality, where scatter graphite to specific solvents or surfactants, and strip the single layer or multilayer graphene through ultrasonic energy to get graphene dispersions [15, 41].

**2.5. Epitaxial growth on substrates**

Obviously, the term "epitaxial growth" means ordered or arranged atomic growth in single crystalline manner over a single-crystalline substrate. When the substrate deposited materials are the same as the substrate, it is called homoepitaxial growth. Oppositely, it is defined as hetero epitaxial growth if substrate differs from the deposited materials. And epitaxial growth of graphite on semiconducting substrates (SiC) was known to us since 1975 [46]. Berger's group successfully synthesized epitaxial graphene sheet [47] by using Si terminated face of single-crystal 6H-SiC by thermal desorption of Si. To summarize, although it is still an expensive approach, this is a promising method and possibly be scalable commercially.

Large-Area Synthesis and Growth Mechanism of Graphene by Chemical Vapor Deposition

http://dx.doi.org/10.5772/intechopen.79959

101

Chemical vapor deposition has been used to prepare inorganic materials in recent years. Besides, it also can be applied to produce graphene. CVD is the process of using gaseous matter to react on a solid surface and form solid deposits. The thermal decomposition and thermal synthesis of the deposition techniques are carried out at high temperature. As for the

Two phenomenological growth mechanisms have been proposed based on an isotope labeling experiments [12] to understand the graphene growth. It is very easy to form a metalcarbide phase due to the strong interaction between carbon and metal [45, 48–50], following the precipitated growth mechanism. During the subsequent cooling process, the pyrolytic carbon atoms from the carbon sources firstly dissolve into the catalyst and then deposit at the metal surface to form graphene layers. On the other hand, the pyrolytic carbon atoms could only diffuse on the catalyst surface as the cases are shown with Cu and Au when the interaction between metal and carbon is fairly weak [22, 51], where both the nucleation and growth of graphene are dominated by the surface diffusion of the decomposed carbon atoms. Multi-layered graphene film formation normally follows the sedimentary growth mechanism and the number of graphene layers can be controlled by adjusting the dissolved C atoms in the metals or the thickness and composition of the substrate or by slowing down the cooling rate. Obviously, only single-layer graphene can be formed in the diffusive growth because the

Meantime, SLG (shown in **Figure 5**) has been synthesized from solid and liquid hydrocarbon sources at different temperature (400–1000°C). Li et al. [52] adopted polymethyl-methacrylate (PMMA) and polystyrene as solid hydrocarbon and benzene as a liquid hydrocarbon source to grow graphene at temperatures lower than 1000°C on copper foil. Jang et al. [53] tried to remove remaining oxidizing impurities (oxygen-free APCVD, which is shown in **Figure 6**) before graphene deposition by using pumping and purging. This method leads to continuous

**3. CVD synthesis mechanism and the choice of carbon source**

synthesis of graphene, three steps are generally required:

**2.** transport of volatile matter to the precipitation area, and

**3.** chemical reaction on the substrate to produce a solid substance.

feedstock cannot get access to the graphene covered area of catalyst surface.

SLG with full coverage of the substrate.

**1.** a volatile precursor,

### **2.3. Reduced graphene oxide (RGO)**

With regard to RGO methods, first, the graphite is oxidized by treatment with KMnO<sup>4</sup> , NaNO<sup>3</sup> , and so on. Second, GO prepared from flake graphite can be readily dispersed in water and has been used on a large scale for preparing large graphitic films, but it would result defects in graphene [42].

#### **2.4. Chemical vapor deposition**

In comparison, CVD is an effective and powerful method of producing graphene. CVD processes typically involve two steps: the activation of gaseous reactants and the chemical reaction of forming a stable solid deposit over a suitable substrate [43, 44]. The deposition process consists of two types of reactions: homogeneous gas-phase reactions, occurring in the gas phase, and heterogeneous chemical reactions that occur on a heated surface. Therefore, CVD method is the use of carbon source (gas, liquid, and solid) decomposing in the high-temperature reaction zone, the release of carbon atoms in the metal substrate, eventually it forms a continuous graphene membrane. And many researchers use CVD method to prepare the high-quality and large-area graphene, as Li et al. [45] pointed (shown in **Figure 4**). Subsequently, CVD can be divided into two types: (I) thermal CVD and (II) plasma-enhanced CVD.

**Figure 4.** Schematic diagram of growth mechanism of graphene on the metal substrate. Reproduced with permission of ref. [45].

#### **2.5. Epitaxial growth on substrates**

**2.2. Liquid-phase exfoliation**

100 Chemical Vapor Deposition for Nanotechnology

NaNO<sup>3</sup>

of ref. [45].

**2.3. Reduced graphene oxide (RGO)**

result defects in graphene [42].

**2.4. Chemical vapor deposition**

Liquid-phase exfoliation cannot produce graphene films with high electronic quality, where scatter graphite to specific solvents or surfactants, and strip the single layer or multilayer

With regard to RGO methods, first, the graphite is oxidized by treatment with KMnO<sup>4</sup>

water and has been used on a large scale for preparing large graphitic films, but it would

In comparison, CVD is an effective and powerful method of producing graphene. CVD processes typically involve two steps: the activation of gaseous reactants and the chemical reaction of forming a stable solid deposit over a suitable substrate [43, 44]. The deposition process consists of two types of reactions: homogeneous gas-phase reactions, occurring in the gas phase, and heterogeneous chemical reactions that occur on a heated surface. Therefore, CVD method is the use of carbon source (gas, liquid, and solid) decomposing in the high-temperature reaction zone, the release of carbon atoms in the metal substrate, eventually it forms a continuous graphene membrane. And many researchers use CVD method to prepare the high-quality and large-area graphene, as Li et al. [45] pointed (shown in **Figure 4**). Subsequently, CVD can be

**Figure 4.** Schematic diagram of growth mechanism of graphene on the metal substrate. Reproduced with permission

, and so on. Second, GO prepared from flake graphite can be readily dispersed in

,

graphene through ultrasonic energy to get graphene dispersions [15, 41].

divided into two types: (I) thermal CVD and (II) plasma-enhanced CVD.

Obviously, the term "epitaxial growth" means ordered or arranged atomic growth in single crystalline manner over a single-crystalline substrate. When the substrate deposited materials are the same as the substrate, it is called homoepitaxial growth. Oppositely, it is defined as hetero epitaxial growth if substrate differs from the deposited materials. And epitaxial growth of graphite on semiconducting substrates (SiC) was known to us since 1975 [46]. Berger's group successfully synthesized epitaxial graphene sheet [47] by using Si terminated face of single-crystal 6H-SiC by thermal desorption of Si. To summarize, although it is still an expensive approach, this is a promising method and possibly be scalable commercially.

## **3. CVD synthesis mechanism and the choice of carbon source**

Chemical vapor deposition has been used to prepare inorganic materials in recent years. Besides, it also can be applied to produce graphene. CVD is the process of using gaseous matter to react on a solid surface and form solid deposits. The thermal decomposition and thermal synthesis of the deposition techniques are carried out at high temperature. As for the synthesis of graphene, three steps are generally required:


Two phenomenological growth mechanisms have been proposed based on an isotope labeling experiments [12] to understand the graphene growth. It is very easy to form a metalcarbide phase due to the strong interaction between carbon and metal [45, 48–50], following the precipitated growth mechanism. During the subsequent cooling process, the pyrolytic carbon atoms from the carbon sources firstly dissolve into the catalyst and then deposit at the metal surface to form graphene layers. On the other hand, the pyrolytic carbon atoms could only diffuse on the catalyst surface as the cases are shown with Cu and Au when the interaction between metal and carbon is fairly weak [22, 51], where both the nucleation and growth of graphene are dominated by the surface diffusion of the decomposed carbon atoms. Multi-layered graphene film formation normally follows the sedimentary growth mechanism and the number of graphene layers can be controlled by adjusting the dissolved C atoms in the metals or the thickness and composition of the substrate or by slowing down the cooling rate. Obviously, only single-layer graphene can be formed in the diffusive growth because the feedstock cannot get access to the graphene covered area of catalyst surface.

Meantime, SLG (shown in **Figure 5**) has been synthesized from solid and liquid hydrocarbon sources at different temperature (400–1000°C). Li et al. [52] adopted polymethyl-methacrylate (PMMA) and polystyrene as solid hydrocarbon and benzene as a liquid hydrocarbon source to grow graphene at temperatures lower than 1000°C on copper foil. Jang et al. [53] tried to remove remaining oxidizing impurities (oxygen-free APCVD, which is shown in **Figure 6**) before graphene deposition by using pumping and purging. This method leads to continuous SLG with full coverage of the substrate.

surfaces, such as quality, continuity, and layer number, is different in each case. The catalyst plays an important role in the CVD process. We generally use Cu [5, 24, 25], Ni [6, 23], Pd [49], Ru [57], and Ir [58], as the metal base material for the preparation of graphene. As we know, different metal base materials have a different effect on the preparation of graphene, so the

Large-Area Synthesis and Growth Mechanism of Graphene by Chemical Vapor Deposition

http://dx.doi.org/10.5772/intechopen.79959

103

The mechanism of growth of graphene by chemical vapor deposition in copper and nickel substrate is different. Studies have shown that the growth of graphene on other metal substrates can also be basically classified into the growth mechanisms of copper and nickel substrates. On a metal substrate with high soluble carbon content, such as nickel, the carbon atoms produced by the carbon source cracking are infiltrated into the metal matrix at high temperature. When the temperature drops, the core is precipitated from the inside, then the graphene is formed. It is difficult to control the precipitation of carbon atoms during the cooling process because the solubility of carbon atoms in nickel increases with temperature, so it has a negative effect on the preparation of graphene. In the case of copper, the solubility of carbon atoms in copper is relatively low at high temperature; therefore, compared with other catalysts, Cu is currently the most widely used catalyst

Hydrogen is often involved in CVD graphene synthesis. Thus, hydrogen has a great impact on graphene reducing substrate surface contamination and defects in the annealing process [59], controlling the graphene domain shape [40, 60, 61], nucleation [62], and layer number [63]. And excess hydrogen could etch graphene to destroy the integrity of the lattice and make the quality of graphene worse [64]. It also affects the adsorption, stability, thickness [65–67], population of active species on the catalyst surface [68, 69] and the morphology [70–79] of the grown graphene. Luo et al. [72] determined the effect of hydrogen on the shape and orientation of graphene using DFT calculation (shown in **Figure 7**). High hydrogen concentration is required to synthesize large single-crystal monolayer graphene domains [40, 80]. Besides the low carbon concentration, etching by hydrogen is also an obstacle to short-time graphene

It is known that oxygen plays an important role in each step of graphene synthesis, especially on copper. The effect of surface oxygen or pre-adsorbed oxygen on methane decomposition

The strong binding of active carbon species with catalyst surface is crucial for nucleation of graphene. The adsorption of active species on the catalyst surface becomes higher in the presence of oxygen. Hao et al. [82] found that the graphene edge with H-termination is better than a bare edge on Cu. In addition, oxygen plays a vital role in reducing the nucleation density not only in the bulk but also on the surface [83].For example, oxygen could remove

on Cu and that on the transition metal surface such as Ni and Pd is different [33].

the unwanted carbon from Cu bulk, [84] which results in unintentional nucleation.

metal substrate is the key factor to determine the growth of graphene.

material [22, 45].

growth [81].

**4.3. Effect of oxygen**

**4.2. Effect of hydrogen**

**Figure 5.** HRTEM image of single-layer graphene.

**Figure 6.** The schematic diagram of APCVD. Reproduced with permission of ref. [54].

As for the multilayer graphene (MLG), the high solubility of carbon in catalysts results in the dissolution of carbon into the bulk at high temperatures, leading to multilayer graphene (MLG) during the cooling process [33, 54–56].

## **4. Toward high-quality graphene: effect of experimental parameters**

#### **4.1. Choice of metal base material**

Up to now, a large number of research have been done about the growth of graphene on the metal surfaces. CVD graphene growth is strongly dependent on the catalyst. Due to the different catalytic activity and atomic packing, the growth behavior of graphene on catalyst surfaces, such as quality, continuity, and layer number, is different in each case. The catalyst plays an important role in the CVD process. We generally use Cu [5, 24, 25], Ni [6, 23], Pd [49], Ru [57], and Ir [58], as the metal base material for the preparation of graphene. As we know, different metal base materials have a different effect on the preparation of graphene, so the metal substrate is the key factor to determine the growth of graphene.

The mechanism of growth of graphene by chemical vapor deposition in copper and nickel substrate is different. Studies have shown that the growth of graphene on other metal substrates can also be basically classified into the growth mechanisms of copper and nickel substrates. On a metal substrate with high soluble carbon content, such as nickel, the carbon atoms produced by the carbon source cracking are infiltrated into the metal matrix at high temperature. When the temperature drops, the core is precipitated from the inside, then the graphene is formed. It is difficult to control the precipitation of carbon atoms during the cooling process because the solubility of carbon atoms in nickel increases with temperature, so it has a negative effect on the preparation of graphene. In the case of copper, the solubility of carbon atoms in copper is relatively low at high temperature; therefore, compared with other catalysts, Cu is currently the most widely used catalyst material [22, 45].

#### **4.2. Effect of hydrogen**

**Figure 5.** HRTEM image of single-layer graphene.

102 Chemical Vapor Deposition for Nanotechnology

(MLG) during the cooling process [33, 54–56].

**Figure 6.** The schematic diagram of APCVD. Reproduced with permission of ref. [54].

**4.1. Choice of metal base material**

As for the multilayer graphene (MLG), the high solubility of carbon in catalysts results in the dissolution of carbon into the bulk at high temperatures, leading to multilayer graphene

Up to now, a large number of research have been done about the growth of graphene on the metal surfaces. CVD graphene growth is strongly dependent on the catalyst. Due to the different catalytic activity and atomic packing, the growth behavior of graphene on catalyst

**4. Toward high-quality graphene: effect of experimental parameters**

Hydrogen is often involved in CVD graphene synthesis. Thus, hydrogen has a great impact on graphene reducing substrate surface contamination and defects in the annealing process [59], controlling the graphene domain shape [40, 60, 61], nucleation [62], and layer number [63]. And excess hydrogen could etch graphene to destroy the integrity of the lattice and make the quality of graphene worse [64]. It also affects the adsorption, stability, thickness [65–67], population of active species on the catalyst surface [68, 69] and the morphology [70–79] of the grown graphene. Luo et al. [72] determined the effect of hydrogen on the shape and orientation of graphene using DFT calculation (shown in **Figure 7**). High hydrogen concentration is required to synthesize large single-crystal monolayer graphene domains [40, 80]. Besides the low carbon concentration, etching by hydrogen is also an obstacle to short-time graphene growth [81].

#### **4.3. Effect of oxygen**

It is known that oxygen plays an important role in each step of graphene synthesis, especially on copper. The effect of surface oxygen or pre-adsorbed oxygen on methane decomposition on Cu and that on the transition metal surface such as Ni and Pd is different [33].

The strong binding of active carbon species with catalyst surface is crucial for nucleation of graphene. The adsorption of active species on the catalyst surface becomes higher in the presence of oxygen. Hao et al. [82] found that the graphene edge with H-termination is better than a bare edge on Cu. In addition, oxygen plays a vital role in reducing the nucleation density not only in the bulk but also on the surface [83].For example, oxygen could remove the unwanted carbon from Cu bulk, [84] which results in unintentional nucleation.

flow rate of 20 sccm adopt, at 80 Pa, and 1000°C, in their CVD method for graphene growth. The methane flow was varied systematically between 0.4 and 15 sccm. The graphene was characterized by Raman spectroscopy, and the results were shown in **Figure 8**. The study found that the ratio of IG/I2D peak ratio did not increase with the increase of methane flow and is less than 0.5. This indicates the formation of single layer graphene and no dependence on methane flow rate. They reported that the methane flow has a significant effect on the structure of graphene and determined the optimal flow rate of methane as 5 sccm. At 0 sccm, ID/I2D peak intensity ratio has a minimum value (0.0395) yielding graphene with the least defects.

Large-Area Synthesis and Growth Mechanism of Graphene by Chemical Vapor Deposition

http://dx.doi.org/10.5772/intechopen.79959

Li et al. [22] synthesize large-area graphene grown by low-pressure chemical vapor deposition in copper-foil enclosures using methane as a precursor single crystal with dimensions of up to 0.5 mm on a side. While Hao et al. [82] enabled repeatable growth of centimeter-scale singlecrystal graphene domains to attain 1 cm single-crystal graphene by controlling of surface oxygen. Correspondingly, Wu et al. [29] prepare a similar to 1.5-inch-large graphene monolayer in 2.5 h by locally feeding carbon precursors to a desired position of a substrate composed of

film without grain boundaries on polished copper(111) foil by Seamless stitching of graphene domains. However, a foundation of the modern technology by Xu et al. [34] that using singlecrystal silicon improve the growth of high-quality single-crystal graphene with diameters up

In 2004, micron-sized graphene was obtained by exfoliation, which was not scalable. In 2006, the scientists obtained graphene by thermal deposition of SiC, which is an expensive process. In 2009, researchers first used Cu-CVD method to get graphene, and subsequently, this CVD

The graphene can be transferred using a wet method [32]. The process involves (1) coating a protective layer (e.g., PMMA) on one side, (2) etching graphene on the other side, (3) etching away the copper and then rinse with water, (4) transferring to target substrate, and (5) removal of protective film, by typically using solvent to dissolve it. With the development of graphene production, there are many ways to transfer graphene, including R2R ([87–89],

**Figure 9.** Schematic diagram of roll to roll production of graphene. Reproduced with permission of ref. [89].

graphene

105

an optimized Cu-Ni alloy. Besides, Nguyen et al. [86] have fabricated the 6 × 3 cm2

**5. Timeline of graphene production**

method was extended to using other metals as substrates.

to 12 inches or larger.

**Figure 7.** Schematic diagram of Etching-controlled growth mechanism of CVD. Reproduced with permission of ref. [71].

**Figure 8.** Raman spectra (a) and Gand 2D peaks (b) of the graphene films grown under various methane fluxes. Reproduced with permission of ref. [85].

#### **4.4. Effect of gas flow rate**

The gas flow has been considered as a crucial factor during graphene synthesis. Optimizing the flow rate of CH<sup>4</sup> and H<sup>2</sup> could improve the quality of graphene. Therefore, Li et al. [22] described the CVD growth of graphene single crystals up to 0.5 mm in size in a quasi-static flow regime, using a copper enclosure in LPCVD. Meantime, Wang et al. [85] have used a H<sup>2</sup>

flow rate of 20 sccm adopt, at 80 Pa, and 1000°C, in their CVD method for graphene growth. The methane flow was varied systematically between 0.4 and 15 sccm. The graphene was characterized by Raman spectroscopy, and the results were shown in **Figure 8**. The study found that the ratio of IG/I2D peak ratio did not increase with the increase of methane flow and is less than 0.5. This indicates the formation of single layer graphene and no dependence on methane flow rate. They reported that the methane flow has a significant effect on the structure of graphene and determined the optimal flow rate of methane as 5 sccm. At 0 sccm, ID/I2D peak intensity ratio has a minimum value (0.0395) yielding graphene with the least defects.

## **5. Timeline of graphene production**

**4.4. Effect of gas flow rate**

Reproduced with permission of ref. [85].

104 Chemical Vapor Deposition for Nanotechnology

and H<sup>2</sup>

the flow rate of CH<sup>4</sup>

The gas flow has been considered as a crucial factor during graphene synthesis. Optimizing

**Figure 8.** Raman spectra (a) and Gand 2D peaks (b) of the graphene films grown under various methane fluxes.

**Figure 7.** Schematic diagram of Etching-controlled growth mechanism of CVD. Reproduced with permission of ref. [71].

described the CVD growth of graphene single crystals up to 0.5 mm in size in a quasi-static flow regime, using a copper enclosure in LPCVD. Meantime, Wang et al. [85] have used a H<sup>2</sup>

could improve the quality of graphene. Therefore, Li et al. [22]

Li et al. [22] synthesize large-area graphene grown by low-pressure chemical vapor deposition in copper-foil enclosures using methane as a precursor single crystal with dimensions of up to 0.5 mm on a side. While Hao et al. [82] enabled repeatable growth of centimeter-scale singlecrystal graphene domains to attain 1 cm single-crystal graphene by controlling of surface oxygen. Correspondingly, Wu et al. [29] prepare a similar to 1.5-inch-large graphene monolayer in 2.5 h by locally feeding carbon precursors to a desired position of a substrate composed of an optimized Cu-Ni alloy. Besides, Nguyen et al. [86] have fabricated the 6 × 3 cm2 graphene film without grain boundaries on polished copper(111) foil by Seamless stitching of graphene domains. However, a foundation of the modern technology by Xu et al. [34] that using singlecrystal silicon improve the growth of high-quality single-crystal graphene with diameters up to 12 inches or larger.

In 2004, micron-sized graphene was obtained by exfoliation, which was not scalable. In 2006, the scientists obtained graphene by thermal deposition of SiC, which is an expensive process. In 2009, researchers first used Cu-CVD method to get graphene, and subsequently, this CVD method was extended to using other metals as substrates.

The graphene can be transferred using a wet method [32]. The process involves (1) coating a protective layer (e.g., PMMA) on one side, (2) etching graphene on the other side, (3) etching away the copper and then rinse with water, (4) transferring to target substrate, and (5) removal of protective film, by typically using solvent to dissolve it. With the development of graphene production, there are many ways to transfer graphene, including R2R ([87–89],

**Figure 9.** Schematic diagram of roll to roll production of graphene. Reproduced with permission of ref. [89].

shown in **Figure 9**) with thermal release tape, R2R with epoxy [90], and bubbling transfer [91]. These developments have led to much more controllable graphene synthesis [19, 33, 89].

more scientific communities are making efforts to achieve the industrialization of graphene. Undoubtedly, the graphene industry is growing at a fast rate. In the next 5–10 years, the technology for graphene-based conductive additives, anticorrosive coatings, touch screen would be mature, along with the industrialization of high-end applications such as super capacitors, sensors, and electronic chips. But mono-crystalline graphene films are extremely difficult to prepare. At present, they can only be made in millimeter size, and they are still quite far away from the practical crystal size, which is difficult to break in the short term. We need to achieve a balance between the advantages and disadvantages of graphene in order to create a fast and healthy way for the sustainable development of graphene. Chemical vapor deposition (CVD) is the most promising preparation technique. The preparation by chemical vapor deposition has become a major method for preparing semiconductor, thin film materials, and industrial production of high-quality graphene materials in large quantity. Meantime, for the CVD method, considerable efforts have been exerted to get low cost on any arbitrary substrate, but

Large-Area Synthesis and Growth Mechanism of Graphene by Chemical Vapor Deposition

http://dx.doi.org/10.5772/intechopen.79959

107

\* and Gui-Ping Dai1,2,3\*

1 School of Resources Environmental and Chemical Engineering, Nanchang University,

3 Institute for Advanced Study, Nanchang University, Nanchang, Jiangxi, China

2 Department of Chemistry and Biochemistry, North Carolina Central University, Durham,

[1] Novoselov KS, Geim AK, Morozov SV, Jiang D, Katsnelson MI, Grigorieva IV, et al. Two-dimensional gas of massless Dirac fermions in graphene. Nature. 2005;438:197-200

[2] Liu Y, Xie B, Zhang Z, et al. Mechanical properties of graphene papers. Journal of the

[3] Stöberl U, Wurstbauer U, Wegscheider W, et al. Morphology and flexibility of graphene and few-layer graphene on various substrates. Applied Physics Letters. 2008;**93**(5):19912

[4] Mišković-Stanković V, Jevremović I, Jung I, et al. Electrochemical study of corrosion behavior of graphene coatings on copper and aluminum in a chloride solution. Carbon.

[5] Raman RKS, Banerjee PC, Lobo DE, et al. Protecting copper from electrochemical degra-

it is still a long way to make it.

, Kizhanipuram Vinodgopal<sup>2</sup>

\*Address all correspondence to: nanodai@gmail.com

Mechanics & Physics of Solids. 2012;**60**(4):591-605

dation by graphene coating. Carbon. 2012;**50**(11):4040-4045

**Author details**

Nanchang, Jiangxi, China

2014;**75**(10):335-344

Chen Wang<sup>1</sup>

NC, USA

**References**

## **6. Development trend and application prospect of CVD graphene**

As for the graphene synthesis by CVD, it is beneficial to maximize the scale while maintaining the homogeneity of large-area films, as indicated by Bae et al. [89]. A creative approach to the CVD synthesis of ultra large-area graphene based on selective Joule heating was reported by Kobayashi et al. [92]. Since the first growth on copper foil a decade ago, inch-sized singlecrystal graphene has been achieved. For most industrialized applications, large single-crystal graphene films are promising for top-down processing. In recent decades, the SCG island size has increased more than four orders of magnitude, from micrometers to inches. Xu et al. [34] have made large-area graphene film (5 cm × 50 cm) within 20 min, which is almost 99% ultra-highly oriented grains and is, to the best of our knowledge, the largest graphene area synthesized and demonstrated by far. The growth steps are described as follows: (1) manufacturing substrate with meter-sized single-crystal Cu(111) foil; (2) epitaxial growth of graphene grains on the Cu(111) surface; (3) seamless merging of graphene domains into a film with high-single crystallinity; and (4) the growth of graphene film in an ultrafast fashion.

At present, the process of preparing graphene by chemical vapor deposition has matured to the point of large-scale production and has been applied to electric ink, lithium battery additives, coatings and composite materials additives, touch screen, and other low-end applications [93].

For lithium battery applications, graphene as a conductive additive can also solve the contradiction between the high-voltage real ratio and the battery performance of the cathode materials such as lithium cobalt oxide [94]. Graphene has extremely high thermal conductivity, high thermal radiation coefficient, and high specific surface area, so that a surface coating of graphene as auxiliary heat dissipation has great potential for application [95]. Graphene can be used as the main filling material for new conductive ink and plastics [96], and it can also be mixed with nanometer silver powder into a new type of conductive adhesive or ink, and the resistivity and adhesion properties of conductive ink can be adjusted by changing the filling ratio of graphene. The two-dimensional structure of graphene allows for conductive thermal channels in graphenebased anticorrosive paint coating and forms a tight labyrinth of physical barriers to prevent corrosion related exposure. Graphene could be applied to improve performance on graphene capacitive screen [96]. Similarly, carbon nanowalls [97] described as a graphite nano-structure with edge structure and arranged perpendicularly to the matrix on a sheet of graphite have been synthesized by the CVD and is widely used in electrochemical devices, catalyst carriers, etc.

## **7. Summary and outlook**

With excellent prosperities of graphene, there are tremendous amount of research on digging out its exceptional prospect in future applications. But challenges including the growth rate of graphene growth, defects, stacking order, and transfer still remain. Nevertheless, more and more scientific communities are making efforts to achieve the industrialization of graphene. Undoubtedly, the graphene industry is growing at a fast rate. In the next 5–10 years, the technology for graphene-based conductive additives, anticorrosive coatings, touch screen would be mature, along with the industrialization of high-end applications such as super capacitors, sensors, and electronic chips. But mono-crystalline graphene films are extremely difficult to prepare. At present, they can only be made in millimeter size, and they are still quite far away from the practical crystal size, which is difficult to break in the short term. We need to achieve a balance between the advantages and disadvantages of graphene in order to create a fast and healthy way for the sustainable development of graphene. Chemical vapor deposition (CVD) is the most promising preparation technique. The preparation by chemical vapor deposition has become a major method for preparing semiconductor, thin film materials, and industrial production of high-quality graphene materials in large quantity. Meantime, for the CVD method, considerable efforts have been exerted to get low cost on any arbitrary substrate, but it is still a long way to make it.

## **Author details**

shown in **Figure 9**) with thermal release tape, R2R with epoxy [90], and bubbling transfer [91]. These developments have led to much more controllable graphene synthesis [19, 33, 89].

As for the graphene synthesis by CVD, it is beneficial to maximize the scale while maintaining the homogeneity of large-area films, as indicated by Bae et al. [89]. A creative approach to the CVD synthesis of ultra large-area graphene based on selective Joule heating was reported by Kobayashi et al. [92]. Since the first growth on copper foil a decade ago, inch-sized singlecrystal graphene has been achieved. For most industrialized applications, large single-crystal graphene films are promising for top-down processing. In recent decades, the SCG island size has increased more than four orders of magnitude, from micrometers to inches. Xu et al. [34] have made large-area graphene film (5 cm × 50 cm) within 20 min, which is almost 99% ultra-highly oriented grains and is, to the best of our knowledge, the largest graphene area synthesized and demonstrated by far. The growth steps are described as follows: (1) manufacturing substrate with meter-sized single-crystal Cu(111) foil; (2) epitaxial growth of graphene grains on the Cu(111) surface; (3) seamless merging of graphene domains into a film with

**6. Development trend and application prospect of CVD graphene**

high-single crystallinity; and (4) the growth of graphene film in an ultrafast fashion.

**7. Summary and outlook**

106 Chemical Vapor Deposition for Nanotechnology

At present, the process of preparing graphene by chemical vapor deposition has matured to the point of large-scale production and has been applied to electric ink, lithium battery additives, coatings and composite materials additives, touch screen, and other low-end applications [93]. For lithium battery applications, graphene as a conductive additive can also solve the contradiction between the high-voltage real ratio and the battery performance of the cathode materials such as lithium cobalt oxide [94]. Graphene has extremely high thermal conductivity, high thermal radiation coefficient, and high specific surface area, so that a surface coating of graphene as auxiliary heat dissipation has great potential for application [95]. Graphene can be used as the main filling material for new conductive ink and plastics [96], and it can also be mixed with nanometer silver powder into a new type of conductive adhesive or ink, and the resistivity and adhesion properties of conductive ink can be adjusted by changing the filling ratio of graphene. The two-dimensional structure of graphene allows for conductive thermal channels in graphenebased anticorrosive paint coating and forms a tight labyrinth of physical barriers to prevent corrosion related exposure. Graphene could be applied to improve performance on graphene capacitive screen [96]. Similarly, carbon nanowalls [97] described as a graphite nano-structure with edge structure and arranged perpendicularly to the matrix on a sheet of graphite have been synthesized by the CVD and is widely used in electrochemical devices, catalyst carriers, etc.

With excellent prosperities of graphene, there are tremendous amount of research on digging out its exceptional prospect in future applications. But challenges including the growth rate of graphene growth, defects, stacking order, and transfer still remain. Nevertheless, more and Chen Wang<sup>1</sup> , Kizhanipuram Vinodgopal<sup>2</sup> \* and Gui-Ping Dai1,2,3\*

\*Address all correspondence to: nanodai@gmail.com

1 School of Resources Environmental and Chemical Engineering, Nanchang University, Nanchang, Jiangxi, China

2 Department of Chemistry and Biochemistry, North Carolina Central University, Durham, NC, USA

3 Institute for Advanced Study, Nanchang University, Nanchang, Jiangxi, China

## **References**


[6] Nayak PK, Hsu CJ, Wang SC, et al. Graphene coated Ni films: A protective coating. Thin Solid Films. 2013;**529**(2):312-316

[23] Kwon H-J et al. Synthesis of flake-like graphene from nickel-coated polyacrylonitrile

Large-Area Synthesis and Growth Mechanism of Graphene by Chemical Vapor Deposition

http://dx.doi.org/10.5772/intechopen.79959

109

[24] Kim M-S et al. Effect of copper surface pre-treatment on the properties of CVD grown

[25] Gnanaprakasa TJ et al. The role of copper pretreatment on the morphology of graphene grown by chemical vapor deposition. Microelectronic Engineering. 2015;**131**:1-7

[26] Vinogradov NA et al. Formation and structure of graphene waves on Fe(110). Physical

[27] Sutter P, Sadowski JT, Sutter E. Graphene on Pt(111): Growth and substrate interaction.

[28] Feng X, Wu J, Bell AT, Salmeron M. An atomicscale view of the nucleation and growth of graphene islands on Pt surfaces. Journal of Physical Chemistry C. 2015;**119**:7124-7129

[29] Wu T et al. Fast growth of inch-sized single-crystalline graphene from a controlled single

[30] Liu X, Fu L, Liu N, Gao T, Zhang Y, Liao L. Segregation growth of graphene on Cu–Ni alloy for precise layer control. Journal of Physical Chemistry C. 2011;**115**:11976-11982

[31] Robertson SD. Graphite formation from low temperature pyrolysis of methane over

[32] Jiao L, Fan B, Xian X, et al. Creation of nanostructures with poly(methyl methacrylate) mediated nanotransfer printing. Journal of the American Chemical Society. 2008;**130**(38):

[33] Addou R et al. Monolayer graphene growth on Ni(111) by low temperature chemical

[34] Xu X, Zhang Z, Dong J, et al. Ultrafast epitaxial growth of metre-sized single-crystal

[35] Wang H, Wang G, Bao P, et al. Controllable synthesis of submillimeter single-crystal monolayer graphene domains on copper foils by suppressing nucleation. Journal of the

[36] Fan L, Li Z, Li X, et al. Controllable growth of shaped graphene domains by atmospheric

[37] Liu L, Zhou H, Cheng R, et al. High-yield chemical vapor deposition growth of highquality large-area AB-stacked bilayer graphene. Acs Nano. 2012;**6**(9):8241-8249

[38] Yu Q, Jauregui LA, Wu W, et al. Control and characterization of individual grains and grain boundaries in graphene grown by chemical vapour deposition. Nature Materials.

pressure chemical vapour deposition. Nanoscale. 2011;**3**(12):4946-4950

polymer. Nanoscale Research Letters. 2014;**9**(1):618

nucleus on Cu–Ni alloys. Nature Materials. 2015;**15**(1):43

vapor deposition. Applied Physics Letters.

graphene. Science Bulletin. 2017;**62**(15):1074-1080

American Chemical Society. 2012;**134**(8):3627

some transition metal surfaces. Nature. 1969;**221**(5185):1044-1046

graphene. AIP Advances. 2014;**4**(12):127107

Review Letters.2012;**109**:026101

Physical Review. 2009;**B80**:245411

12612-12613

2010;**10**(6):443-449


[23] Kwon H-J et al. Synthesis of flake-like graphene from nickel-coated polyacrylonitrile polymer. Nanoscale Research Letters. 2014;**9**(1):618

[6] Nayak PK, Hsu CJ, Wang SC, et al. Graphene coated Ni films: A protective coating. Thin

[7] Ma Y, Kim D, Jang H, et al. Characterization of low temperature graphene synthesis in inductively coupled plasma chemical vapor deposition process with optical emission

[8] Ning G, Xu C, Cao Y, et al. Chemical vapor deposition derived flexible graphene paper and its application as high performance anodes for lithium rechargeable batteries.

[9] Chen Z, Ren W, Gao L, et al. Three-dimensional flexible and conductive interconnected graphene networks grown by chemical vapour deposition. Nature Materials.

[10] Yuan GD, Zhang WJ, Yang Y, et al. Graphene sheets via microwave chemical vapor

[11] Zheng Q, Kim JK. Synthesis, structure, and properties of graphene and graphene oxide.

[12] Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, et al. Electric field

[15] Hernandez Y, Nicolosi V, Lotya M, et al. High-yield production of graphene by liquid-

[16] Ciesielski A, Samori P. Graphene via sonication assisted liquid-phase exfoliation.

[17] Dikin DA, Stankovich S, Zimney EJ, Piner RD, Dommett GHB, Evmenenko G, et al. Preparation and characterization of graphene oxide paper. Nature. 2007;**448**:457-460 [18] Eda G, Fanchini G, Chhowalla M. Large-area ultrathin films of reduced graphene oxide as a transparent and flexible electronic material. Nature Nanotechnology. 2008;**3**:270-274

[19] Li M, Liu D, Wei D, et al. Controllable synthesis of graphene by plasma-enhanced chemical vapor deposition and its related applications. Advanced Science. 2016;**3**(11):1600003

[20] Qi Y, Eskelsen JR, Mazur U, et al. Fabrication of graphene with CuO islands by chemical vapor deposition. Langmuir the ACS Journal of Surfaces & Colloids. 2012;**28**(7):3489-3493

[21] Sun J, Lindvall N, Cole MT, et al. Low partial pressure chemical vapor deposition of graphene on copper. IEEE Transactions on Nanotechnology. 2012;**11**(2):255-260

[22] Li X, Cai W, An J, et al. Large-area synthesis of high-quality and uniform graphene films

spectroscopy. Journal of Nanoscience & Nanotechnology. 2014;**14**(12):9065

Journal of Materials Chemistry A. 2012;**1**(2):408-414

deposition. Chemical Physics Letters. 2009;**467**(4):361-364

effect in atomically thin carbon films. Science. 2004;**306**:666-669

[13] Geim AK. Graphene: Status and prospects. Science. 2009;**324**:1530-1534

[14] Schwierz F. Graphene transistors. Nature Nanotechnology. 2010;**5**:487-496

phase exfoliation of graphite. Nature Nanotechnology. 2008;**3**(9):563-568

Journal of Experimental Medicine. 2015;**23**(2):29-94

Chemical Society Reviews. 2014;**43**:381-398

on copper foils. Science. 2009;**324**(5932):1312

Solid Films. 2013;**529**(2):312-316

108 Chemical Vapor Deposition for Nanotechnology

2011;**10**(6):424


[39] Luo B, Chen B, Meng L, Geng D, et al. Layer-stacking growth and electrical transport of hierarchical graphene architectures. Advanced Materials. 2014;**26**:3218-3224

[54] Jiang B-B, Pan M, Wang C, Wu MH, Vinodgopal K, Dai G-P. Controllable synthesis of circular graphene domains by atmosphere pressure chemical vapor deposition. Journal

Large-Area Synthesis and Growth Mechanism of Graphene by Chemical Vapor Deposition

http://dx.doi.org/10.5772/intechopen.79959

111

[55] Weatherup RS, Dlubak B, Hofmann S. Kinetic control of catalytic CVD for high-quality

[56] Yokoyama H, Numakura H, Koiwa M. The solubility and diffusion of carbon in pal-

[57] Sutter PW, Flege JI, Sutter EA. Epitaxial graphene on ruthenium. Nature Materials.

[58] Shu N, Bartelt NC, Starodub E, Walter A, Bostwick A, Rotenberg E. Growth from below: Graphene multilayers on Ir(111). United States. 2011. DOI: 10.1021/nn103582g

[59] Loginova E, Bartelt NC, Feibelman PJ, McCarty KF. Evidence for graphene growth by C

[60] Yan Z, Lin J, Peng ZW, Sun ZZ, Zhu Y, Li L, Xiang CS, Samuel EL, Kittrell C, Tour JM.

[62] Meca E, Lowengrub J, Kim H, Mattevi C, Shenoy VB. Epitaxial graphene growth and shape dynamics on copper: Phase-field modeling and experiments. Nano Letters. 2013;

[63] Jacobberger RM, Arnold MS. Graphene growth dynamics on epitaxial copper thin films.

[64] Park HJ et al. Growth and properties of few-layer graphene prepared by chemical vapor

[65] Zhang X, Wang L, Xin JH, et al. The role of hydrogen in graphene chemical vapor deposition (CVD) growth on copper surface. Journal of the American Chemical Society.

[66] Weiss SJ, Young J, Lobuglio AF, et al. Role of hydrogen peroxide in neutrophilmediated destruction of cultured endothelial cells. Journal of Clinical Investigation.

[67] Liu Q, Gong Y, Wilt JS, et al. Synchronous growth of AB-stacked bilayer graphene on Cu by simply controlling hydrogen pressure in CVD process. Carbon. 2015;**93**:199-206

[68] Shu H, Tao XM, Ding F. What are the active carbon species during graphene chemical

[69] Li K, He C, Jiao M, et al. A first-principles study on the role of hydrogen in early stage of graphene growth during the CH4 dissociation on Cu(111) and Ni(111) surfaces. Carbon.

vapor deposition growth? Nanoscale. 2015;**7**(5):1627-1634

graphene at low temperatures. ACS Nano. 2012;**6**(11):9996-10003

cluster attachment. New Journal of Physics. 2009;**11**:063046 (093026)

[61] Zhang Y, Li Z, Kim P, Zhang LY, Zhou CW. ACS Nano. 2012;**7**(6):871-877

of Physical Chemistry C. 2018;**122**:13572-13578

ladium. Acta Materials. 1998;**46**(8):2823-2830

Chemistry of Materials. 2013;**6**(25):3040-3047

deposition. Carbon. 2010;**48**(4):1088

2008;**7**(5):406-411

**11**(13):5692-5697

2014;**136**(8):3040-3047

1981;**68**(3):714-721

2014;**74**(10):255-265

ACS Nano. 2012;**10**(6):126-132


[54] Jiang B-B, Pan M, Wang C, Wu MH, Vinodgopal K, Dai G-P. Controllable synthesis of circular graphene domains by atmosphere pressure chemical vapor deposition. Journal of Physical Chemistry C. 2018;**122**:13572-13578

[39] Luo B, Chen B, Meng L, Geng D, et al. Layer-stacking growth and electrical transport of

[40] Wu B, Geng D, Xu Z et al. Self-organized graphene crystal patterns. NPG Asia Materials.

[41] Lotya M, Hernandez Y, King PJ, et al. Liquid phase production of graphene by exfoliation of graphite in surfactant/water solutions. Journal of the American Chemical Society.

[42] Hummers WS, Offeman RE. Preparation of graphitic oxide. Journal of the American

[43] Song HS, Li SL, Miyazaki H, Sato S, Hayashi K, Yamada A, et al. Origin of the relatively low transport mobility of graphene grown through chemical vapor deposition. Scientific

[44] Muñoz R, Gómez-Aleixandre C. Review of CVD synthesis of graphene. Chemical Vapor

[45] Li X, Cai W, Colombo L, Ruoff RS. Evolution of graphene growth on Ni and Cu by

[46] Van Bommel AJ, Crombeen JE, Van Tooren A. LEED and Auger electron observations of

[47] Berger C et al. Ultrathin epitaxial graphite: 2D electron gas properties and a route toward graphene-based nanoelectronics. The Journal of Physical Chemistry B. 2004;**108**(52):

[48] Zhang X, Li H, Ding F. Self-assembly of carbon atomson transition metal surfaces— Chemical vapor deposition growth mechanism of graphene. Advanced Materials.

[49] Kwon S-Y, Ciobanu CV, Petrova V, Shenoy VB, Bareno J, Gambin V, et al. Growth of

[50] Losurdo M, Giangregorio MM, Capezzuto P, Bruno G. Graphene CVD growth on copper and nickel: Role of hydrogen in kinetics and structure. Physical Chemistry Chemical

[51] Nie S, Bartelt NC, Wofford JM, Dubon OD, McCarty KF, Thurmer K. Scanning tunneling microscopy study of graphene on Au(111): Growth mechanisms and substrate interac-

[52] Li Z et al. Low-temperature growth of graphene by chemical vapor deposition using

[53] Jang J et al. Low-temperature-growth continuous graphene films from benzene by chemical vapor deposition at ambient pressure. Scientific Reports. 2015;**5**:17955

semiconducting graphene on palladium. Nano Letters. 2009;**9**:3985-3390

solid and liquid carbon sources. ACS Nano. 2011;**5**(4):3385-3390

hierarchical graphene architectures. Advanced Materials. 2014;**26**:3218-3224

2013;**S**:e36

2009;**131**(10):3611-3620

110 Chemical Vapor Deposition for Nanotechnology

Reports. UK. 2012;**2**:337

19912-19916

2014;**26**:5488-5495

Physics. 2011;**13**:20836-20843

tions. Physical Review B. 2012;**85**:205406

Chemical Society. 1958;**80**(6):1339

Deposition. 2013;**19**(10-11-12):297-322

carbon isotope labeling. Nano Letters. 2009;**9**:4268-4272

the SiC(0001) surface. Surface Science. 1975;**48**(2):463-472


[70] Gan CK, Srolovitz DJ. First-principles study of graphene edge properties and flake shapes. 2010;**81**(12):760-762

[87] Yamada T, Ishihara M, Hasegawa M. Large area coating of graphene at low temperature using a roll-to-roll microwave plasma chemical vapor deposition. Thin Solid Films.

Large-Area Synthesis and Growth Mechanism of Graphene by Chemical Vapor Deposition

http://dx.doi.org/10.5772/intechopen.79959

113

[88] Juang ZY, Wu CY, Lu AY, et al. Graphene synthesis by chemical vapor deposition and

[89] Bae S, Kim H, Lee Y, Xu X, Park J-S, Zheng Y, et al. Roll-to-roll production of 30-inch graphene films for transparent electrodes. Nature Nanotechnology. 2010;**5**:574

[90] Kraus J, Böbel M, Günther S. Suppressing graphene nucleation during CVD on polycrystalline Cu bycontrolling the carbon content of the support foils. Carbon. 2016;**96**:153-165

[91] Zhang H, Zhang Y, Wang B, Chen Z, Sui Y, Zhang Y, et al. Journal of Electronic Materials.

[92] Kobayashi T, Bando M, Kimura N, Shimizu K, Kadono K, Umezu N, et al. Production of a 100-m-long high-quality graphene transparent conductive film by roll-to-roll chemical

[93] Choi W, Lahiri I, Seelaboyina R, Kang YS. Synthesis of graphene and its applications: A review. Critical Reviews in Solid State and Materials Sciences. 2010;**35**:52-71

[94] Li X, Sun X, et al. A simple one-pot strategy for synthesizing ultrafine SnS2 nanoparticle/graphene composites as anodes for lithium/sodium-ion batteries. ChemSuschem.

[95] Molina-Valdovinos S et al. Low-dimensional thermoelectricity in graphene: The case of gated graphene superlattices. Physica E: Low-dimensional Systems and Nanostructures.

[96] Arapov K et al. Conductive screen printing inks by gelation of graphene dispersions.

[97] Wang JJ, Zhu MY, Outlaw RA, Zhao X, Manos DM. Free-standing sub-nanometer graph-

Advanced Functional Materials. 2016;**26**(4):586-593

ite sheets. Applied Physics Letters. 2004;**85**:1265

vapor deposition and transfer process. Applied Physics Letters. 2013;**102**:023112

transfer by a roll-to-roll process. Carbon. 2010;**48**(11):3169-3174

2013;**532**(532):89-93

2015;**44**:79-86

2018;**11**(9):1390-1575

2018;**101**:188-196


[87] Yamada T, Ishihara M, Hasegawa M. Large area coating of graphene at low temperature using a roll-to-roll microwave plasma chemical vapor deposition. Thin Solid Films. 2013;**532**(532):89-93

[70] Gan CK, Srolovitz DJ. First-principles study of graphene edge properties and flake

[71] Geng D, Wu B, Guo Y, et al. From the cover: Uniform hexagonal graphene flakes and films grown on liquid copper surface. Proceedings of the National Academy of Sciences

[72] Luo B, Gao E, Geng D, et al. Etching-controlled growth of graphene by chemical vapor

[73] Zhang H, Zhang Y, Zhang Y, et al. Realizing controllable graphene nucleation by regulating the competition of hydrogen and oxygen during chemical vapor deposition heat-

[74] Ma T, Ren W, Liu Z, et al. Repeated growth-etching-regrowth for large-area defect-free single-crystal graphene by chemical vapor deposition. ACS Nano. 2014;**8**(12):12806-12813

[75] Da HJ, Kang C, Kim M, et al. Effects of hydrogen partial pressure in the annealing process on graphene growth. Journal of Physical Chemistry C. 2015;**118**(7):3574-3580 [76] Hu B, Jin Y, Guan D, et al. H2-dependent carbon dissolution and diffusion-out in graphene chemical vapor deposition growth. Journal of Physical Chemistry C. 2015;**119**(42):

[77] Jin Y, Hu B, Wei Z, et al. Roles of H2 in annealing and growth times of graphene CVD synthesis over copper foil. Journal of Materials Chemistry A. 2014;**2**(38):16208-16216 [78] Wu B, Geng D, Guo Y, et al. Equiangular hexagon-shape-controlled synthesis of gra-

[79] Zhao P, Cheng Y, Zhao D, et al. The role of hydrogen in oxygen-assisted chemical vapor deposition growth of millimeter-sized graphene single crystals. Nanoscale. 2016;**8**(14):

[81] Zhang XY, Wang L, Xin J, Yakobson BI, Ding F. Journal of American Chemical Society.

[82] Hao Y, Ruoff RS. The role of surface oxygen in the growth of large single-crystal gra-

[83] Liang T, Luan C, Chen H, et al. Exploring oxygen in graphene chemical vapor deposition

[85] Wang ZG, Chen YF, Li PJ, et al. Effects of methane flux on structural and transport prop-

[86] Nguyen VL, Shin BG, Duong DL, et al. Seamless stitching of graphene domains on pol-

of the United States of America. 2012;**109**(21):7992-7996

deposition. Chemistry of Materials. 2017;**29**(3):1022-1027

ing. Physical Chemistry Chemical Physics. 2016;**18**(34):23638-23642

phene on copper surface. Advanced Materials. 2011;**23**(31):3522-3525

shapes. 2010;**81**(12):760-762

112 Chemical Vapor Deposition for Nanotechnology

585080264

7646

2014;**18**:136

[80] Shin YC, Kong J. Carbon. 2013;**59**:439

synthesis. Nanoscale. 2017;**9**(11):3719

phene on copper. Science. 2013;**342**(6159):720-723

[84] Zhang Y, Li Z, Kim P, Zhang LY, Zhou CW. ACS Nano. 2012;**7**:6

erties of CVD-grown graphene films. Vacuum. 2012;**86**(7):895

ished copper (111) foil. Advanced Materials. 2015;**27**(8):1376-1382


**Chapter 6**

Provisional chapter

**Atmospheric Pressure Chemical Vapor Deposition of**

DOI: 10.5772/intechopen.81293

Recently, graphene has gained significant interest owing to its outstanding conductivity, mechanical strength, thermal stability, etc. Among various graphene synthesis methods, atmospheric pressure chemical vapor deposition (APCVD) is one of the best syntheses due to very low diffusivity coefficient and a critical step for graphene-based device fabrication. High-temperature APCVD processes for thin film productions are being recognized in many diversity technologies such as solid state electronic devices, in particular, high quality epitaxial semiconductor films for silicon bipolar and metal oxide semiconductor (MOS) transistors. Graphene-based devices exhibit high potential for applications in flexible electronics, optoelectronics, and energy harvesting. In this chapter, recent advances of APCVD-based graphene synthesis and their related applications will be addressed.

Keywords: graphene, atmospheric pressure chemical vapor deposition (APCVD), single-layer graphene (SLG), bilayer graphene (BLG), atmosphere pressure, large-scale

Single-layer graphene (SLG), bilayer graphene (BLG), and multi-layer graphene (MLG) films have been regarded as optimal materials for electronics and optoelectronics owing to their excellent electrical properties and their ability to integrate with current top-down device fabrication technology [1–95]. Since the beginning of the twenty-first century, the interest in graphene materials has drastically increased, which is apparent in the number of annual publications on graphene (Figure 1). Until now, various strategies including chemical vapor deposition (CVD) [28], liquid and mechanical exfoliation from graphite [23, 29, 30], epitaxial growth on crystal substrate [31–34], or solution-based processes on graphene oxides (GOs)

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

Atmospheric Pressure Chemical Vapor Deposition of

**Graphene**

Graphene

Phuong V. Pham

Phuong V. Pham

Abstract

1. Introduction

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.81293

#### **Atmospheric Pressure Chemical Vapor Deposition of Graphene** Atmospheric Pressure Chemical Vapor Deposition of Graphene

DOI: 10.5772/intechopen.81293

#### Phuong V. Pham Phuong V. Pham

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.81293

#### Abstract

Recently, graphene has gained significant interest owing to its outstanding conductivity, mechanical strength, thermal stability, etc. Among various graphene synthesis methods, atmospheric pressure chemical vapor deposition (APCVD) is one of the best syntheses due to very low diffusivity coefficient and a critical step for graphene-based device fabrication. High-temperature APCVD processes for thin film productions are being recognized in many diversity technologies such as solid state electronic devices, in particular, high quality epitaxial semiconductor films for silicon bipolar and metal oxide semiconductor (MOS) transistors. Graphene-based devices exhibit high potential for applications in flexible electronics, optoelectronics, and energy harvesting. In this chapter, recent advances of APCVD-based graphene synthesis and their related applications will be addressed.

Keywords: graphene, atmospheric pressure chemical vapor deposition (APCVD), single-layer graphene (SLG), bilayer graphene (BLG), atmosphere pressure, large-scale

## 1. Introduction

Single-layer graphene (SLG), bilayer graphene (BLG), and multi-layer graphene (MLG) films have been regarded as optimal materials for electronics and optoelectronics owing to their excellent electrical properties and their ability to integrate with current top-down device fabrication technology [1–95]. Since the beginning of the twenty-first century, the interest in graphene materials has drastically increased, which is apparent in the number of annual publications on graphene (Figure 1). Until now, various strategies including chemical vapor deposition (CVD) [28], liquid and mechanical exfoliation from graphite [23, 29, 30], epitaxial growth on crystal substrate [31–34], or solution-based processes on graphene oxides (GOs)

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Among the utilized synthesis methods, APCVD has been considered as the most potential and medium cost one for large-scale high-quality graphene on various metallic substrates including Pt [54], Ir [42], Ni [7], and Cu [45]. Especially, Cu is considered as the best choice owing to low-carbon solubility, well-controlled surface, and inexpensive for growing monolayer graphene [45, 55]. Many efforts have made for obtaining large-scale single crystal graphene with as less grain boundaries as possible. There are two general methods to realize: the first method associates with the growth of single domains with possible enlargement [54, 56–63]. Despite the centimeter-scale domains have achieved, this method is not the bright candidate in practice for large-scale growth because of the difficult quantity control of nucleation seeds as well as unclear self-limiting growth factors resulting worse and requiring very long growth time (over 24 h) [64, 65]. The second method is the alignment of graphene domain orientations on arbitrary substrates and then to atomically stitch them to form uniform single crystalline

Atmospheric Pressure Chemical Vapor Deposition of Graphene

http://dx.doi.org/10.5772/intechopen.81293

117

The CVD method creates large-scale graphene but polycrystalline morphology, which includes different oriented domains. Such the orientated disorders inevitably lead to graphene grain boundaries (GGBs) formation at intercalated interface of domain [67–69]. GGBs include a series of nonhexagonal rings of pentagons, heptagons, and octagons. Intrinsic graphene has high conductivity and chemical inert, however, the appearance of atomically defect lines and GGBs in graphene could remarkably modulate its features such as mechanics [69, 70], electrics [71, 72], chemistry [73, 74], and magnets [75]. These defect lines have extraordinary features and rely on the atomical configuration at GGBs and the crystallinity of mono-grains; moreover, they could be modulated through different function groups [75]. Therefore, the investigation on the orientations and boundaries of the grains is the key to comprehend the underlying

Conventionally, graphene is grown on metal foils (Cu, Ni, Pt, etc.) through APCVD, LPCVD, or PECVD. The temperatures, pressures, and concentrations of precursor gases inside the furnace play key roles on the graphene growth and quality. These factors need be optimized to get the desirable growth results. In general, surface morphology is studied via optical microscope (OM) and scanning electron microscopy (SEM); the graphene quality is examined through Raman spectra, UV-visible spectroscopy, and transmission electron microscopy (TEM); and sheet resistance (Rs) of synthesized graphene is measured utilizing four-point probe technique. Almost graphene films synthesized via APCVD are monolayer with better quality in atmospheric condition compared with LPCVD or PECVD. The synthesis study using APCVD would be greatly significant in the desirable growth of graphene and other related materials. In this chapter, recent advances of APCVD-based graphene synthesis with practical concerns about chemical vapor transport, deposition process as well as their device applica-

APCVD growth of graphene is a chemical synthesis process at atmosphere pressure for the formation of SLG or FLG on an arbitrary substrate by exposing the substrate to the gas-phase

graphene [5, 66]. This seems to be ideal for growing large scale.

features as well as to realize compatible applications of graphene.

2. Growth mechanism of APCVD-based graphene

tions will be addressed.

Figure 1. Graphene publications from 2000 to 2018. Source: ISI Web of Science (search: Topic = Graphene).

[35–41]. They have been investigated for obtaining graphene layers. In particular, recent advances in CVD growth have successfully led to large-scale graphene production on metal substrates [1, 27, 42–48], driven by the high demand for utilizing graphene in possible applications of current complementary metal-oxide-semiconductor (CMOS) technology such as radio-frequency transistors, optical devices, and deposition processes [2].

High-quality large-scale graphene has been synthesized on conducting metallic substrates by using the catalytic CVD growth approach, which promoted a wide range of graphene-based device applications [1, 28, 42–49]. However, graphene grown on a metal substrate needs to be transferred onto dielectric substrates for electronic applications. Although various approaches, such as wet etching/transfer [45], mechanical exfoliation/transfer [23, 29, 30], bubbling transfer [50], electrochemical delamination [51–53], for transferring from the catalytic metal growth substrates to dielectric device substrates have been developed; none of these approaches is free from degradation of the transferred graphene. For example, 'wet etching and transfer,' the most widely used transfer approach, is a serial process, which includes encapsulation of the graphene surface with polymer support.

Growing through CVD is for the production of films or for coating of metal, semiconductor, crystalline, and vitreous formed-compounds in either, occupying high-purity as well as desirable characteristics. In addition, the creation of controllable film with varying stoichiometry results CVD uniquely among deposition approaches. Other advantages include reasonably low cost of the equipment and operating expenses, suitability for semicontinuous operation. Consequently, the variants of CVD have been developed recently such as low-pressure chemical vapor deposition (LPCVD), APCVD, plasma-enhanced chemical vapor deposition (PECVD), and laser-enhanced chemical vapor deposition (LECVD). The hybrid system represents for both CVD and physical vapor deposition (PVD) have also discovered.

Among the utilized synthesis methods, APCVD has been considered as the most potential and medium cost one for large-scale high-quality graphene on various metallic substrates including Pt [54], Ir [42], Ni [7], and Cu [45]. Especially, Cu is considered as the best choice owing to low-carbon solubility, well-controlled surface, and inexpensive for growing monolayer graphene [45, 55]. Many efforts have made for obtaining large-scale single crystal graphene with as less grain boundaries as possible. There are two general methods to realize: the first method associates with the growth of single domains with possible enlargement [54, 56–63]. Despite the centimeter-scale domains have achieved, this method is not the bright candidate in practice for large-scale growth because of the difficult quantity control of nucleation seeds as well as unclear self-limiting growth factors resulting worse and requiring very long growth time (over 24 h) [64, 65]. The second method is the alignment of graphene domain orientations on arbitrary substrates and then to atomically stitch them to form uniform single crystalline graphene [5, 66]. This seems to be ideal for growing large scale.

The CVD method creates large-scale graphene but polycrystalline morphology, which includes different oriented domains. Such the orientated disorders inevitably lead to graphene grain boundaries (GGBs) formation at intercalated interface of domain [67–69]. GGBs include a series of nonhexagonal rings of pentagons, heptagons, and octagons. Intrinsic graphene has high conductivity and chemical inert, however, the appearance of atomically defect lines and GGBs in graphene could remarkably modulate its features such as mechanics [69, 70], electrics [71, 72], chemistry [73, 74], and magnets [75]. These defect lines have extraordinary features and rely on the atomical configuration at GGBs and the crystallinity of mono-grains; moreover, they could be modulated through different function groups [75]. Therefore, the investigation on the orientations and boundaries of the grains is the key to comprehend the underlying features as well as to realize compatible applications of graphene.

Conventionally, graphene is grown on metal foils (Cu, Ni, Pt, etc.) through APCVD, LPCVD, or PECVD. The temperatures, pressures, and concentrations of precursor gases inside the furnace play key roles on the graphene growth and quality. These factors need be optimized to get the desirable growth results. In general, surface morphology is studied via optical microscope (OM) and scanning electron microscopy (SEM); the graphene quality is examined through Raman spectra, UV-visible spectroscopy, and transmission electron microscopy (TEM); and sheet resistance (Rs) of synthesized graphene is measured utilizing four-point probe technique. Almost graphene films synthesized via APCVD are monolayer with better quality in atmospheric condition compared with LPCVD or PECVD. The synthesis study using APCVD would be greatly significant in the desirable growth of graphene and other related materials. In this chapter, recent advances of APCVD-based graphene synthesis with practical concerns about chemical vapor transport, deposition process as well as their device applications will be addressed.

## 2. Growth mechanism of APCVD-based graphene

[35–41]. They have been investigated for obtaining graphene layers. In particular, recent advances in CVD growth have successfully led to large-scale graphene production on metal substrates [1, 27, 42–48], driven by the high demand for utilizing graphene in possible applications of current complementary metal-oxide-semiconductor (CMOS) technology such as

Figure 1. Graphene publications from 2000 to 2018. Source: ISI Web of Science (search: Topic = Graphene).

High-quality large-scale graphene has been synthesized on conducting metallic substrates by using the catalytic CVD growth approach, which promoted a wide range of graphene-based device applications [1, 28, 42–49]. However, graphene grown on a metal substrate needs to be transferred onto dielectric substrates for electronic applications. Although various approaches, such as wet etching/transfer [45], mechanical exfoliation/transfer [23, 29, 30], bubbling transfer [50], electrochemical delamination [51–53], for transferring from the catalytic metal growth substrates to dielectric device substrates have been developed; none of these approaches is free from degradation of the transferred graphene. For example, 'wet etching and transfer,' the most widely used transfer approach, is a serial process, which includes encapsulation of the

Growing through CVD is for the production of films or for coating of metal, semiconductor, crystalline, and vitreous formed-compounds in either, occupying high-purity as well as desirable characteristics. In addition, the creation of controllable film with varying stoichiometry results CVD uniquely among deposition approaches. Other advantages include reasonably low cost of the equipment and operating expenses, suitability for semicontinuous operation. Consequently, the variants of CVD have been developed recently such as low-pressure chemical vapor deposition (LPCVD), APCVD, plasma-enhanced chemical vapor deposition (PECVD), and laser-enhanced chemical vapor deposition (LECVD). The hybrid system represents for both

radio-frequency transistors, optical devices, and deposition processes [2].

CVD and physical vapor deposition (PVD) have also discovered.

graphene surface with polymer support.

116 Chemical Vapor Deposition for Nanotechnology

APCVD growth of graphene is a chemical synthesis process at atmosphere pressure for the formation of SLG or FLG on an arbitrary substrate by exposing the substrate to the gas-phase precursors at controlled reaction conditions [76]. Owing to the versatile nature of APCVD, intricately mixed homogeneous gas-phase and heterogeneous surface reactions are involved [77]. In general, as the partial pressure and/or temperature in the reaction substances are increased, homogeneous gas-phase reactions and the resulting homogeneous nucleation became significant [77]. To grow a high-quality graphene layer, this homogeneous nucleation needs to be minimized [77]. A general mechanism for APCVD-based graphene growth on catalytic metal substrates, for the growth of uniform and highly crystalline graphene layer on the surface, includes eight steps as follow: (1) mass transport of the reactant, (2) reaction of the film precursor, (3) diffusion of gas molecules, (4) adsorption of the precursor, (5) diffusion of the precursor into substrate, (6) surface reaction, (7) desorption of the product, and (8) removal of the byproduct (Figure 2) [78, 96].

It has been a general observation in most of the experimental works that the LPCVD produces nonuniform thick graphene layers. However, as high-quality single-layer graphene growth, APCVD would be a better choice due to reduced mean free path/diffusion coefficient of the reactive species on the catalyst [79]. APCVD shows a much lower diffusivity coefficient: Dg 1/(total pressure) compare with LPCVD. As the result, the small multilayer graphene islands is less appearing on a full large monolayer graphene island via APCVD growth process [79] compared with LPCVD.

seeds formed, almost the active carbon species are incorporated and captured at surface/interface of graphene lattice. In addition, H2 precursor plays more role as an etchant and controls the size as well as shape of graphene domains via reactions: Hs + graphene ➔ (graphene-C) + (CHx)s with

Figure 3. Schematic for graphene growth mechanism on Cu substrate. Reproduced with permission from [89]. Copyright

Atmospheric Pressure Chemical Vapor Deposition of Graphene

http://dx.doi.org/10.5772/intechopen.81293

119

Typically, APCVD growth of 2D materials (e.g. graphene) involves catalytic activation of chemical reactions of precursors at the growth substrate surface/interface in a properly designed environment. In general, the roles of precursors, conditions (e.g. fast growth rates, large domain size, or very high crystalline quality), atmosphere, substrates, and catalysts are the key factors affecting the final quality of the grown 2D materials. So far, significant efforts have been made to prepare high crystalline 2D materials (e.g. graphene), but many challenges are still ahead. For example, due to the rough feature of catalytic metal surface, growth of uniform and high-quality graphene is considerably difficult. The 2D material research community is also interested in new precursors (e.g. solid precursor only, gas precursor, or solid precursor mixed with certain solvents) that could form uniformly high-quality graphene with minimal defect density. Another question is the effect of growth rate on the catalytic metal surface on the quality of graphene. Currently, it is difficult to give an exact answer, as investigations are progressing at an exponential rate. To date, the understanding of the concept of the general mechanism of the APCVD growth of graphene is still not yet adequate, neither experimentally nor theoretically. Thus, understanding the graphene growth mechanism and the effect of various growth conditions will be of significant interest to the 2D material research

CVD is a thin solid film deposition process of vapor species through suitable chemical reaction. The deposition needs low-carbon solubility substrate in high-temperature region [80]. To date, CVD is the sole approach, which could produce high-quality graphene with ultra-large size [45, 81]. For the first experiment, researchers successfully synthesized graphene films via CVD on Ni and Cu catalytic substrates [44, 82, 83]. The significant progress for large-size and high-

community to obtain large scale, high-quality graphene.

quality graphene films has also well done [81, 83].

3. APCVD growth of graphene

x = 1, 2, 3 [89].

2012, American Chemical Society.

In another graphene growth mechanism model, hydrocarbon molecules are absorbed as well as dissociated on Cu forming active carbon species by dehydrogenation reaction (Figure 3). These species are diffused on two sides of Cu foil and agglomerated on its active sites to form graphene nucleation seeds. Actually, introducing H2 gas is mandatory as a key role in graphene growth for most CVD approaches. The overall processes of graphene growth on Cu are described (Figure 3) [89]. Generally, there are three main expected steps: (i) adsorp-decompose, (ii) diffuse-desorb, and (iii) nucleate-grow (Figure 3) [89]. The active carbon species are commonly not stable and easily agglomerate with thermodynamically stable species on active sites to form graphene nucleation seeds by the reactions: (CHx)s + graphene ➔ (graphene-C) + (CHx)s with x = 1, 2, 3 [89]. Till those

Figure 2. Diagram of CVD growth mechanism (APCVD and LPCVD) of graphene: transport and reaction processes. Reproduced with permission from [78, 96]. Copyright 2011, Freund Publishing.

Figure 3. Schematic for graphene growth mechanism on Cu substrate. Reproduced with permission from [89]. Copyright 2012, American Chemical Society.

seeds formed, almost the active carbon species are incorporated and captured at surface/interface of graphene lattice. In addition, H2 precursor plays more role as an etchant and controls the size as well as shape of graphene domains via reactions: Hs + graphene ➔ (graphene-C) + (CHx)s with x = 1, 2, 3 [89].

Typically, APCVD growth of 2D materials (e.g. graphene) involves catalytic activation of chemical reactions of precursors at the growth substrate surface/interface in a properly designed environment. In general, the roles of precursors, conditions (e.g. fast growth rates, large domain size, or very high crystalline quality), atmosphere, substrates, and catalysts are the key factors affecting the final quality of the grown 2D materials. So far, significant efforts have been made to prepare high crystalline 2D materials (e.g. graphene), but many challenges are still ahead. For example, due to the rough feature of catalytic metal surface, growth of uniform and high-quality graphene is considerably difficult. The 2D material research community is also interested in new precursors (e.g. solid precursor only, gas precursor, or solid precursor mixed with certain solvents) that could form uniformly high-quality graphene with minimal defect density. Another question is the effect of growth rate on the catalytic metal surface on the quality of graphene. Currently, it is difficult to give an exact answer, as investigations are progressing at an exponential rate. To date, the understanding of the concept of the general mechanism of the APCVD growth of graphene is still not yet adequate, neither experimentally nor theoretically. Thus, understanding the graphene growth mechanism and the effect of various growth conditions will be of significant interest to the 2D material research community to obtain large scale, high-quality graphene.

## 3. APCVD growth of graphene

precursors at controlled reaction conditions [76]. Owing to the versatile nature of APCVD, intricately mixed homogeneous gas-phase and heterogeneous surface reactions are involved [77]. In general, as the partial pressure and/or temperature in the reaction substances are increased, homogeneous gas-phase reactions and the resulting homogeneous nucleation became significant [77]. To grow a high-quality graphene layer, this homogeneous nucleation needs to be minimized [77]. A general mechanism for APCVD-based graphene growth on catalytic metal substrates, for the growth of uniform and highly crystalline graphene layer on the surface, includes eight steps as follow: (1) mass transport of the reactant, (2) reaction of the film precursor, (3) diffusion of gas molecules, (4) adsorption of the precursor, (5) diffusion of the precursor into substrate, (6) surface reaction, (7) desorption of the product, and (8) removal of the by-

It has been a general observation in most of the experimental works that the LPCVD produces nonuniform thick graphene layers. However, as high-quality single-layer graphene growth, APCVD would be a better choice due to reduced mean free path/diffusion coefficient of the reactive species on the catalyst [79]. APCVD shows a much lower diffusivity coefficient: Dg 1/(total pressure) compare with LPCVD. As the result, the small multilayer graphene islands is less appearing on a full large monolayer graphene island via APCVD growth process [79]

In another graphene growth mechanism model, hydrocarbon molecules are absorbed as well as dissociated on Cu forming active carbon species by dehydrogenation reaction (Figure 3). These species are diffused on two sides of Cu foil and agglomerated on its active sites to form graphene nucleation seeds. Actually, introducing H2 gas is mandatory as a key role in graphene growth for most CVD approaches. The overall processes of graphene growth on Cu are described (Figure 3) [89]. Generally, there are three main expected steps: (i) adsorp-decompose, (ii) diffuse-desorb, and (iii) nucleate-grow (Figure 3) [89]. The active carbon species are commonly not stable and easily agglomerate with thermodynamically stable species on active sites to form graphene nucleation seeds by the reactions: (CHx)s + graphene ➔ (graphene-C) + (CHx)s with x = 1, 2, 3 [89]. Till those

Figure 2. Diagram of CVD growth mechanism (APCVD and LPCVD) of graphene: transport and reaction processes.

Reproduced with permission from [78, 96]. Copyright 2011, Freund Publishing.

product (Figure 2) [78, 96].

118 Chemical Vapor Deposition for Nanotechnology

compared with LPCVD.

CVD is a thin solid film deposition process of vapor species through suitable chemical reaction. The deposition needs low-carbon solubility substrate in high-temperature region [80]. To date, CVD is the sole approach, which could produce high-quality graphene with ultra-large size [45, 81]. For the first experiment, researchers successfully synthesized graphene films via CVD on Ni and Cu catalytic substrates [44, 82, 83]. The significant progress for large-size and highquality graphene films has also well done [81, 83].

APCVD requires high temperature (1000C) for graphene synthesis. It is experimentally expensive; therefore, it requires the CVD equipment more sophisticatedly. Unfortunately, this is an obstacle for direct-growth of graphene on insulating device substrate (e.g., SiO2), for instance, it can produce unavoidable physical damages around 1000C and dramatically degrades the quality of synthesized graphene. As a result, the deposition of graphene on insulating substrates at reduced temperatures becomes very necessary [84, 85]. The features of graphene will change corresponding with the number of layers. Hence, the electronic, optical, mechanical, and other features of graphene could be tuned by controlling the number of layers, or by adjusting the experimental conditions. Currently, it is still the challenge to exactly atomic-scale control the number of graphene layers [86–88]. The proposed synthesis method is composed by four stages (cleaning, precursor injection, reaction time, and cooling). The temperature and flow rate of carbon precursor were the studied parameters. In Figure 4a, b, the common CVD systems and differences are shown [90].

In 2016, Wang et al. obviously showed millimeter-scale graphene single crystals synthesized on Cu through APCVD (Figure 5) [91]. The possible mechanism will be based on graphene nucleation and kinetic growth. At the stage of nucleation, thermal decomposition of oxide layer leads to O2 desorption at high temperature at front side of Cu and dominates the temperature dependence of nucleation density. The graphene island growth is edge-limited on two sides of Cu at various enlargement rates. The roughness of support substrates (quartz, sapphire) also affects the graphene deposition. After optimized annealing and polished

support substrate, the isolated graphene islands (3 mm) were produced with a growth rate (25 μm/min). The domains were uniformly single-crystalline graphene with good mobility

Figure 5. (a) Schematic for APCVD-grown graphene on Cu located on quartz or sapphire substrates. Insets in (a) are OM images of graphene/Cu at front side and back side. OM images of grown graphene on Cu foils with various pre-annealing times of (b) 0, (c) 5, (d) 30, (e) 60, (f) 120 min at the same growth time (60 min). (g) Evolution of nucleation density and size of graphene domains as a function of pre-annealing times. Reproduced with permission from [91]. Copyright 2016,

Cu for 60 min growth at various Ar pre-annealing times. The growth and cooling parameters maintained the same. Here, the Ar heating induced the growth of individual graphene

of oxide layer on top of Cu. Through annealing in 30 min (Figure 5d), the nucleation density

(Figure 5e). But the extending of annealing (120 min) caused the increase of nucleation density with reduced domain size (0.6 mm) (Figure 5f). The evolutions of nucleation densities and domain sizes via annealing are shown in Figure 5g. The nucleation density reveals a nonmonotonic dependence of Ar-passivated annealing. Since the surface becomes flatter after

domains with relatively low nucleation density (102 cm<sup>2</sup>

/V.s) at room temperature. Figure 5b-f shows OM images of graphene domains/

, and a grain size of 1 mm was obtained after 60 min growth

) owing to the catalyst passivation

Atmospheric Pressure Chemical Vapor Deposition of Graphene

http://dx.doi.org/10.5772/intechopen.81293

121

(4900 cm2

reduced 12 nuclei/cm<sup>2</sup>

American Chemical Society.

Figure 4. Schematic of graphene synthesis by (a) APCVD using polycrystalline Cu substrate and gas discontinuous flow and (b) high vacuum using single crystalline Cu(111) substrate and gas continuous flow. Reproduced with permission from [90]. Copyright 2018, Hindawi Publishing.

APCVD requires high temperature (1000C) for graphene synthesis. It is experimentally expensive; therefore, it requires the CVD equipment more sophisticatedly. Unfortunately, this is an obstacle for direct-growth of graphene on insulating device substrate (e.g., SiO2), for instance, it can produce unavoidable physical damages around 1000C and dramatically degrades the quality of synthesized graphene. As a result, the deposition of graphene on insulating substrates at reduced temperatures becomes very necessary [84, 85]. The features of graphene will change corresponding with the number of layers. Hence, the electronic, optical, mechanical, and other features of graphene could be tuned by controlling the number of layers, or by adjusting the experimental conditions. Currently, it is still the challenge to exactly atomic-scale control the number of graphene layers [86–88]. The proposed synthesis method is composed by four stages (cleaning, precursor injection, reaction time, and cooling). The temperature and flow rate of carbon precursor were the studied parameters. In Figure 4a,

In 2016, Wang et al. obviously showed millimeter-scale graphene single crystals synthesized on Cu through APCVD (Figure 5) [91]. The possible mechanism will be based on graphene nucleation and kinetic growth. At the stage of nucleation, thermal decomposition of oxide layer leads to O2 desorption at high temperature at front side of Cu and dominates the temperature dependence of nucleation density. The graphene island growth is edge-limited on two sides of Cu at various enlargement rates. The roughness of support substrates (quartz, sapphire) also affects the graphene deposition. After optimized annealing and polished

Figure 4. Schematic of graphene synthesis by (a) APCVD using polycrystalline Cu substrate and gas discontinuous flow and (b) high vacuum using single crystalline Cu(111) substrate and gas continuous flow. Reproduced with permission

from [90]. Copyright 2018, Hindawi Publishing.

b, the common CVD systems and differences are shown [90].

120 Chemical Vapor Deposition for Nanotechnology

Figure 5. (a) Schematic for APCVD-grown graphene on Cu located on quartz or sapphire substrates. Insets in (a) are OM images of graphene/Cu at front side and back side. OM images of grown graphene on Cu foils with various pre-annealing times of (b) 0, (c) 5, (d) 30, (e) 60, (f) 120 min at the same growth time (60 min). (g) Evolution of nucleation density and size of graphene domains as a function of pre-annealing times. Reproduced with permission from [91]. Copyright 2016, American Chemical Society.

support substrate, the isolated graphene islands (3 mm) were produced with a growth rate (25 μm/min). The domains were uniformly single-crystalline graphene with good mobility (4900 cm2 /V.s) at room temperature. Figure 5b-f shows OM images of graphene domains/ Cu for 60 min growth at various Ar pre-annealing times. The growth and cooling parameters maintained the same. Here, the Ar heating induced the growth of individual graphene domains with relatively low nucleation density (102 cm<sup>2</sup> ) owing to the catalyst passivation of oxide layer on top of Cu. Through annealing in 30 min (Figure 5d), the nucleation density reduced 12 nuclei/cm<sup>2</sup> , and a grain size of 1 mm was obtained after 60 min growth (Figure 5e). But the extending of annealing (120 min) caused the increase of nucleation density with reduced domain size (0.6 mm) (Figure 5f). The evolutions of nucleation densities and domain sizes via annealing are shown in Figure 5g. The nucleation density reveals a nonmonotonic dependence of Ar-passivated annealing. Since the surface becomes flatter after annealing, the O-species also gradually desorb at surface, which leaves pristine Cu as active nucleation sites following the growth steps.

Thermodynamics of CVD-based graphene synthesis using catalytic Cu at particular temperature is same irrespective at atmosphere pressure (AP), low pressure (LP), or ultrahigh vacuum (UHV) [79]. But, the kinetics will vary on different steps. The kinetics of cooling rate, the pressure of CVD furnace has major ramification on the graphene growth rate, large-scale thickness uniformity, and the defect density. Figure 6A illustrates a steady state flow of a mixture of CH4, H2, and Ar precursors on Cu surface at 1000C [79]. The boundary layer because of steady state gas flow is stagnant. Firstly, the carbon species (1) diffuse via the boundary layer to the surface, then (2) will be adsorbed on the surface, (3) decompose for formation of active carbon species, (4) diffuse on/into the catalyst surface and forming the graphene lattice, (5) inactive species (e.g. H2) get desorbed, forming H2, then (6) diffuse away through boundary layer and swept away by the bulk gas flow [79]. Processes, which occur on/ close the surface, are highly affected by substrate temperature. Generally, there are two fluxes of active species: flux of active species via boundary layer and at metal substrate surface forming graphene lattice (Figure 6B) [79]. The equations of these fluxes are Fmass transport = hg(Cg – Cs ) and Fsurface reaction = KsCs where, Fmass-transport is the flux of active species through the boundary layer, Fsurface-reaction is the flux of consumed active species at surface, hg is the mass transport coefficient, Ks is the surface reaction constant, Cg is the concentration of gas in bulk, and Cs is the concentration of active species at the surface [79]. At high temperatures, under APCVD parameters, mass transport via boundary layer is rate limiting (Ks » hg), and under LP and UHV parameters, the surface reaction is the rate limiting step (hg » Ks) [79].

a lot. The temperature, the time, and flow rate permit a sophisticated controlling on carbon

Figure 7. Graphene growth mechanism on new designed APCVD with a bubbler. A single gas inlet is split into two paths (P1 and P2) using glass. During heating/annealing/cooling, gas is passed through P1; while for graphene growth, gas flows through P2 and bubbler containing alcohol. Inside the tubular furnace a combustion boat is placed in the center

Atmospheric Pressure Chemical Vapor Deposition of Graphene

http://dx.doi.org/10.5772/intechopen.81293

123

The additional oxide substrate (SiO2, Al2O3) to continuously supply the oxygen (O2) to Cu surface of APCVD graphene synthesis (Figure 8a,b) [93]. Cu was placed on oxide substrates away in 15 μm gap. Last report proved that the oxide shall slowly release O2 at above 800C) [97]. To clear it, Xu et al. carried out Auger electron spectroscopy (AES) element measurement on SiO2, as the result, O2 escaped from SiO2 after annealing in UHV or CVD (Figure 8c) [93]. Despite the small amount of O2 released, the O2 concentration between the narrow gap (15 μm) of Cu and oxide substrate could be high because of the trapping effect; hence, the O2 attachment to Cu surface significantly trapped. Beside AP condition, high CH4 flow (5 sccm) and high CH4/H2 ratio (1) ensure a sufficient supply of carbon for ultrafast domain synthesis. The large domains (0.3 mm) appeared on the back surface of Cu (Figure 8d). Contrarily, graphene domains at front surface of Cu are 20 times smaller (15 μm) (Figure 8e). All these domains are star-shaped. For further demonstration, when graphite is used as the supporting substrate, graphene domains on both sides of Cu are also star-shaped similar with the case of

Figure 8h,i revealed OM images of graphene/Cu in 60 and 80 min under CH4 0.000075 Pa [94]. The graphenes needed to place in air (200C, 1 min) for chemical oxidation between Cu and air to emerge the nonoxidized-graphene domains for the observation by OM and human-eye. Graphene partly covered the Cu (70%) after 60 min. Some isolated domains were 1.5 mm (Figure 8h) [94]. But the color as well as contrast of sample were of no change after 80 min implying that Cu got full coverage by graphene via OM image (Figure 8i) [94]. The histogram in Figure 8j showed the growth distribution of domain sizes for 50–60 min [94]. The distribution of domain sizes increased via growth time; the domain sizes were 0.4–0.8 mm for 50–60 min. The time evolution of average domain size and coverage were clearly described in

A growth at APCVD in Mohsin et al. is to avoid Cu evaporation from the foil substrate, which commonly appears in LPCVD case [95, 98]. Regarding Figure 9, firstly Cu is placed on a tungsten (W) foil for preventing the dewetting of liquid Cu on quartz. Then, it was heated up 1100C, which is higher melting point of Cu for 30 min under Ar (940 sccm) and H2 (60 sccm).

Figure 8k [94]. The domain sizes for 100% coverage after 80 min was 1 mm [94].

concentration toward tubular furnace (Figure 7) [92].

containing a Cu foil. Reproduced with permission from [92]. Copyright 2013, Elsevier.

SiO2 (Figure 8g) [93].

A diagram of designed-APCVD for graphene synthesis is described in Figure 7. This splits one gas inlet in two paths by means of glass valves: one for heating, annealing, and cooling (P1); and the other for graphene synthesis (P2). The synthesis temperature is 980–990C in tubular furnace; during heating, the Ar-H2 (5% of H2) flow is 0.2 l/min via P1. Then Cu is annealed in 20–30 min. Then, the flow passed via alcohol container (P2) to desired growth rate, and the varied time depended on precursor type. Keeping the flow rate, the flux is to P1 and the sample is annealed in 10 min. As the result, this annealing step improved the graphene quality

Figure 6. (A) Schematic for graphene growth mechanism using low carbon solid solubility catalysts (Cu) at atmosphere pressure (AP), low pressure (LP), or ultrahigh vacuum (UHV) environments. (B) Mass transport and surface reaction fluxes under steady state conditions. Reproduced with permission from [79]. Copyright 2010, American Chemical Society.

annealing, the O-species also gradually desorb at surface, which leaves pristine Cu as active

Thermodynamics of CVD-based graphene synthesis using catalytic Cu at particular temperature is same irrespective at atmosphere pressure (AP), low pressure (LP), or ultrahigh vacuum (UHV) [79]. But, the kinetics will vary on different steps. The kinetics of cooling rate, the pressure of CVD furnace has major ramification on the graphene growth rate, large-scale thickness uniformity, and the defect density. Figure 6A illustrates a steady state flow of a mixture of CH4, H2, and Ar precursors on Cu surface at 1000C [79]. The boundary layer because of steady state gas flow is stagnant. Firstly, the carbon species (1) diffuse via the boundary layer to the surface, then (2) will be adsorbed on the surface, (3) decompose for formation of active carbon species, (4) diffuse on/into the catalyst surface and forming the graphene lattice, (5) inactive species (e.g. H2) get desorbed, forming H2, then (6) diffuse away through boundary layer and swept away by the bulk gas flow [79]. Processes, which occur on/ close the surface, are highly affected by substrate temperature. Generally, there are two fluxes of active species: flux of active species via boundary layer and at metal substrate surface forming graphene lattice (Figure 6B) [79]. The equations of these fluxes are Fmass transport = hg(Cg

) and Fsurface reaction = KsCs where, Fmass-transport is the flux of active species through the boundary layer, Fsurface-reaction is the flux of consumed active species at surface, hg is the mass transport coefficient, Ks is the surface reaction constant, Cg is the concentration of gas in bulk, and Cs is the concentration of active species at the surface [79]. At high temperatures, under APCVD parameters, mass transport via boundary layer is rate limiting (Ks » hg), and under LP

A diagram of designed-APCVD for graphene synthesis is described in Figure 7. This splits one gas inlet in two paths by means of glass valves: one for heating, annealing, and cooling (P1); and the other for graphene synthesis (P2). The synthesis temperature is 980–990C in tubular furnace; during heating, the Ar-H2 (5% of H2) flow is 0.2 l/min via P1. Then Cu is annealed in 20–30 min. Then, the flow passed via alcohol container (P2) to desired growth rate, and the varied time depended on precursor type. Keeping the flow rate, the flux is to P1 and the sample is annealed in 10 min. As the result, this annealing step improved the graphene quality

Figure 6. (A) Schematic for graphene growth mechanism using low carbon solid solubility catalysts (Cu) at atmosphere pressure (AP), low pressure (LP), or ultrahigh vacuum (UHV) environments. (B) Mass transport and surface reaction fluxes under steady state conditions. Reproduced with permission from [79]. Copyright 2010, American Chemical Society.

and UHV parameters, the surface reaction is the rate limiting step (hg » Ks) [79].

nucleation sites following the growth steps.

122 Chemical Vapor Deposition for Nanotechnology

– Cs

Figure 7. Graphene growth mechanism on new designed APCVD with a bubbler. A single gas inlet is split into two paths (P1 and P2) using glass. During heating/annealing/cooling, gas is passed through P1; while for graphene growth, gas flows through P2 and bubbler containing alcohol. Inside the tubular furnace a combustion boat is placed in the center containing a Cu foil. Reproduced with permission from [92]. Copyright 2013, Elsevier.

a lot. The temperature, the time, and flow rate permit a sophisticated controlling on carbon concentration toward tubular furnace (Figure 7) [92].

The additional oxide substrate (SiO2, Al2O3) to continuously supply the oxygen (O2) to Cu surface of APCVD graphene synthesis (Figure 8a,b) [93]. Cu was placed on oxide substrates away in 15 μm gap. Last report proved that the oxide shall slowly release O2 at above 800C) [97]. To clear it, Xu et al. carried out Auger electron spectroscopy (AES) element measurement on SiO2, as the result, O2 escaped from SiO2 after annealing in UHV or CVD (Figure 8c) [93]. Despite the small amount of O2 released, the O2 concentration between the narrow gap (15 μm) of Cu and oxide substrate could be high because of the trapping effect; hence, the O2 attachment to Cu surface significantly trapped. Beside AP condition, high CH4 flow (5 sccm) and high CH4/H2 ratio (1) ensure a sufficient supply of carbon for ultrafast domain synthesis. The large domains (0.3 mm) appeared on the back surface of Cu (Figure 8d). Contrarily, graphene domains at front surface of Cu are 20 times smaller (15 μm) (Figure 8e). All these domains are star-shaped. For further demonstration, when graphite is used as the supporting substrate, graphene domains on both sides of Cu are also star-shaped similar with the case of SiO2 (Figure 8g) [93].

Figure 8h,i revealed OM images of graphene/Cu in 60 and 80 min under CH4 0.000075 Pa [94]. The graphenes needed to place in air (200C, 1 min) for chemical oxidation between Cu and air to emerge the nonoxidized-graphene domains for the observation by OM and human-eye. Graphene partly covered the Cu (70%) after 60 min. Some isolated domains were 1.5 mm (Figure 8h) [94]. But the color as well as contrast of sample were of no change after 80 min implying that Cu got full coverage by graphene via OM image (Figure 8i) [94]. The histogram in Figure 8j showed the growth distribution of domain sizes for 50–60 min [94]. The distribution of domain sizes increased via growth time; the domain sizes were 0.4–0.8 mm for 50–60 min. The time evolution of average domain size and coverage were clearly described in Figure 8k [94]. The domain sizes for 100% coverage after 80 min was 1 mm [94].

A growth at APCVD in Mohsin et al. is to avoid Cu evaporation from the foil substrate, which commonly appears in LPCVD case [95, 98]. Regarding Figure 9, firstly Cu is placed on a tungsten (W) foil for preventing the dewetting of liquid Cu on quartz. Then, it was heated up 1100C, which is higher melting point of Cu for 30 min under Ar (940 sccm) and H2 (60 sccm).

Figure 8. (a) Schematic of graphene growth on Cu using O2-assisted APCVD. (b) Side view of (a). (c) Auger electron spectroscopy (AES) of SiO2 before/after annealing at 1000C in UHV and APCVD systems. The disappearance of O2 peak after annealing proves the emission of O2 from the oxide at high temperature. (d, e) OM images of graphene domains on the back (d) and front (e) Cu surfaces using fused silica as the supporting substrate. (f, g) OM images of graphene domains on the back Cu surfaces using sapphire (Al2O3) (f) or graphite (g). Reproduced with permission from [93]. Copyright 2016, Nature Publishing Group. (h, i) OM images of graphene/Cu after 60 and 80 min, respectively. APCVD conditions: 1 atm, 1030C, 0.3 sccm CH4, 80 sccm H2, and 3920 sccm Ar. (j) Histogram of domain size for 50 and 60 min growth. (k) The graphene coverage and the average domain size as a function of growth time. Reproduced with permission from [94]. Copyright 2016, Nature Publishing Group.

APCVD-based bilayer graphene growth is another great issue for optimum electronic and photonic devices because of higher carrier mobility and wider band gap by perpendicular electric field compared with single-layer graphene [99, 100]. The synthesis of bilayer graphene faced many drawbacks owing to limited grain size and nonsynchronic growth between the first and the second graphene layer. In 2016, Sun et al. reported a cooling-APCVD to growth of bilayer graphene on polycrystalline Cu (Figure 10) [99]. Here, the surface adsorption for decomposed carbons and phase segregation for dissolved carbon were shown. Consequently, the millimeter-scale hexagonal bilayer graphene (1.0 mm) was produced. This study opens the possibility to grow centimeter-scale bilayer graphene for optimum graphene-based appli-

Figure 10. (a) Schematic of bilayer graphene growth using a cooling APCVD. (b, c) SEM and OM images of bilayer graphene domains, respectively. (d) Rama spectra of bilayer graphene. Reproduced with permission from [99]. Copyright

Figure 9. (a) Simple method to grow millimeter-size graphene single crystals on melted and resolidified Cu using APCVD. (b) OM image of the synthesized graphene domains. Reproduced with permission from [95]. Copyright 2013,

Atmospheric Pressure Chemical Vapor Deposition of Graphene

http://dx.doi.org/10.5772/intechopen.81293

125

Strategies for direct graphene growth using APCVD method have been reviewed. The prospects of APCVD-grown graphene are bright and currently receiving considerable attention

cations in recent future.

2016, American Chemical Society.

American Chemical Society.

4. Conclusions

Then, the temperature is lowered to 1075C, and the Cu resolidified. Growth was carried out at this temperature, with 0.1% dilute CH4 in Ar. On the right side of Figure 9, it shows OM image of two hexagon domains of graphene (>1 mm) which has similar morphology [95]. These domains obtained in Mohsin et al. are hexagons with very less roughness edges compared to previous reports [60, 89].

Figure 9. (a) Simple method to grow millimeter-size graphene single crystals on melted and resolidified Cu using APCVD. (b) OM image of the synthesized graphene domains. Reproduced with permission from [95]. Copyright 2013, American Chemical Society.

Figure 10. (a) Schematic of bilayer graphene growth using a cooling APCVD. (b, c) SEM and OM images of bilayer graphene domains, respectively. (d) Rama spectra of bilayer graphene. Reproduced with permission from [99]. Copyright 2016, American Chemical Society.

APCVD-based bilayer graphene growth is another great issue for optimum electronic and photonic devices because of higher carrier mobility and wider band gap by perpendicular electric field compared with single-layer graphene [99, 100]. The synthesis of bilayer graphene faced many drawbacks owing to limited grain size and nonsynchronic growth between the first and the second graphene layer. In 2016, Sun et al. reported a cooling-APCVD to growth of bilayer graphene on polycrystalline Cu (Figure 10) [99]. Here, the surface adsorption for decomposed carbons and phase segregation for dissolved carbon were shown. Consequently, the millimeter-scale hexagonal bilayer graphene (1.0 mm) was produced. This study opens the possibility to grow centimeter-scale bilayer graphene for optimum graphene-based applications in recent future.

#### 4. Conclusions

Then, the temperature is lowered to 1075C, and the Cu resolidified. Growth was carried out at this temperature, with 0.1% dilute CH4 in Ar. On the right side of Figure 9, it shows OM image of two hexagon domains of graphene (>1 mm) which has similar morphology [95]. These domains obtained in Mohsin et al. are hexagons with very less roughness edges compared to

Figure 8. (a) Schematic of graphene growth on Cu using O2-assisted APCVD. (b) Side view of (a). (c) Auger electron spectroscopy (AES) of SiO2 before/after annealing at 1000C in UHV and APCVD systems. The disappearance of O2 peak after annealing proves the emission of O2 from the oxide at high temperature. (d, e) OM images of graphene domains on the back (d) and front (e) Cu surfaces using fused silica as the supporting substrate. (f, g) OM images of graphene domains on the back Cu surfaces using sapphire (Al2O3) (f) or graphite (g). Reproduced with permission from [93]. Copyright 2016, Nature Publishing Group. (h, i) OM images of graphene/Cu after 60 and 80 min, respectively. APCVD conditions: 1 atm, 1030C, 0.3 sccm CH4, 80 sccm H2, and 3920 sccm Ar. (j) Histogram of domain size for 50 and 60 min growth. (k) The graphene coverage and the average domain size as a function of growth time. Reproduced with permission from [94].

previous reports [60, 89].

Copyright 2016, Nature Publishing Group.

124 Chemical Vapor Deposition for Nanotechnology

Strategies for direct graphene growth using APCVD method have been reviewed. The prospects of APCVD-grown graphene are bright and currently receiving considerable attention from the 2D material research community. Yet, understanding the growth process and conditions that affect the quality of graphene is not adequate.

[2] Kim K, Choi JY, Kim T, Cho SH, Chung HJ. A role for graphene in silicon-based semi-

Atmospheric Pressure Chemical Vapor Deposition of Graphene

http://dx.doi.org/10.5772/intechopen.81293

127

[3] Duong DL, Han GH, Lee SM, Gunes F, Kim ES, Kim ST, et al. Probing graphene grain boundaries with optical miscroscopy. Nature. 2012;490:235-239. DOI: 10.1038/nature11562

[4] Choi J-Y. Graphene transfer: A stamp for all substrates. Nature Nanotechnology. 2013;8:

[5] Lee JH, Lee EK, Joo WJ, Jang Y, Kim BS, Lim JY, et al. Wafer-scale growth of single-crystal monolayer graphene on reusable hydrogen-terminated germanium. Science. 2014;344:

[6] Shin HJ, Choi WM, Yoon SM, Han GH, Woo YS, Kim ES, et al. Transfer-free growth of fewlayer graphene by self-assembled monolayers. Advanced Materials. 2011;23:4392-4397.

[7] Chae SJ, Güneş F, Kim KK, Kim ES, Han GH, Kim SM, et al. Synthesis of large-area graphene layers on poly-nickel substrate by chemical vapor deposition: Wrinkle forma-

[8] Güneş F, Shin HJ, Biswas C, Han GH, Kim ES, Chae SJ, et al. Layer-by-layer doping of few-layer graphene film. ACS Nano. 2010;4:4595-4600. DOI: 10.1021/nn1008808

[9] Pham VP, Jang HS, Whang D, Choi JY. Direct growth of graphene on rigid and flexible substrates: Progress, applications, and challenges. Chemical Society Reviews. 2017;46:

[10] Pham VP. A library of doped-graphene images via transmission electron microscopy. C.

[11] Pham VP, Nguyen MT, Park JW, Kwak SS, Nguyen DHT, Mun MK, et al. Chlorinetrapped CVD bilayer graphene for resistive pressure sensor with high detection limit

and high sensitivity. 2D Materials. 2017;4:025049. DOI: 10.1088/2053-1583/aa6390

[12] Pham VP, Kim KN, Jeon MH, Kim KS, Yeom GY. Cyclic chlorine trap-doping for transparent, conductive, thermally stable and damage-free graphene. Nanoscale. 2014;6:

[13] Pham VP, Kim KH, Jeon MH, Lee SH, Kim KN, Yeom GY. Low damage pre-doping on CVD graphene/Cu using a chlorine inductively coupled plasma. Carbon. 2015;95:

[14] Pham VP, Mishra A, Yeom GY. The enhancement of hall mobility and conductivity of CVD graphene through radical doping and vacuum annealing. RSC Advances. 2017;7:

[15] Pham VP. Cleaning of graphene surfaces by low pressure air plasma. Royal Society Open

tion. Advanced Materials. 2009;21:2328-2333. DOI: 10.1002/adma.200803016

conductor devices. Nature. 2011;479:338-344. DOI: 10.1038/nature10680

311-312. DOI: 10.1038/nnano.2013.74

286-289. DOI: 10.1126/science.1252268

DOI: 10.1002/adma.201102526

6276-6300. DOI: 10.1039/c7cs00224f

2018;4:34. DOI: 10.3390/c4020034

15301-15308. DOI: 10.1039/c4nr04387a

664-671. DOI: 10.1016/j.carbon.2015.08.070

16104-16108. DOI: 10.1039/c7ra01330b

Science. 2018;5:172395. DOI: 10.1098/rsos.172395

Because APCVD growth depends on thermal decomposition of carbon resource, the growth rate is usually low and size of the graphene domain is small, resulting in growth of defective graphene layer. To date, the challenge remains for this research directions and large-scale highquality graphene productions are still hard. In order to obtain more advanced results, an indepth mechanism understanding of APCVD-based graphene growth is essential.

Cu is considered as the best substrate owing to low-carbon solubility, well-controlled surface, and inexpensive for growing monolayer graphene. Generally, graphene is synthesized on Cu via APCVD parameters utilizing CH4 precursor revealed that the growth is different from single layer graphene at low CH4 concentration to multi-layer domain on a single layer at higher CH4 concentration. In particular, APCVD-based graphene growth at higher CH4 concentration has no self-limiting compared with LPCVD, indicating that further investigation is needed to point out the growth mechanism. The kinetic processes utilizing low carbon solubility catalyst via APCVD parameters were presented.

Finally, APCVD-grown graphene on flexible/metal/dielectric substrates assisted by metal powder precursors (solid, gas, and solution) contained inside a sub-chamber for direct evaporation into an innovative APCVD main chamber allowing hexagonal domain formation on flexible/ metal/dielectric substrates is the secret topic and needs to be exploited in the coming time.

## Conflict of interest

There are no conflicts of interest to declare.

## Author details

Phuong V. Pham1,2\*

\*Address all correspondence to: pvphuong@skku.edu

1 SKKU Advanced Institute of Nano Technology (SAINT), Sungkyunkwan University (SKKU), Suwon, Gyeonggi-do, South Korea

2 Center for Multidimensional Carbon Materials, Institute for Basic Science, Ulsan, South Korea

## References

[1] Kim KS, Zhao Y, Jang H, Lee SY, Kim JM, Kim KS, et al. Large-scale pattern growth of graphene films for stretchable. Nature. 2009;457:706-710. DOI: 10.1038/nature07719

[2] Kim K, Choi JY, Kim T, Cho SH, Chung HJ. A role for graphene in silicon-based semiconductor devices. Nature. 2011;479:338-344. DOI: 10.1038/nature10680

from the 2D material research community. Yet, understanding the growth process and condi-

Because APCVD growth depends on thermal decomposition of carbon resource, the growth rate is usually low and size of the graphene domain is small, resulting in growth of defective graphene layer. To date, the challenge remains for this research directions and large-scale highquality graphene productions are still hard. In order to obtain more advanced results, an in-

Cu is considered as the best substrate owing to low-carbon solubility, well-controlled surface, and inexpensive for growing monolayer graphene. Generally, graphene is synthesized on Cu via APCVD parameters utilizing CH4 precursor revealed that the growth is different from single layer graphene at low CH4 concentration to multi-layer domain on a single layer at higher CH4 concentration. In particular, APCVD-based graphene growth at higher CH4 concentration has no self-limiting compared with LPCVD, indicating that further investigation is needed to point out the growth mechanism. The kinetic processes utilizing low carbon solu-

Finally, APCVD-grown graphene on flexible/metal/dielectric substrates assisted by metal powder precursors (solid, gas, and solution) contained inside a sub-chamber for direct evaporation into an innovative APCVD main chamber allowing hexagonal domain formation on flexible/ metal/dielectric substrates is the secret topic and needs to be exploited in the coming time.

1 SKKU Advanced Institute of Nano Technology (SAINT), Sungkyunkwan University

[1] Kim KS, Zhao Y, Jang H, Lee SY, Kim JM, Kim KS, et al. Large-scale pattern growth of graphene films for stretchable. Nature. 2009;457:706-710. DOI: 10.1038/nature07719

2 Center for Multidimensional Carbon Materials, Institute for Basic Science, Ulsan,

depth mechanism understanding of APCVD-based graphene growth is essential.

tions that affect the quality of graphene is not adequate.

126 Chemical Vapor Deposition for Nanotechnology

bility catalyst via APCVD parameters were presented.

Conflict of interest

Author details

Phuong V. Pham1,2\*

South Korea

References

There are no conflicts of interest to declare.

(SKKU), Suwon, Gyeonggi-do, South Korea

\*Address all correspondence to: pvphuong@skku.edu


[16] Pham VP, Kim DS, Kim KS, Park JW, Yang KC, Lee SH, et al. Low energy BCl3 plasma doping of few-layer graphene. Science of Advanced Materials. 2016;8:884-890. DOI: 10.1166/sam.2016.2549

[31] Sutter PW, Flege JI, Sutter EA. Epitaxial graphene on ruthenium. Nature Materials. 2008;

Atmospheric Pressure Chemical Vapor Deposition of Graphene

http://dx.doi.org/10.5772/intechopen.81293

129

[32] Berger C, Song Z, Li X, Wu X, Brown N, Naud C, et al. Electronic confinement and coherence in patterned epitaxial graphene. Science. 2006;312:1191-1196. DOI: 10.1126/

[33] Emtsev KV, Speck F, Seyller T, Ley L, Riley JD. Interaction, growth, and ordering of epitaxial graphene on SiC{0001} surfaces: A comparative photoelectron spectroscopy

[34] Hass J, WAd H, Conrad EH. The growth and morphology of epitaxial multilayer graphene. Journal of Physics. Condensed Matter. 2008;20:323202. DOI: 10.1088/0953-8984/20/

[35] Su CY, Xu Y, Zhang W, Zhao J, Liu A, Tang X, et al. Highly efficient restoration of graphitic structure in graphene oxide using alcohol vapors. ACS Nano. 2010;4:5285-5292.

[36] Williams G, Seger B, Kamat PV. TiO2-graphene nanocomposites UV-assisted photocatalytic reduction of graphene oxide. ACS Nano. 2008;2:1487-1491. DOI: 10.1021/nn800251f [37] Green AA, Hersam MC. Solution phase production of graphene with controlled thickness via density differentiation. Nano Letters. 2009;9:4031-4036. DOI: 10.1021/nl902200b

[38] Cote LJ, Kim F, Huang J. Langmuir-Blodgett assembly of graphite oxide singl layers. Journal of the American Chemical Society. 2008;131:1043-1049. DOI: 10.1021/ja806262m

[39] Li D, Müller MB, Gilje S, Kaner RB, Wallace GG. Processable aquaous dispersions of graphene nanosheets. Nature Nanotechnology. 2008;3:101-105. DOI: 10.1038/nnano.

[40] Gao W, Alemany LB, Ci L, Ajayan PM. New insights into the structure and reduction of

[41] Joshi RK, Alwarappan S, Yoshimura M, Sahajwalla V. Graphene oxide: The new membrane material. Applied Materials Today. 2015;1:1-12. DOI: 10.1016/j.apmt.2015.06.002

[42] Coraux J, Diaye ATN, Busse C, Michely T. Structural coherency of graphene on Ir(111).

[43] Lee Y, Bae S, Jang H, Jang S, Zhu SE, Sim SH, et al. Wafer-scale synthesis and transfer of

[44] Reina A, Jia X, Ho J, Nezich D, Son H, Bulovic V, et al. Large area, few-layer graphene films on arbitrary substrates by chemical vapor deposition. Nano Letters. 2008;9:30-35.

[45] Li X, Cai W, An J, Kim S, Nah J, Yang D, et al. Large-area synthesis of high-quality and uniform graphene films on copper foils. Science. 2009;324:1312-1314. DOI: 10.1126/science.

graphite oxide. Nature Chemistry. 2009;1:403-408. DOI: 10.1038/nchem.281

graphene films. Nano Letters. 2010;10:490-493. DOI: 10.1021/nl903272n

Nano Letters. 2008;8:565-570. DOI: 10.1021/nl0728874

study. Physical Review B. 2008;77:155303. DOI: 10.1103/PhysRevB.77.155303

7:406-411. DOI: 10.1038/nmat2166

science.1125925

32/323202

2007.451

DOI: 10.1021/nn10169m

DOI: 10.1021/nl801827v

1171245


[31] Sutter PW, Flege JI, Sutter EA. Epitaxial graphene on ruthenium. Nature Materials. 2008; 7:406-411. DOI: 10.1038/nmat2166

[16] Pham VP, Kim DS, Kim KS, Park JW, Yang KC, Lee SH, et al. Low energy BCl3 plasma doping of few-layer graphene. Science of Advanced Materials. 2016;8:884-890. DOI:

[17] Kim KN, Pham VP, Yeom GY. Chlorine radical doping of a few layer graphene with low damage. ECS Journal of Solid State Science and Technology. 2015;4:N5095-N5097. DOI:

[18] Pham VP. Chemical vapor deposited graphene synthesis with same-oriented hexagonal

[19] Pham VP. How can the nanomaterial surfaces be highly cleaned? Edelweiss Applied

[20] Pham VP. Layer-by-layer thinning of 2D materials. Edelweiss Applied Science and Tech-

[21] Pham VP. Plasma-related graphene etching: A mini review. Journal of Materials Science and Engineering with Advanced Technology. 2018;17:91-106. DOI: 10.18642/jmseat\_

[22] Pham VP. Graphene Etching: How Could it be Etched? Current Graphene Science; 2018

[23] Geim AK, Novoselov KS. The rise of graphene. Nature Materials. 2007;6:183-191. DOI:

[24] Neto AHC, Guinea F, Peres NMR, Novoselov KS, Geim AK. The electronic properties of graphene. Reviews of Modern Physics. 2009;81:109-162. DOI: 10.1103/RevModPhys.81.109

[25] Schwierz F. Graphene transistor. Nature Nanotechnology. 2010;5:487-496. DOI: 10.1038/

[26] Wilson NR, Macpherson JV. Carbon nanotube tips for atomic force microscopy. Nature

[27] Lin YM, Dimitrakopoulos C, Jenkins KA, Farmer DB, Chiu HY, Grill A, et al. 100GHz transistor from wafer-scale epitaxial graphene. Science. 2010;327:662. DOI: 10.1126/science.

[28] Yan Z, Peng Z, Tour JM. Chemical vapor deposition of graphene single crystals.

[29] Su CY, Lu AY, Xu Y, Chen FR, Khlobystov AN, Li LJ. High-quality thin graphene films from fast electrochemical exfoliation. ACS Nano. 2011;5:2332-2339. DOI: 10.1021/

[30] Hernandez Y, Nicolosi V, Lotya M, Blighe FM, Sun Z, De S, et al. High-yield production of graphene by liquid-phase exfoliation of graphite. Nature Nanotechnology. 2008;3:

Accounts of Chemical Research. 2014;47:1327-1337. DOI: 10.1021/ar4003043

Nanotechnology. 2009;4:483-491. DOI: 10.1038/nnano.2009.154

domains. Engineering Press. 2018;1:39-42. DOI: 10.28964/EngPress-1-107

10.1166/sam.2016.2549

128 Chemical Vapor Deposition for Nanotechnology

10.1149/2.0141506jss

nology. 2018;2:36-37

7100121943

(Accepted)

10.1038/nmat1849

nnano.2010.89

1184289

nn200025p

563-568. DOI: 10.1038/nnano.2008.215

Science and Technology. 2018;2:184-186


[46] Lee S, Lee K, Zhong Z. Wafer scale homogeneous bilayer graphene films by chemical vapor deposition. Nano Letters. 2010;10:4702-4707. DOI: 10.1021/nl1029978

[60] Chen S, Ji H, Chou H, Li Q, Li H, Suk JW, et al. Milimeter-size single-crystal graphene by suppressing evaporative loss of Cu during low pressure chemical vapor deposition.

Atmospheric Pressure Chemical Vapor Deposition of Graphene

http://dx.doi.org/10.5772/intechopen.81293

131

[61] Mohsin A, Liu L, Liu P, Deng W, Ivanov IN, Li G, et al. Synthesis of miloimeter-size hexagonshaped graphene single crystals on resolidified copper. ACS Nano. 2013;7:8924-8931. DOI:

[62] Wu T, Ding G, Shen H, Wang H, Sun L, Jiang D, et al. Triggering the continuous growth of growth toward milimeter-sized grains. Advanced Functional Materials. 2013;23:

[63] Gan L, Luo Z. Turning off hydrogen to realize seeded growth of subcentimeter singlecrystal graphene grains on copper. ACS Nano. 2013;7:9480-9488. DOI: 10.1021/nn404

[64] Zhou H, Yu WJ, Liu L, Cheng R, Chen Y, Huang X, et al. Chemical vapour deposition growth of large single crystals of monolayer and bilayer graphene. Nature Communica-

[65] Hao Y, Bharathi MS, Wang L, Liu Y, Chen H, Nie S, et al. The role of surface oxygen in the growth of large single-crystal gtaphene on copper. Science. 2013;342:720-723. DOI:

[66] Nguyen VL, Shin BG, Duong DL, Kim ST, Perello D, Lim YJ, et al. Seamless stiching of graphene domains on polished copper (111) foil. Advanced Materials. 2015;27:1376-1382.

[67] Kim K, Lee Z, Regan W, Kisielowski C, Crommie MF, Zettl A. Grain bourdary mapping

[68] Huang PY, Vargas CSR, van der Zande AM, Whitney WS, Levendorf MP, Kevek JW, et al. Grains and grain boundaries in single-layer graphene atomic patchwork quilts.

[69] Rasool HI, Ophus C, Klug WS, Zettl A, Gimzewski JK. Measurement of the intrinsic strength of crystalline and polycrystalline graphene. Nature Communications. 2013;4:

[70] Lee C, Wei X, Kysar JW, Hone J. Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science. 2008;321:385-388. DOI: 10.1126/science.1157996

[71] Tuan DV, Kotakoski J, Louvet T, Ortmann F, Meyer JC, Roche S. Scaling properties of charge transport in polycrystalline graphene. Nano Letters. 2013;13:1730-1735. DOI:

[72] Zhang H, Lee G, Gong C, Colombo L, Cho K. Grain boundary effect on electrical transport properties of graphene. Journal of Physical Chemistry C. 2014;118:2338-2343.

Advanced Materials. 2013;25:2062-2065. DOI: 10.1002/adma.201204000

10.1021/nn4034019

10.1126/science.1243879

DOI: 10.1002/adma.201404541

Nature. 2011;469:389-392

10.1021/nl400321r

DOI: 10.1021/jp411464w

2811. DOI: 10.1038/ncomms3811

393b

198-203. DOI: 10.1002/adfm.201201577

tions. 2013;4:2096. DOI: 10.1038/ncomms3096

in polycrystalline graphene. ACS Nano. 2011;5:2142-2146


[60] Chen S, Ji H, Chou H, Li Q, Li H, Suk JW, et al. Milimeter-size single-crystal graphene by suppressing evaporative loss of Cu during low pressure chemical vapor deposition. Advanced Materials. 2013;25:2062-2065. DOI: 10.1002/adma.201204000

[46] Lee S, Lee K, Zhong Z. Wafer scale homogeneous bilayer graphene films by chemical

[47] Yan K, Peng H, Zhou Y, Li H, Liu Z. Formation of bilayer Bernal graphene: Layer-bylayer epitaxy via chemical vapor deposition. Nano Letters. 2011;11:1106-1110. DOI:

[48] Sun Z, Yan Z, Yao J, Beitler E, Zhu Y, Tour JM. Growth of graphene from solid carbon

[49] Chen Y, Gong XZ, Gai JG. Progress and challenges in transfer of large-area graphene films. Advancement of Science. 2016;3:1500343. DOI: 10.1002/advs.201500343

[50] Sun J, Deng S, Guo W, Zhan Z, Deng J, Xu C, et al. Electrochemical bubbling transfer of graphene using a polymer support with encapsulated air gap as permeation stopping

[51] Cherian CT, Giustiniano F, Martin-Fernandez I, Andersen H, Balakrishnan J, Özyilmaz B. Bubble-free electrochemical delamination of CVD graphene films. Small. 2015;11:

[52] Mafra D, Ming T, Kong J. Facial graphene transfer directly to target substrates with a reusable metal catalyst. Nanoscale. 2015;7:14807-14812. DOI: 10.1039/C5NR3892H [53] Wang Y, Zheng Y, Xu X, Dubuisson E, Bao Q, Lu J, et al. Electrochemical delamination of CVD-grown graphene film: Toward the recyclable use of copper catalyst. ACS Nano.

[54] Gao L, Ren W, Xu H, Jin L, Wang Z, Ma T, et al. Repeated growth and bubbling transfer of graphene with millimetre-size single-crystal grains using platinum. Nature Communica-

[55] Li X, Cai W, Colombo L, Ruoff RS. Evolution of graphene growth on Ni and Cu by carbon isotope labeling. Nano Letters. 2009;9:4268-4272. DOI: 10.1021/nl902515k

[56] Wang H, Wang G, Bao P, Yang S, Zhu W, Xie X, et al. Controllable synthesis of submilimeter single-crystal monolayer graphene domains on copper foils by suppressing nucleation. Journal of the American Chemical Society. 2012;134:3627-3630. DOI: 10.1021/ja2105976 [57] Eres G, Regmi M, Rouleau CM, Chen J, Ivanov IN, Puretzky AA, et al. Cooperative island growth of large-area single-crystal graphene on copper using chemical vapor

[58] Wang C, Chen W, Han C, Wang G, Tang B, Tang C, et al. Growth of millimeter-size single crystal graphene on Cu foils by circumfluence chemical vapor deposition. Scientific

[59] Li X, Magnuson CW, Venugopal A, Tromp RM, Hannon JB, Vogel EM, et al. Large-area graphene single crystals grown by low-pressure chemical vapor deposition of methane on copper. Journal of the American Chemical Society. 2011;133:2816-2819. DOI: 10.1021/

deposition. ACS Nano. 2014;8:5657-5669. DOI: 10.1021/nn500209d

layer. Journal of Nanomaterials. 2016;2016:1-7. DOI: 10.1155/2016/7024246

vapor deposition. Nano Letters. 2010;10:4702-4707. DOI: 10.1021/nl1029978

sources. Nature. 2010;468:549-552. DOI: 10.1038/nature09579

10.1021/nl104000b

130 Chemical Vapor Deposition for Nanotechnology

189-194. DOI: 10.1002/smll.201402024

2011;5:9927-9933. DOI: 10.1021/nn203700w

tions. 2012;3:699. DOI: 10.1038/ncomms1702

Reports. 2014;4:4537. DOI: 10.1038/srep04537

ja109793s


[73] Boukhvalov DW, Katsnelson MI. Chemical functionalization of graphene with defects. Nano Letters. 2008;8:4373-4379. DOI: 10.1021/nl802234n

[87] Tu Z, Liu Z, Li Y, Yang F, Zhang L, Zhao Z, et al. Controllable growth of 1-7 layers of graphene by chemical vapour deposition. Carbon. 2014;73:252-258. DOI: 10.1016/j.car-

Atmospheric Pressure Chemical Vapor Deposition of Graphene

http://dx.doi.org/10.5772/intechopen.81293

133

[88] Suk JW, Kitt A, Magnuson CW, Hao Y, Ahmed S, An J, et al. Transfer of CVD-grown monolayer graphene onto arbitrary substrates. ACS Nano. 2011;5:6916-6924. DOI: 10.1021/

[89] Yan Z, Lin J, Peng Z, Sun Z, Zhu Y, Li L, et al. Toward the synthesis of wafer-scale singlecrystal graphene on copper foils. ACS Nano. 2012;6:9110-9117. DOI: 10.1021/nn303352k

[90] Barcenas AM, Robles JFP, Vorobiev YV, Soto NO, Mexicano A, Garcia AG. Graphene synthesis using a CVD reactor and a discontinuous feed of gas precursor at atmospheric

[91] Wang S, Hibino H, Suzuki S, Yamamoto H. Atmospheric pressure chemical vapor deposition growth of millimeter-scale single graphene on the copper surface with a native oxide layer. Chemistry of Materials. 2016;28:4893-4900. DOI: 10.1021/acs.chemmater.

[92] Campos-Delgado J, Botello-Mendez AR, Algara-Siller G, Hackens B, Pardoen T, Kaiser U, et al. CVD synthesis of mono and few-layer graphene using alcohols at low hydrogen concentration and atmospheric pressure. Chemical Physics Letters. 2013;584:142-146.

[93] Xu X, Zhang Z, Qiu L, Zhuang J, Zhang L, Wang H, et al. Ultrafast growth of singlecrystal graphene assisted by a continuous oxygen supply. Nature Nanotechnology. 2016;

[94] Wu X, Zhong G, D'Arsie L, Sugime H, Esconjauregui S, Robertson AW, et al. Growth of continuous monolayer graphene with millimeter-sized domains using industrially safe

[95] Mohsin A, Liu L, Liu P, Deng W, Ivanov IN, Li G, et al. Synthesis of millimeter-size hexagonshaped graphene single crystals on resolidified copper. ACS Nano. 2013;7:8924-8931. DOI:

[96] Hess DW, Jensen KF, Anderson TJ. Chemical Vapor Deposition. 2011. DOI: 10.1515/

[97] Ishizaka A, Shiraki Y. Low-temperature surface cleaning of silicon and itsapplication to silicon MBE. Journal of the Electrochemical Society. 1986;133:666-671. DOI: 10.1149/

[98] Kidambi PR, Ducati C, Dlubak B, Gardiner D, Weatherup RS, Martin MB, et al. The parameter space of graphene chemical vapor deposition on polycrystalline Cu. Journal

of Physical Chemistry C. 2012;116:22492-22501. DOI: 10.1021/jp303597m

conditions. Scientific Reports. 2016;6:21152. DOI: 10.1038/srep21152

pressure. Journal of Nanomaterials. 2018;2018:1-11. DOI: 10.1155/2018/3457263

bon.2014.02.061

nn201207c

6b00252

DOI: 10.1016/j.cplett.2013.08.031

10.1021/nn4034019

revce.1985.3.2.97

1.2108651

11:930-935. DOI: 10.1038/nnano.2016.132


[87] Tu Z, Liu Z, Li Y, Yang F, Zhang L, Zhao Z, et al. Controllable growth of 1-7 layers of graphene by chemical vapour deposition. Carbon. 2014;73:252-258. DOI: 10.1016/j.carbon.2014.02.061

[73] Boukhvalov DW, Katsnelson MI. Chemical functionalization of graphene with defects.

[74] Cummings AW, Duong DL, Nguyen VL, Tuan DV, Kotakoski J, Vargas JEB, et al. Charge transport in polycrystalline graphene: Challenges and opportunities. Advanced Mate-

[75] Chen JH, Li L, Cullen WG, Williams ED, Fuhrer MS. Tunable Kondo effect in graphene

[76] Gogotsi Y. Nanomaterials Handbook. Boca Raton, Florida, United States: CRC Press;

[77] Blocher JM, Browning MF, Barrett DM. Emergent Process Methods for High-Technology.

[79] Bhaviripudi S, Jia X, Dresselhaus MS, Kong J. Role of kinetic factors in chemical vapor deposition synthesis of uniform large area graphene using copper catalyst. Nano Letters.

[80] Miao C, Zheng C, Liang O, Xie YH. Physics and Applications of Graphene-Experiments,

[81] Bae S, Kim H, Lee Y, Xu X, Park JS, Zheng Y, et al. Roll-to-roll production of 30-inch graphene films for transparent electrodes. Nature Nanotechnology 2010;5:574-578. DOI:

[82] Yu Q, Lian J, Siriponglert S, Li H, Chen YP, Pei SS. Graphene segregated on Ni surfaces and transferred to insulators. Applied Physics Letters. 2008;93:113103. DOI: 10.1063/

[83] Kobayashi T, Bando M, Kimura N, Shimizu K, Kadono K, Umezu N, et al. Production of a 100-m-long high-quality graphene transparent film by roll-to-roll chemical vapor deposition and transfer process. Applied Physics Letters. 2013;102:023112. DOI: 10.1063/

[84] Boyd DA, Lin WH, Hsu CC, Teague ML, Chen CC, Lo YY, et al. Single step deposition of high-mobility graphene at reduced temperatures. Nature Communications. 2015;6:6620.

[85] Jang J, Son M, Chung S, Kim K, Cho C, Hun LB, et al. Low-temperature-grown continuous graphene films from benzene by chemical vapor deposition at ambient pressure.

[86] Gong Y, Zhang X, Liu G, Wu L, Geng X, Long M, et al. Layer-controlled and wafer-scale synthesis of uniform and high-quality graphene films on a polycrystalline nickel catalyst. Advanced Functional Materials. 2012;22:3153-3159. DOI: 10.1002/adfm.201200388

Scientific Reports. 2015;5:17955. DOI: 10.1038/srep17955

Book Chapter. Chemical Vapor Deposition of Graphene. US: InTech; 2011

[78] Morosanu CE. Thin Films by Chemical Vapour Deposition. Elsevier; 1990. pp. 1-717

with defects. Nature Physics. 2011;7:535-538. DOI: 10.1038/nphys1962

Nano Letters. 2008;8:4373-4379. DOI: 10.1021/nl802234n

rials. 2014;26:5079-5094. DOI: 10.1002/adma.201401389

Berlin, Germany: Ceramics. Springer; 1984. pp. 299-316

2010;10:4128-4133. DOI: 10.1021/nl102355e

10.1038/nnano.2010.132

132 Chemical Vapor Deposition for Nanotechnology

DOI: 10.1038/ncomms7620

1.2982585

1.4776707

2006


[99] Sun H, Han Y, Wu J, Lu Y, Xu J, Luo Y, et al. Cooling growth of milimeter-size single0 crystal bilayer graphene at atmospheric pressure. Journal of Physical Chemistry C. 2016; 120:13596-13603. DOI: 10.1021/acs.jpcc.6b04105

**Section 4**

**CVD of Nanofibers**

[100] Qaisi RM, Smith CE, Hussain MM. Atmospheric pressure chemical vapor deposition (APCVD) grown bi-layer graphene transistor characteristics at high temperature. Physica Status Solidi RRL: Rapid Research Letters. 2014;8:621-624. DOI: 10.1002/pssr.201409100

**Section 4**

**CVD of Nanofibers**

[99] Sun H, Han Y, Wu J, Lu Y, Xu J, Luo Y, et al. Cooling growth of milimeter-size single0 crystal bilayer graphene at atmospheric pressure. Journal of Physical Chemistry C. 2016;

[100] Qaisi RM, Smith CE, Hussain MM. Atmospheric pressure chemical vapor deposition (APCVD) grown bi-layer graphene transistor characteristics at high temperature. Physica Status Solidi RRL: Rapid Research Letters. 2014;8:621-624. DOI: 10.1002/pssr.201409100

120:13596-13603. DOI: 10.1021/acs.jpcc.6b04105

134 Chemical Vapor Deposition for Nanotechnology

**Chapter 7**

,

**Provisional chapter**

**Chemical Vapor Deposition of Helical Carbon**

**Chemical Vapor Deposition of Helical Carbon** 

DOI: 10.5772/intechopen.81676

Helical carbon nanofibers (HCNFs), including carbon nanocoils (CNCs), carbon nanotwists, and multi-walled CNCs, can be synthesized by chemical vapor deposition (CVD). HCNFs are predicted to have a high mechanical strength and hence are expected to have a use in nanodevices such as electromagnetic wave absorbers and sensors. For nanodevice applications, it is necessary to synthesize HCNFs in high yield and purity. In this chapter, I focus on CNCs and describe its history, expected application, and synthesis method. Finally, I introduce the author's recent studies on the improvement of the purity of CNCs by improving CVD conditions. A CNC layer with a thickness of larger than 40 μm was grown from a triple

**Keywords:** helical carbon nanofibers, carbon nanocoils, electromagnetic wave absorber, sensing devices, energy devices, binary catalyst, tetramethyltin, ferrocene, iron oxide

Carbon is an element with various allotropes because of its variety of chemical bonding: sp1

dimensional substance, carbon nanotube (CNT) is a one-dimensional substance, and graphene is a two-dimensional substance. Helical carbon nanofiber (HCNF) is a helical-shaped carbon nanofiber with coil and fiber diameters in the range of 20–1000 nm and 5–400 nm, respectively. HCNFs are recognized as a three-dimensional substance according to the above definition.

. If we classify each carbon nanomaterial by its dimension, fullerene is a zero-

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

catalyst on a substrate, and the CNC purity increased to 81 ± 2%.

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.81676

**Nanofibers**

**Nanofibers**

Yoshiyuki Suda

**Abstract**

layer of SnO<sup>2</sup>

fine powder

**1. Introduction**

, and sp<sup>3</sup>

sp<sup>2</sup>

/Fe<sup>2</sup> O3 /SnO<sup>2</sup>

**1.1. Helical carbon nanofibers and carbon nanocoils**

Yoshiyuki Suda

#### **Chemical Vapor Deposition of Helical Carbon Nanofibers Chemical Vapor Deposition of Helical Carbon Nanofibers**

DOI: 10.5772/intechopen.81676

#### Yoshiyuki Suda Yoshiyuki Suda

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.81676

#### **Abstract**

Helical carbon nanofibers (HCNFs), including carbon nanocoils (CNCs), carbon nanotwists, and multi-walled CNCs, can be synthesized by chemical vapor deposition (CVD). HCNFs are predicted to have a high mechanical strength and hence are expected to have a use in nanodevices such as electromagnetic wave absorbers and sensors. For nanodevice applications, it is necessary to synthesize HCNFs in high yield and purity. In this chapter, I focus on CNCs and describe its history, expected application, and synthesis method. Finally, I introduce the author's recent studies on the improvement of the purity of CNCs by improving CVD conditions. A CNC layer with a thickness of larger than 40 μm was grown from a triple layer of SnO<sup>2</sup> /Fe<sup>2</sup> O3 /SnO<sup>2</sup> catalyst on a substrate, and the CNC purity increased to 81 ± 2%.

**Keywords:** helical carbon nanofibers, carbon nanocoils, electromagnetic wave absorber, sensing devices, energy devices, binary catalyst, tetramethyltin, ferrocene, iron oxide fine powder

## **1. Introduction**

#### **1.1. Helical carbon nanofibers and carbon nanocoils**

Carbon is an element with various allotropes because of its variety of chemical bonding: sp1 , sp<sup>2</sup> , and sp<sup>3</sup> . If we classify each carbon nanomaterial by its dimension, fullerene is a zerodimensional substance, carbon nanotube (CNT) is a one-dimensional substance, and graphene is a two-dimensional substance. Helical carbon nanofiber (HCNF) is a helical-shaped carbon nanofiber with coil and fiber diameters in the range of 20–1000 nm and 5–400 nm, respectively. HCNFs are recognized as a three-dimensional substance according to the above definition.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

There are two types of HCNFs: the first type, called CNCs, has an inner diameter and the second type does not have an inner diameter, and it is called carbon nanotwists. CNCs are classified into two types by their size and crystalline structure: CNCs and multi-walled CNCs (or coiled CNTs) which are the multi-walled CNTs with a coil geometry. In this chapter, I focus on CNCs (**Figure 1(a)**).

### **1.2. History**

As far as I know, Davis et al. are the group who discovered CNCs. They have found minute vermicular growths of carbon through the experimental work on the deposition of carbon in the brickwork of blast furnaces in 1953 [1]. Two years later from the discovery, Hofer et al. reported the growth of carbon filaments with fiber diameters of 10–200 nm. They also found that the filaments were extended in two directions from central catalyst particles [2]. They used a catalytic chemical vapor deposition (CVD) technique of carbon from carbon monoxide.

After a while, Baker et al. developed a catalytic CVD using C<sup>2</sup> H2 gas over catalyst surfaces and studied the effect of Sn on the growth of filaments from the Fe/C<sup>2</sup> H2 system. They found that almost all the filaments had a spiral shape with a constant pitch and the Fe/Sn alloy worked to form spiral filaments [3, 4]. At the same time as Bakers' study, Boehm developed a continuous process of carbon filaments by feeding metal carbonyl into a CO stream before it passed a heated tube. He found that double or even triple helices were formed in the twisted filaments [5].

Carbon nanomaterials have been discovered one after another since the 1980s. Although fullerene [6], CNT [7], and carbon nanohorn (CNH) [8] were firstly synthesized by an arc discharge, CVD became a dominant technique in the research of carbon nanomaterials including HCNFs. In 1990, Motojima et al. have grown regularly coiled carbon filaments with a coil diameter of several micrometer by a catalytic CVD at 350–750°C using C<sup>2</sup> H2 gas feedstock and Ni as a catalyst [9]. Concurrently, Ivanov et al. have grown multi-walled CNC (or coiled CNT) which has a hollow structure with several concentric graphitic layers [10]. The CVD of single-walled CNTs (SWCNTs) was successfully realized in 1998 [11].

It is commonly accepted in CVD process that hydrocarbon precursors dissociate on catalyst particles, and the formed carbon species dissolve into catalyst particles and recombine to form carbon filaments. Models have been proposed to explain the formation of a helical structure in CNCs. Baker et al. postulated a bulk-diffusion model of carbon through catalyst particles based on the temperature gradient through a catalyst particle [3]. Another model was proposed by Amelinckx et al. and Fonseca et al. [12, 13]. The mismatch between extrusion velocities of different faces of a catalyst particle results in the deformation of carbon deposits.

#### **1.3. Expected applications**

Because of the structure of a solenoid coil, the application of electromagnetic wave absorber using CNCs [14–18] has been studied for the longest as an application of CNC. This is an application as a structural material with excellent electromagnetic wave absorption characteristics by mixing CNC with resin. It is comparatively easy and feasible for CNC application.

**Figure 1.** Scanning electron micrograph of HCNFs: (a) CNC, (b) carbon nanotwist, and (c) multi-walled CNC.

Chemical Vapor Deposition of Helical Carbon Nanofibers

http://dx.doi.org/10.5772/intechopen.81676

139

There are two types of HCNFs: the first type, called CNCs, has an inner diameter and the second type does not have an inner diameter, and it is called carbon nanotwists. CNCs are classified into two types by their size and crystalline structure: CNCs and multi-walled CNCs (or coiled CNTs) which are the multi-walled CNTs with a coil geometry. In this chapter, I

As far as I know, Davis et al. are the group who discovered CNCs. They have found minute vermicular growths of carbon through the experimental work on the deposition of carbon in the brickwork of blast furnaces in 1953 [1]. Two years later from the discovery, Hofer et al. reported the growth of carbon filaments with fiber diameters of 10–200 nm. They also found that the filaments were extended in two directions from central catalyst particles [2]. They used a catalytic chemical vapor deposition (CVD) technique of carbon from carbon monoxide.

that almost all the filaments had a spiral shape with a constant pitch and the Fe/Sn alloy worked to form spiral filaments [3, 4]. At the same time as Bakers' study, Boehm developed a continuous process of carbon filaments by feeding metal carbonyl into a CO stream before it passed a heated tube. He found that double or even triple helices were formed in the twisted

Carbon nanomaterials have been discovered one after another since the 1980s. Although fullerene [6], CNT [7], and carbon nanohorn (CNH) [8] were firstly synthesized by an arc discharge, CVD became a dominant technique in the research of carbon nanomaterials including HCNFs. In 1990, Motojima et al. have grown regularly coiled carbon filaments with a coil

and Ni as a catalyst [9]. Concurrently, Ivanov et al. have grown multi-walled CNC (or coiled CNT) which has a hollow structure with several concentric graphitic layers [10]. The CVD of

It is commonly accepted in CVD process that hydrocarbon precursors dissociate on catalyst particles, and the formed carbon species dissolve into catalyst particles and recombine to form carbon filaments. Models have been proposed to explain the formation of a helical structure in CNCs. Baker et al. postulated a bulk-diffusion model of carbon through catalyst particles based on the temperature gradient through a catalyst particle [3]. Another model was proposed by Amelinckx et al. and Fonseca et al. [12, 13]. The mismatch between extrusion velocities of different faces of a catalyst particle results in the deformation of carbon deposits.

Because of the structure of a solenoid coil, the application of electromagnetic wave absorber using CNCs [14–18] has been studied for the longest as an application of CNC. This is an application as a structural material with excellent electromagnetic wave absorption characteristics by mixing CNC with resin. It is comparatively easy and feasible for CNC application.

H2

H2

gas over catalyst surfaces

H2

gas feedstock

system. They found

After a while, Baker et al. developed a catalytic CVD using C<sup>2</sup>

and studied the effect of Sn on the growth of filaments from the Fe/C<sup>2</sup>

diameter of several micrometer by a catalytic CVD at 350–750°C using C<sup>2</sup>

single-walled CNTs (SWCNTs) was successfully realized in 1998 [11].

focus on CNCs (**Figure 1(a)**).

138 Chemical Vapor Deposition for Nanotechnology

**1.2. History**

filaments [5].

**1.3. Expected applications**

**Figure 1.** Scanning electron micrograph of HCNFs: (a) CNC, (b) carbon nanotwist, and (c) multi-walled CNC.

As a similar example, there is a fabrication of a sensing device in which plural CNCs are randomly integrated [19]. CNC-based biosensors [19], thermal sensors [20], and pressure and strain sensors [21–23] are reported one after another, and further development is expected.

Now, in order to synthesize CNC, the element which does not dissolve carbon inside such as Sn is mixed with Fe. The author tried various binary catalysts and showed that CNC can be synthesized from Fe/Sn catalyst with high efficiency [39]. Pan et al. synthesized CNC with high efficiency using a mixed catalyst of Fe and indium tin oxide (ITO) and proposed a model that the coil shape is formed due to the nonuniformity of the segregation speed of carbon from the catalyst nanoparticles [40]. There are several catalysts suitable for CNC growth, and

It is now possible to produce 2–3 g of HCNFs on a catalyst-supported graphite substrate

The main problem of CNC synthesis is that CNCs can grow with high purity on the surface of carbon deposits, but interlayers with very low CNC purity are present inside the carbon deposit [39, 41, 42]. Here I introduce two techniques to improve CNC purity in CVD and the evaluation method of CNC purity. The CNC purity in most reports was evaluated by observing the surface of the carbon deposit. The variations in the CNC purity inside carbon deposits

In this study, the Sn/Fe catalysts were supplied using the following four materials: a thin Sn

CNC purity averaged over the overall carbon deposit was increased 1.5-fold by the TMT supply. A maximum CNC purity of 72% using a combination of TMT and ferrocene vapors was

Various experimental methods have been proposed for the production of high-purity CNCs [44–47]. However, the supply of vapor-phase catalysts during the synthesis of CNCs has been reported in only a few previous studies [48, 49]. In this study, we performed experiments in

The experiments were performed in a horizontal CVD reactor using the following steps: (1)

tigating the effects of changing the catalytic vapor supply, adding tetramethyltin (TMT) and/or

Sn and Fe vapor catalysts were introduced into the CVD reactor using a tube pump and

H5 )2

/Si substrate, (2) drop-coating an Fe<sup>2</sup>

H2 /N<sup>2</sup>

, purity: >96%, CAS No.: 594–27-4, Tokyo Chemical Industry

/Si substrate and performing calcination, (3) mounting the Sn/Fe-coated SiO<sup>2</sup>

which different catalyst metal vapors were supplied successively in a CVD reactor.

feedstock [39]. However, problems of low CNC purity remain.

Chemical Vapor Deposition of Helical Carbon Nanofibers

http://dx.doi.org/10.5772/intechopen.81676

141

, tetramethyltin (TMT) vapor, and ferrocene vapor. The

O3

purity: > 95%, CAS No.: 102–54-5, Tokyo

gas mixture, and (5) inves-

solution on the

/Si

there is a possibility that new combinations can be found in the future.

**3. Recent improvement on CNC growth technique**

H2

**3.1. Supplying catalyst molecules from the vapor phase**

obtained, with Sn/Fe deposition on the substrate [43].

substrate in the CVD reactor, (4) performing CVD using a C<sup>2</sup>

)4

Co., Ltd., Tokyo, Japan) and ferrocene (Fe(C5

gas mixture during CVD.

O3

have not been yet understood completely.

film, a drop-coated solution of Fe<sup>2</sup>

*3.1.1. Experimental setup and procedure*

vacuum-evaporating Sn on the SiO<sup>2</sup>

H2 /N<sup>2</sup>

a vaporizer. TMT (Sn(CH<sup>3</sup>

Sn-coated SiO<sup>2</sup>

ferrocene to C<sup>2</sup>

within an hour, using C<sup>2</sup>

On the other hand, there is no movement to use a single CNC as a nanospring or a nanoinductor, and it remains within the scope of basic research that it measures the spring constant [24] and estimates the inductance by simulation [25]. This is considered to be attributable to the fact that it takes a lot of trouble to arrange a single CNC in a desired place and direction and suitable application is not developed.

I am currently paying attention to the application of CNC to energy devices such as fuel cells [26–32] and electric double layer capacitors [33–37]. In this field, although carbon black and activated carbon, whose mass-production processes have been established, are used as a major carbon material, the applications of carbon nanomaterials including CNCs are progressing little by little. It is difficult at present to produce CNC at the same cost as these materials; its electrochemical properties higher than existing materials are however reported at the laboratory level. Future application of CNC is greatly expected.

## **2. Growth method**

As fullerenes and CNTs were initially synthesized in high density plasmas such as laser vaporization and arc discharge, plasma processes have been used for synthesis of carbon nanomaterials. Meanwhile, CVD method is the mainstream of CNC since it was introduced in the 1970s. From the viewpoint of the carbon species to be supplied as a raw material, the method using plasma and the CVD method are summarized; a major difference is in between atomic and/or molecular carbon and hydrocarbon molecules.

Most hydrocarbons and alcohols are gaseous or liquid at ordinary temperature and normal pressure and can be easily introduced into the reaction furnace by using differential pressure or carrier gas. The hydrocarbon molecules supplied into the reactor are thermally decomposed by the catalytic reaction to change to the desired carbon nanomaterial. This process is called CVD method. Since synthesis of CNC by CVD method has much in common with synthesis of CNT, synthesis of CNT will be described first. As common to both materials, it is very important to arrange the supply of carbon feedstock and the synthesis site: catalyst and substrate.

In 1998, Dai's group succeeded in synthesizing single-walled CNTs by fixing Fe<sup>2</sup> O3 nanoparticles on a substrate and using CH4 gas as a raw material [11]. This is explained by that hydrocarbon molecules are decomposed on the surface of Fe nanoparticles reduced in a high-temperature CH4 atmosphere, carbon dissolves inside the nanoparticles (or diffuses on the surface), and carbon segregates from the nanoparticles to form CNT. This concept is basically the same as the synthesis process of carbon fiber, which was known before the discovery of CNTs [38]. Dai's group realized the growth of single-walled CNTs by maintaining the size of Fe nanoparticles to a minimum. On the other hand, in the synthesis of graphene, Cu in which the solubility of carbon is low is used as a catalyst. In this way, new catalysts have been developed.

Now, in order to synthesize CNC, the element which does not dissolve carbon inside such as Sn is mixed with Fe. The author tried various binary catalysts and showed that CNC can be synthesized from Fe/Sn catalyst with high efficiency [39]. Pan et al. synthesized CNC with high efficiency using a mixed catalyst of Fe and indium tin oxide (ITO) and proposed a model that the coil shape is formed due to the nonuniformity of the segregation speed of carbon from the catalyst nanoparticles [40]. There are several catalysts suitable for CNC growth, and there is a possibility that new combinations can be found in the future.

## **3. Recent improvement on CNC growth technique**

As a similar example, there is a fabrication of a sensing device in which plural CNCs are randomly integrated [19]. CNC-based biosensors [19], thermal sensors [20], and pressure and strain sensors [21–23] are reported one after another, and further development is expected.

On the other hand, there is no movement to use a single CNC as a nanospring or a nanoinductor, and it remains within the scope of basic research that it measures the spring constant [24] and estimates the inductance by simulation [25]. This is considered to be attributable to the fact that it takes a lot of trouble to arrange a single CNC in a desired place and direction and

I am currently paying attention to the application of CNC to energy devices such as fuel cells [26–32] and electric double layer capacitors [33–37]. In this field, although carbon black and activated carbon, whose mass-production processes have been established, are used as a major carbon material, the applications of carbon nanomaterials including CNCs are progressing little by little. It is difficult at present to produce CNC at the same cost as these materials; its electrochemical properties higher than existing materials are however reported

As fullerenes and CNTs were initially synthesized in high density plasmas such as laser vaporization and arc discharge, plasma processes have been used for synthesis of carbon nanomaterials. Meanwhile, CVD method is the mainstream of CNC since it was introduced in the 1970s. From the viewpoint of the carbon species to be supplied as a raw material, the method using plasma and the CVD method are summarized; a major difference is in between

Most hydrocarbons and alcohols are gaseous or liquid at ordinary temperature and normal pressure and can be easily introduced into the reaction furnace by using differential pressure or carrier gas. The hydrocarbon molecules supplied into the reactor are thermally decomposed by the catalytic reaction to change to the desired carbon nanomaterial. This process is called CVD method. Since synthesis of CNC by CVD method has much in common with synthesis of CNT, synthesis of CNT will be described first. As common to both materials, it is very important to arrange the supply of carbon feedstock and the synthesis site: catalyst and substrate.

hydrocarbon molecules are decomposed on the surface of Fe nanoparticles reduced in a

on the surface), and carbon segregates from the nanoparticles to form CNT. This concept is basically the same as the synthesis process of carbon fiber, which was known before the discovery of CNTs [38]. Dai's group realized the growth of single-walled CNTs by maintaining the size of Fe nanoparticles to a minimum. On the other hand, in the synthesis of graphene, Cu in which the solubility of carbon is low is used as a catalyst. In this way, new catalysts

O3

gas as a raw material [11]. This is explained by that

atmosphere, carbon dissolves inside the nanoparticles (or diffuses

nanopar-

In 1998, Dai's group succeeded in synthesizing single-walled CNTs by fixing Fe<sup>2</sup>

at the laboratory level. Future application of CNC is greatly expected.

atomic and/or molecular carbon and hydrocarbon molecules.

suitable application is not developed.

140 Chemical Vapor Deposition for Nanotechnology

ticles on a substrate and using CH4

high-temperature CH4

have been developed.

**2. Growth method**

It is now possible to produce 2–3 g of HCNFs on a catalyst-supported graphite substrate within an hour, using C<sup>2</sup> H2 feedstock [39]. However, problems of low CNC purity remain. The main problem of CNC synthesis is that CNCs can grow with high purity on the surface of carbon deposits, but interlayers with very low CNC purity are present inside the carbon deposit [39, 41, 42]. Here I introduce two techniques to improve CNC purity in CVD and the evaluation method of CNC purity. The CNC purity in most reports was evaluated by observing the surface of the carbon deposit. The variations in the CNC purity inside carbon deposits have not been yet understood completely.

#### **3.1. Supplying catalyst molecules from the vapor phase**

In this study, the Sn/Fe catalysts were supplied using the following four materials: a thin Sn film, a drop-coated solution of Fe<sup>2</sup> O3 , tetramethyltin (TMT) vapor, and ferrocene vapor. The CNC purity averaged over the overall carbon deposit was increased 1.5-fold by the TMT supply. A maximum CNC purity of 72% using a combination of TMT and ferrocene vapors was obtained, with Sn/Fe deposition on the substrate [43].

Various experimental methods have been proposed for the production of high-purity CNCs [44–47]. However, the supply of vapor-phase catalysts during the synthesis of CNCs has been reported in only a few previous studies [48, 49]. In this study, we performed experiments in which different catalyst metal vapors were supplied successively in a CVD reactor.

### *3.1.1. Experimental setup and procedure*

The experiments were performed in a horizontal CVD reactor using the following steps: (1) vacuum-evaporating Sn on the SiO<sup>2</sup> /Si substrate, (2) drop-coating an Fe<sup>2</sup> O3 solution on the Sn-coated SiO<sup>2</sup> /Si substrate and performing calcination, (3) mounting the Sn/Fe-coated SiO<sup>2</sup> /Si substrate in the CVD reactor, (4) performing CVD using a C<sup>2</sup> H2 /N<sup>2</sup> gas mixture, and (5) investigating the effects of changing the catalytic vapor supply, adding tetramethyltin (TMT) and/or ferrocene to C<sup>2</sup> H2 /N<sup>2</sup> gas mixture during CVD.

Sn and Fe vapor catalysts were introduced into the CVD reactor using a tube pump and a vaporizer. TMT (Sn(CH<sup>3</sup> )4 , purity: >96%, CAS No.: 594–27-4, Tokyo Chemical Industry Co., Ltd., Tokyo, Japan) and ferrocene (Fe(C5 H5 )2 purity: > 95%, CAS No.: 102–54-5, Tokyo Chemical Industry Co., Ltd., Tokyo, Japan) were used as the feedstocks for the Sn and Fe catalytic vapors, respectively. These catalyst molecules were mixed in ethanol, supplied to a vaporizer via a tube pump, and vaporized in a vaporizer. The CVD experimental conditions and catalytic vapor supply conditions are listed in **Tables 1** and **2**.

The synthesized CNCs and the catalyst nanoparticles were observed using SEM (S-4500II and SU8000, Hitachi High-Technologies Corp., Tokyo, Japan). The CNC purity was evaluated by analyzing SEM images of (1) the top surface and (2) the cross section of the samples. The carbon deposit was solidified by dropping a Nafion® solution onto the samples; then, the carbon deposit was cut to observe the cross section.

#### *3.1.2. Results and discussion*

In the experiment, the Fe catalyst was formed on the substrate before CVD and TMT vapors were supplied during CVD. The authors confirmed the growth of CNCs in all of the TMT concentration conditions (0.1–2% in ethanol) used in this study. The CNC number density was evaluated from the SEM images obtained at each TMT concentration, as shown in **Figure 2**. The CNC number density was highest at a TMT concentration of 0.2%. As the TMT concentration was higher than 0.2%, the CNC number density tended to decrease. The amount of carbon deposits showed the same tendency as the CNC number density.

By the above experimental results, the TMT concentration in ethanol was fixed at 0.2%, and the thickness of the Sn thin film was changed. CNC grew with all the Sn film thicknesses. Particularly, high-purity CNCs were obtained with the thicknesses of 40 and 60 nm. The CNC purity reached maximum (about 62%) for an Sn film thickness of 40 nm.

**Figure 3** shows the as-grown carbon deposits on the substrate with a Sn film thickness of 40 nm and SEM micrographs of their cross sections. It was shown that the CNC purity inside


**Table 1.** CVD conditions.


the carbon deposit was lower than that of the surface from the SEM observation, regardless of TMT supply. The average CNC purities (average over the total carbon deposit) evaluated from the SEM image of the cross section of the carbon deposit were 42% for TMT and 28% without TMT. The CNC purity increased by 1.5 times by TMT supply. As shown in **Figure 4**, by the energy dispersive spectroscopy (EDS) analysis of the cross section of the carbon deposit,

**Figure 3.** Photographs of as-grown carbon deposits and SEM micrographs of their cross sections: (a) with and (b) without TMT. The TMT concentration in ethanol, Sn film thickness, and growth time were 0.2%, 40 nm, and 600 s, respectively.

**Figure 2.** Dependence of the CNC number density (open squares) and amount of carbon deposits (open triangles) on the

/Si substrates. The growth time was 600 s. The other

Chemical Vapor Deposition of Helical Carbon Nanofibers

http://dx.doi.org/10.5772/intechopen.81676

143

TMT concentration in ethanol. No Sn film was deposited on the SiO<sup>2</sup>

The other growth conditions were the same as described in **Tables 1** and **2**.

growth conditions were the same as described in **Tables 1** and **2**.

**Table 2.** Catalytic vapor supply conditions.

Chemical Industry Co., Ltd., Tokyo, Japan) were used as the feedstocks for the Sn and Fe catalytic vapors, respectively. These catalyst molecules were mixed in ethanol, supplied to a vaporizer via a tube pump, and vaporized in a vaporizer. The CVD experimental conditions

The synthesized CNCs and the catalyst nanoparticles were observed using SEM (S-4500II and SU8000, Hitachi High-Technologies Corp., Tokyo, Japan). The CNC purity was evaluated by analyzing SEM images of (1) the top surface and (2) the cross section of the samples. The carbon deposit was solidified by dropping a Nafion® solution onto the samples; then, the

In the experiment, the Fe catalyst was formed on the substrate before CVD and TMT vapors were supplied during CVD. The authors confirmed the growth of CNCs in all of the TMT concentration conditions (0.1–2% in ethanol) used in this study. The CNC number density was evaluated from the SEM images obtained at each TMT concentration, as shown in **Figure 2**. The CNC number density was highest at a TMT concentration of 0.2%. As the TMT concentration was higher than 0.2%, the CNC number density tended to decrease. The amount of

By the above experimental results, the TMT concentration in ethanol was fixed at 0.2%, and the thickness of the Sn thin film was changed. CNC grew with all the Sn film thicknesses. Particularly, high-purity CNCs were obtained with the thicknesses of 40 and 60 nm. The CNC

**Figure 3** shows the as-grown carbon deposits on the substrate with a Sn film thickness of 40 nm and SEM micrographs of their cross sections. It was shown that the CNC purity inside

gases 50/1000 mL/min

/Si

and catalytic vapor supply conditions are listed in **Tables 1** and **2**.

carbon deposits showed the same tendency as the CNC number density.

purity reached maximum (about 62%) for an Sn film thickness of 40 nm.

Total gas pressure 1.013 × 105 Pa

Synthesis temperature 700°C Growth time 30–600 s Substrate SiO<sup>2</sup>

Sn film thickness on substrate 0–60 nm

solution (3% concentration) dropped on substrate 10 μL

TMT concentration in ethanol 0.1–2% Flow rate of TMT solution 0.144 mL/min

Temperature inside the carburetor 80°C

**Table 2.** Catalytic vapor supply conditions.

carbon deposit was cut to observe the cross section.

*3.1.2. Results and discussion*

142 Chemical Vapor Deposition for Nanotechnology

Flow rates of C<sup>2</sup>

Amount of Fe<sup>2</sup>

H2 /N<sup>2</sup>

O3

**Table 1.** CVD conditions.

**Figure 2.** Dependence of the CNC number density (open squares) and amount of carbon deposits (open triangles) on the TMT concentration in ethanol. No Sn film was deposited on the SiO<sup>2</sup> /Si substrates. The growth time was 600 s. The other growth conditions were the same as described in **Tables 1** and **2**.

**Figure 3.** Photographs of as-grown carbon deposits and SEM micrographs of their cross sections: (a) with and (b) without TMT. The TMT concentration in ethanol, Sn film thickness, and growth time were 0.2%, 40 nm, and 600 s, respectively. The other growth conditions were the same as described in **Tables 1** and **2**.

the carbon deposit was lower than that of the surface from the SEM observation, regardless of TMT supply. The average CNC purities (average over the total carbon deposit) evaluated from the SEM image of the cross section of the carbon deposit were 42% for TMT and 28% without TMT. The CNC purity increased by 1.5 times by TMT supply. As shown in **Figure 4**, by the energy dispersive spectroscopy (EDS) analysis of the cross section of the carbon deposit,

When ferrocene is added, not only Sn but also Fe is supplied as steam. Then, it is possible that these catalysts will grow CNC in the gas phase. In order to confirm this, carbon deposits on the inner wall of the CVD apparatus were analyzed after the CVD experiment. Several CNFs were observed by SEM, but the presence of CNC was not confirmed. Since ferrocene and TMT could be decomposed at the CVD growth temperature used in this study [50, 51], it seems reasonable that CNFs were grown from catalyst nanoparticles in the vapor phase. Li et al. however argue that it is necessary for the CNC catalyst nanoparticles to be fixed on a substrate

In this section, the experimental results that increase the purity of CNC by improving the structure of the catalyst layer and CVD conditions are introduced [52]. By forming a triple-

tions, a CNC layer with a thickness of 40 μm or more was grown, and the CNC purity inside

There is a carbon layer that does not contain CNCs between the upper and lower CNC layers; this was called the carbon layer. Both the CNC and carbon layers were grown from a catalyst that was deposited on the substrate. In the author's experiment, when this carbon layer was taken into account, the CNC purity over the whole deposits grown became almost 55%, very low compared to the purity of the CNC layer. Yokota et al. [39] reported that the thickness of the carbon layer depends on the thickness of the catalyst film on the

In order to increase the CNC purity and reduce the thickness of the carbon layer, formation of the catalyst layer by using a spin coating method and Fe fine powder was attempted. CNCs

The CVD conditions in this study are listed in **Table 3**. The detail of the automatic CVD

In this experiment, the authors replaced a Fe powder (diameter: 1–3 μm) with a Fe fine powder (diameter: 20 nm) and used the spin coating method for coating Fe on the sub-

(0.13 M) was separately coated on the Si substrate. The spin coating speed was varied between 1000 rpm and 2500 rpm. After coating one kind of solution, the substrate was dried at 80°C for 5 minutes to suppress mixture of the solutions. The following three kinds

> O3 /SnO<sup>2</sup>

The purity of CNCs over the entire deposit was evaluated. As illustrated in **Figure 6**, the carbon deposit is sheet-like in shape, and CNCs were grown on both sides of the carbon layer.

. By observing the catalyst structure with SEM, the optimum catalyst structure for CNC growth and the influence of this structure on the thickness of the carbon layer were

, (b) SnO<sup>2</sup>

/Fe<sup>2</sup> O3

strate in order to reduce the catalyst film thickness. Solution of Fe<sup>2</sup>

catalyst on the substrate and optimizing the CVD condi-

Chemical Vapor Deposition of Helical Carbon Nanofibers

http://dx.doi.org/10.5772/intechopen.81676

145

O3

(0.1 M) and SnO<sup>2</sup>

/Fe<sup>2</sup> O3 /

, and (c) SnO<sup>2</sup>

to form the CNCs' helical structure [41].

this layer successfully raised to 81 ± 2%.

*3.2.1. Experimental setup and procedure*

apparatus is described elsewhere [53].

of catalyst structures were formed: (a) Fe<sup>2</sup>

layer structure of SnO<sup>2</sup>

substrate.

SnO<sup>2</sup>

investigated [52].

**3.2. Using iron oxide fine powder as a catalyst**

/Fe<sup>2</sup> O3 /SnO<sup>2</sup>

were synthesized using an automatic CVD apparatus.

**Figure 4.** Relationship between the Sn/Fe ratio at the cross-sectional surfaces of the carbon deposits and the height from the substrate. The TMT concentration in ethanol, Sn film thickness, and growth time were 0.2%, 40 nm, and 600 s, respectively. The other growth conditions were the same as described in **Tables 1** and **2**.

every 100 μm from the substrate surface to a height of 1000 μm revealed that the molar ratio of Sn to Fe (Sn/Fe ratio) obtained with TMT supply was always higher than those without TMT supply. This suggests that Sn catalyst could be supplied by TMT to the interior of the carbon deposit as well as the substrate surface.

Next, the result of adding ferrocene to the TMT solution is explained. The molar ratio of Fe: Sn in the solution containing ferrocene was set to 3: 1. As shown in **Figure 5**, the CNC purity on the surface of carbon deposits obtained with different Sn film thicknesses was evaluated. It was clearly shown that the supply of ferrocene increases CNC purity. When the Sn film thickness was 40 nm, the purity of CNC reached a maximum (72%).

**Figure 5.** Purity of CNCs grown on the surface of carbon deposits under a supply of TMT and ferrocene. The TMT concentration in ethanol and growth time was 0.2% and 600 s, respectively. The other growth conditions were the same as described in **Tables 1** and **2**.

When ferrocene is added, not only Sn but also Fe is supplied as steam. Then, it is possible that these catalysts will grow CNC in the gas phase. In order to confirm this, carbon deposits on the inner wall of the CVD apparatus were analyzed after the CVD experiment. Several CNFs were observed by SEM, but the presence of CNC was not confirmed. Since ferrocene and TMT could be decomposed at the CVD growth temperature used in this study [50, 51], it seems reasonable that CNFs were grown from catalyst nanoparticles in the vapor phase. Li et al. however argue that it is necessary for the CNC catalyst nanoparticles to be fixed on a substrate to form the CNCs' helical structure [41].

#### **3.2. Using iron oxide fine powder as a catalyst**

In this section, the experimental results that increase the purity of CNC by improving the structure of the catalyst layer and CVD conditions are introduced [52]. By forming a triplelayer structure of SnO<sup>2</sup> /Fe<sup>2</sup> O3 /SnO<sup>2</sup> catalyst on the substrate and optimizing the CVD conditions, a CNC layer with a thickness of 40 μm or more was grown, and the CNC purity inside this layer successfully raised to 81 ± 2%.

There is a carbon layer that does not contain CNCs between the upper and lower CNC layers; this was called the carbon layer. Both the CNC and carbon layers were grown from a catalyst that was deposited on the substrate. In the author's experiment, when this carbon layer was taken into account, the CNC purity over the whole deposits grown became almost 55%, very low compared to the purity of the CNC layer. Yokota et al. [39] reported that the thickness of the carbon layer depends on the thickness of the catalyst film on the substrate.

In order to increase the CNC purity and reduce the thickness of the carbon layer, formation of the catalyst layer by using a spin coating method and Fe fine powder was attempted. CNCs were synthesized using an automatic CVD apparatus.

#### *3.2.1. Experimental setup and procedure*

every 100 μm from the substrate surface to a height of 1000 μm revealed that the molar ratio of Sn to Fe (Sn/Fe ratio) obtained with TMT supply was always higher than those without TMT supply. This suggests that Sn catalyst could be supplied by TMT to the interior of the carbon

**Figure 4.** Relationship between the Sn/Fe ratio at the cross-sectional surfaces of the carbon deposits and the height from the substrate. The TMT concentration in ethanol, Sn film thickness, and growth time were 0.2%, 40 nm, and 600 s,

Next, the result of adding ferrocene to the TMT solution is explained. The molar ratio of Fe: Sn in the solution containing ferrocene was set to 3: 1. As shown in **Figure 5**, the CNC purity on the surface of carbon deposits obtained with different Sn film thicknesses was evaluated. It was clearly shown that the supply of ferrocene increases CNC purity. When the Sn film

**Figure 5.** Purity of CNCs grown on the surface of carbon deposits under a supply of TMT and ferrocene. The TMT concentration in ethanol and growth time was 0.2% and 600 s, respectively. The other growth conditions were the same

thickness was 40 nm, the purity of CNC reached a maximum (72%).

respectively. The other growth conditions were the same as described in **Tables 1** and **2**.

deposit as well as the substrate surface.

144 Chemical Vapor Deposition for Nanotechnology

as described in **Tables 1** and **2**.

The CVD conditions in this study are listed in **Table 3**. The detail of the automatic CVD apparatus is described elsewhere [53].

In this experiment, the authors replaced a Fe powder (diameter: 1–3 μm) with a Fe fine powder (diameter: 20 nm) and used the spin coating method for coating Fe on the substrate in order to reduce the catalyst film thickness. Solution of Fe<sup>2</sup> O3 (0.1 M) and SnO<sup>2</sup> (0.13 M) was separately coated on the Si substrate. The spin coating speed was varied between 1000 rpm and 2500 rpm. After coating one kind of solution, the substrate was dried at 80°C for 5 minutes to suppress mixture of the solutions. The following three kinds of catalyst structures were formed: (a) Fe<sup>2</sup> O3 /SnO<sup>2</sup> , (b) SnO<sup>2</sup> /Fe<sup>2</sup> O3 , and (c) SnO<sup>2</sup> /Fe<sup>2</sup> O3 / SnO<sup>2</sup> . By observing the catalyst structure with SEM, the optimum catalyst structure for CNC growth and the influence of this structure on the thickness of the carbon layer were investigated [52].

The purity of CNCs over the entire deposit was evaluated. As illustrated in **Figure 6**, the carbon deposit is sheet-like in shape, and CNCs were grown on both sides of the carbon layer.


experiment in which the gas flow rate was varied, the flow rate of C<sup>2</sup>

 gas to 1400 mL/min. The thickness ratio of the CNC layer and the carbon layer was examined by changing the

Then, the influence of the spin coating velocity was investigated. The experimental conditions different from the experiment of the gas flow rate dependence are as follows: catalyst

flow rate, 1400 mL/min. Other conditions are the same as those shown in **Table 2**. The thicknesses of the CNC layer and the carbon layer with respect to the spin coating velocity are shown in **Figure 7**. The thickness of the carbon layer decreased with increasing spin coating velocity. It is believed that the amount of liquid remained on the substrate was reduced by increasing the spin coating velocity from 1000 to 2500 rpm. Since the CNC layer was hardly formed at a spin coating rate of 1500 rpm or more, the upper limit of the spin coating velocity seems to be 1000 rpm. From the experimental results, the optimum spin coating velocity was 1000 rpm. At this velocity, the CNC purity became highest, but the thickness of the

The effect of synthesis time on CNC growth was also examined. As synthesis time increased from 30 s to 10 min, the CNC purity increased constantly. However, the CNC purity remained almost constant at 75% for synthesis times between 10 and 30 min, indicating saturation tendency.

Next, the influence of the catalyst structure on carbon deposition was investigated using three types of catalyst structures. Different conditions from the above two experiments are as follows: spin coating velocity, 1000 rpm and synthesis time, 30 min. Other conditions are the

); C<sup>2</sup> H2

O3 /SnO<sup>2</sup> H2 /N<sup>2</sup>

gas flow ratio was greater than 0.18. Also, the difference in thickness

gas flow rate ratio was in the range of 0.15–0.25, the purity of CNC in the

250 mL/min, and the flow rate of N<sup>2</sup>

changed by fixing the flow rate of N<sup>2</sup>

H2

that of the carbon layer was maximized with a C<sup>2</sup>

gas and N<sup>2</sup>

**Figure 7.** Thicknesses of CNC and carbon layers vs. spin coating velocity.

between the CNC and the carbon layer was maximized at a C<sup>2</sup>

flow ratio between C<sup>2</sup>

H2 /N<sup>2</sup>

H2 /N<sup>2</sup>

CNC layer was 95% or more.

carbon layer was also highest.

structure on substrate, two layers (Fe<sup>2</sup>

when the C<sup>2</sup>

When the C<sup>2</sup>

H2

http://dx.doi.org/10.5772/intechopen.81676

Chemical Vapor Deposition of Helical Carbon Nanofibers

gas flow ratio of 0.18 and decreased

gas flow rate, 250 mL/min; and N<sup>2</sup>

gas was varied. Similarly, the flow rate of C<sup>2</sup>

gas. As a result, the thickness of the CNC layer as well as

H2 /N<sup>2</sup> gas was fixed at

gas was

147

gas

H2

gas flow ratio of 0.18.

**Table 3.** Catalytic CVD conditions.

**Figure 6.** Illustration of the carbon deposit.

The CNC purity was defined by Eq. (1) under the assumption that the CNC purities in the CNC and the carbon layers are 100% and 0%, respectively. CNC purity <sup>=</sup> Thickness of CNC layer \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ Thicknesses of CNC and carbon layers <sup>×</sup> <sup>100</sup> (%) (1)

$$\text{CNC purity} = \frac{\text{Thickness of CNC layer}}{\text{Thickness of CNC and carbon layers}} \times 100 \text{ (\%)}\tag{1}$$

The assumption is consistent with the SEM observations of the deposits. The thickness of the CNC layer was obtained as the summation of the upper and lower layers.

#### *3.2.2. Results and discussion*

Firstly, the effect of flow rates of C<sup>2</sup> H2 feedstock and N<sup>2</sup> dilution gases on CNC growth was examined. The conditions in this experiment are as follows: catalyst structure, triple layer (SnO<sup>2</sup> /Fe<sup>2</sup> O3 /SnO<sup>2</sup> ); Fe/Sn molar ratio, 1:2.6; spin coating velocity, 1000 rpm; and synthesis time, 10 min. The other conditions are the same as the conditions shown in **Table 2**. In the experiment in which the gas flow rate was varied, the flow rate of C<sup>2</sup> H2 gas was fixed at 250 mL/min, and the flow rate of N<sup>2</sup> gas was varied. Similarly, the flow rate of C<sup>2</sup> H2 gas was changed by fixing the flow rate of N<sup>2</sup> gas to 1400 mL/min.

The thickness ratio of the CNC layer and the carbon layer was examined by changing the flow ratio between C<sup>2</sup> H2 gas and N<sup>2</sup> gas. As a result, the thickness of the CNC layer as well as that of the carbon layer was maximized with a C<sup>2</sup> H2 /N<sup>2</sup> gas flow ratio of 0.18 and decreased when the C<sup>2</sup> H2 /N<sup>2</sup> gas flow ratio was greater than 0.18. Also, the difference in thickness between the CNC and the carbon layer was maximized at a C<sup>2</sup> H2 /N<sup>2</sup> gas flow ratio of 0.18. When the C<sup>2</sup> H2 /N<sup>2</sup> gas flow rate ratio was in the range of 0.15–0.25, the purity of CNC in the CNC layer was 95% or more.

Then, the influence of the spin coating velocity was investigated. The experimental conditions different from the experiment of the gas flow rate dependence are as follows: catalyst structure on substrate, two layers (Fe<sup>2</sup> O3 /SnO<sup>2</sup> ); C<sup>2</sup> H2 gas flow rate, 250 mL/min; and N<sup>2</sup> gas flow rate, 1400 mL/min. Other conditions are the same as those shown in **Table 2**. The thicknesses of the CNC layer and the carbon layer with respect to the spin coating velocity are shown in **Figure 7**. The thickness of the carbon layer decreased with increasing spin coating velocity. It is believed that the amount of liquid remained on the substrate was reduced by increasing the spin coating velocity from 1000 to 2500 rpm. Since the CNC layer was hardly formed at a spin coating rate of 1500 rpm or more, the upper limit of the spin coating velocity seems to be 1000 rpm. From the experimental results, the optimum spin coating velocity was 1000 rpm. At this velocity, the CNC purity became highest, but the thickness of the carbon layer was also highest.

The effect of synthesis time on CNC growth was also examined. As synthesis time increased from 30 s to 10 min, the CNC purity increased constantly. However, the CNC purity remained almost constant at 75% for synthesis times between 10 and 30 min, indicating saturation tendency.

Next, the influence of the catalyst structure on carbon deposition was investigated using three types of catalyst structures. Different conditions from the above two experiments are as follows: spin coating velocity, 1000 rpm and synthesis time, 30 min. Other conditions are the

**Figure 7.** Thicknesses of CNC and carbon layers vs. spin coating velocity.

**Figure 6.** Illustration of the carbon deposit.

Catalyst precursor Fe<sup>2</sup>

Catalyst structure Fe<sup>2</sup>

Feedstock gas (flow rate) C<sup>2</sup>

Dilution gas (flow rate) N<sup>2</sup>

Synthesis temperature 780°C Synthesis time 0.5–30 min Annealing temperature 780°C Annealing time 5 min

**Table 3.** Catalytic CVD conditions.

Spin coating velocity 1000–2500 rpm

146 Chemical Vapor Deposition for Nanotechnology

Catalyst molar ratio Fe:Sn = 1:2.6–1:13

O3

O3 /SnO<sup>2</sup> , SnO<sup>2</sup> /Fe<sup>2</sup> O3 /SnO<sup>2</sup> , SnO<sup>2</sup> /Fe<sup>2</sup> O3

H2

(100–400 mL/min)

(1000–1800 mL/min)

SnO<sup>2</sup>

*3.2.2. Results and discussion*

(SnO<sup>2</sup> /Fe<sup>2</sup> O3 /SnO<sup>2</sup>

Firstly, the effect of flow rates of C<sup>2</sup>

CNC and the carbon layers are 100% and 0%, respectively.

The CNC purity was defined by Eq. (1) under the assumption that the CNC purities in the

fine powder (diameter: 20 nm, Nilaco, Tokyo, Japan)

drop-coating solution (0.13 M, Kojundo Chemical Laboratory, Sakado, Japan)

CNC purity <sup>=</sup> Thickness of CNC layer \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ Thicknesses of CNC and carbon layers <sup>×</sup> <sup>100</sup> (%) (1)

The assumption is consistent with the SEM observations of the deposits. The thickness of the

feedstock and N<sup>2</sup>

); Fe/Sn molar ratio, 1:2.6; spin coating velocity, 1000 rpm; and synthesis

examined. The conditions in this experiment are as follows: catalyst structure, triple layer

time, 10 min. The other conditions are the same as the conditions shown in **Table 2**. In the

dilution gases on CNC growth was

CNC layer was obtained as the summation of the upper and lower layers.

H2

same as those listed in **Table 2**. Cross-sectional SEM micrographs of carbon deposits containing CNC and carbon layers grown from three types of catalyst structures and SEM micrographs of the surface of the CNC layers are shown in **Figures 8** and **9**, respectively.

As shown in **Figure 8**, a CNC layer was formed on the catalyst structures of (a) Fe<sup>2</sup> O3 /SnO<sup>2</sup> and (b) SnO<sup>2</sup> /Fe<sup>2</sup> O3 /SnO<sup>2</sup> . The purities of CNCs in (a) and (b) were estimated to be 69% ± 2% and 81% ± 2%, respectively, according to Eq. (1). The reason for that the CNC purity differs depending on the catalyst structure was examined using a CNC growth model as shown in **Figure 10**.

When C<sup>2</sup>

SnO<sup>2</sup>

Fe<sup>2</sup> O3 /SnO<sup>2</sup>

the SnO<sup>2</sup>

SnO<sup>2</sup>

H2

**Figure 10.** Growth models of (a) Fe<sup>2</sup>

, and SnO<sup>2</sup>

the surface of the Fe<sup>2</sup>

ability to decompose C<sup>2</sup>

/Fe<sup>2</sup> O3 /SnO<sup>2</sup>

source gas molecule is supplied on the Fe<sup>2</sup>

H2

O3 /SnO<sup>2</sup>

catalyst structure becomes thinner than in the case of Fe<sup>2</sup>

H2

molecules, an Fe-Sn alloy is formed on the upper and lower surfaces of SnO<sup>2</sup>

thickness of the layer is greater than the thickness of the layer formed from the Fe<sup>2</sup>

Fe-Sn alloy ((4) in **Figure 10b**), the thickness of the carbon layer grown inside the SnO<sup>2</sup>

H2

and (b) SnO<sup>2</sup>

which is much lower than the CVD temperature, a part of the reduced Sn stays as a liquid phase, and the other part diffuses inside the catalyst ((2) in **Figure 10a**). This phenomenon is explained by previous research results that molten Sn diffuses through the pores in the Fe<sup>2</sup>

/Fe<sup>2</sup> O3 /SnO<sup>2</sup>

thin film by capillary action [43]. However, since the amount of the Fe-Sn alloy formed on

deposits are formed inside the Fe-Sn structure ((4) in **Figure 10a**). From the above, in the

, and a very thick carbon layer is formed.

catalyst structure due to the much amount of Sn on the surface. Since CNC grows from the

process for the growth of the CNC layer. This leads to a difference in thickness between the

forms nanoparticles ((1) in **Figure 10a**). A part of C<sup>2</sup>

O3 /SnO<sup>2</sup>

catalyst structure, C<sup>2</sup>

On the other hand, the amount of C<sup>2</sup>

the same as in the case of the Fe<sup>2</sup>

H2

explains the experimental results shown in **Figure 8**.

In the both models described above, the diffusion of C<sup>2</sup>

is partially reduced by C<sup>2</sup>

O3 /SnO<sup>2</sup>

> O3 /SnO<sup>2</sup>

structure is limited ((3) in **Figure 10a**), a lot of sheets of carbon

catalyst structures.

H2

molecules are directly supplied to Fe<sup>2</sup>

catalyst structure is limited ((1) in **Figure 10b**). By supplying more C<sup>2</sup>

H2

molecules reaching the Fe<sup>2</sup>

catalyst, Fe<sup>2</sup>

molecules. Since the melting point of Sn is 232°C.,

O3

catalyst structure ((3) in **Figure 10b**). However, the

O3 /SnO<sup>2</sup>

molecule passes through the Fe<sup>2</sup>

Chemical Vapor Deposition of Helical Carbon Nanofibers

http://dx.doi.org/10.5772/intechopen.81676

149

O3

O3

in the catalyst film is an important

in the catalyst

having a strong

catalyst nanoparticles in

/Fe<sup>2</sup> O3 /SnO<sup>2</sup> ,

. This consideration

O3 /SnO<sup>2</sup>

> /Fe<sup>2</sup> O3 /

O3 to

O3

H2

**Figure 8.** Cross-sectional SEM micrographs of carbon deposits grown with different catalyst structures: (a) Fe<sup>2</sup> O3 /SnO<sup>2</sup> , (b) SnO<sup>2</sup> /Fe<sup>2</sup> O3 /SnO<sup>2</sup> , and (c) SnO<sup>2</sup> /Fe<sup>2</sup> O3 . The red and blue arrows represent the thicknesses of the CNC and carbon layers, respectively.

**Figure 9.** SEM micrographs of the surfaces of CNC layers grown from different catalyst structures: (a) Fe<sup>2</sup> O3 /SnO<sup>2</sup> and (b) SnO<sup>2</sup> /Fe<sup>2</sup> O3 /SnO<sup>2</sup> . No CNCs were observed on the SnO<sup>2</sup> /Fe<sup>2</sup> O3 catalyst structure.

**Figure 10.** Growth models of (a) Fe<sup>2</sup> O3 /SnO<sup>2</sup> and (b) SnO<sup>2</sup> /Fe<sup>2</sup> O3 /SnO<sup>2</sup> catalyst structures.

same as those listed in **Table 2**. Cross-sectional SEM micrographs of carbon deposits containing CNC and carbon layers grown from three types of catalyst structures and SEM micro-

81% ± 2%, respectively, according to Eq. (1). The reason for that the CNC purity differs depending on the catalyst structure was examined using a CNC growth model as shown in **Figure 10**.

**Figure 8.** Cross-sectional SEM micrographs of carbon deposits grown with different catalyst structures: (a) Fe<sup>2</sup>

**Figure 9.** SEM micrographs of the surfaces of CNC layers grown from different catalyst structures: (a) Fe<sup>2</sup>

/Fe<sup>2</sup> O3

catalyst structure.

. The red and blue arrows represent the thicknesses of the CNC and carbon

. The purities of CNCs in (a) and (b) were estimated to be 69% ± 2% and

O3 /SnO<sup>2</sup>

and

O3 /SnO<sup>2</sup> ,

O3 /SnO<sup>2</sup> and

graphs of the surface of the CNC layers are shown in **Figures 8** and **9**, respectively.

As shown in **Figure 8**, a CNC layer was formed on the catalyst structures of (a) Fe<sup>2</sup>

(b) SnO<sup>2</sup>

(b) SnO<sup>2</sup>

(b) SnO<sup>2</sup> /Fe<sup>2</sup> O3 /SnO<sup>2</sup>

/Fe<sup>2</sup> O3 /SnO<sup>2</sup>

layers, respectively.

, and (c) SnO<sup>2</sup>

/Fe<sup>2</sup> O3

. No CNCs were observed on the SnO<sup>2</sup>

/Fe<sup>2</sup> O3 /SnO<sup>2</sup>

148 Chemical Vapor Deposition for Nanotechnology

When C<sup>2</sup> H2 source gas molecule is supplied on the Fe<sup>2</sup> O3 /SnO<sup>2</sup> catalyst, Fe<sup>2</sup> O3 in the catalyst forms nanoparticles ((1) in **Figure 10a**). A part of C<sup>2</sup> H2 molecule passes through the Fe<sup>2</sup> O3 to SnO<sup>2</sup> , and SnO<sup>2</sup> is partially reduced by C<sup>2</sup> H2 molecules. Since the melting point of Sn is 232°C., which is much lower than the CVD temperature, a part of the reduced Sn stays as a liquid phase, and the other part diffuses inside the catalyst ((2) in **Figure 10a**). This phenomenon is explained by previous research results that molten Sn diffuses through the pores in the Fe<sup>2</sup> O3 thin film by capillary action [43]. However, since the amount of the Fe-Sn alloy formed on the surface of the Fe<sup>2</sup> O3 /SnO<sup>2</sup> structure is limited ((3) in **Figure 10a**), a lot of sheets of carbon deposits are formed inside the Fe-Sn structure ((4) in **Figure 10a**). From the above, in the Fe<sup>2</sup> O3 /SnO<sup>2</sup> catalyst structure, C<sup>2</sup> H2 molecules are directly supplied to Fe<sup>2</sup> O3 having a strong ability to decompose C<sup>2</sup> H2 , and a very thick carbon layer is formed.

On the other hand, the amount of C<sup>2</sup> H2 molecules reaching the Fe<sup>2</sup> O3 catalyst nanoparticles in the SnO<sup>2</sup> /Fe<sup>2</sup> O3 /SnO<sup>2</sup> catalyst structure is limited ((1) in **Figure 10b**). By supplying more C<sup>2</sup> H2 molecules, an Fe-Sn alloy is formed on the upper and lower surfaces of SnO<sup>2</sup> /Fe<sup>2</sup> O3 /SnO<sup>2</sup> , the same as in the case of the Fe<sup>2</sup> O3 /SnO<sup>2</sup> catalyst structure ((3) in **Figure 10b**). However, the thickness of the layer is greater than the thickness of the layer formed from the Fe<sup>2</sup> O3 /SnO<sup>2</sup> catalyst structure due to the much amount of Sn on the surface. Since CNC grows from the Fe-Sn alloy ((4) in **Figure 10b**), the thickness of the carbon layer grown inside the SnO<sup>2</sup> /Fe<sup>2</sup> O3 / SnO<sup>2</sup> catalyst structure becomes thinner than in the case of Fe<sup>2</sup> O3 /SnO<sup>2</sup> . This consideration explains the experimental results shown in **Figure 8**.

In the both models described above, the diffusion of C<sup>2</sup> H2 in the catalyst film is an important process for the growth of the CNC layer. This leads to a difference in thickness between the upper and lower CNC layers. As the experimental results show, the thickness of the upper CNC layer was always thicker than the thickness of the lower CNC layer. This also explains the reason for that CNC hardly grows on the SnO<sup>2</sup> /Fe<sup>2</sup> O3 catalyst structure. This indicates that when the SnO<sup>2</sup> layer is too thick, the amount of C<sup>2</sup> H2 supplied to Fe<sup>2</sup> O3 is appreciably limited. Based on the above experimental results, the authors concluded that the optimum catalyst structure for growing CNC in high yield and high purity is the SnO<sup>2</sup> /Fe<sup>2</sup> O3 /SnO<sup>2</sup> catalyst structure.

**References**

DOI: 10.1038/171756a0

DOI: 10.1021/j150533a010

DOI: 10.1038/253037b0

354056a0

(98)00745-3

6223(95)00150-3

DOI: 10.1016/0008-6223(73)90323-0

1985;**318**:162. DOI: 10.1038/318162a0

170. DOI: 10.1016/S0009-2614(99)00642-9

DOI: 10.1016/0009-2614(94)00467-6

DOI: 10.1126/science.265.5172.635

[1] Davis WR, Slawson RJ, Rigby GR. An unusual form of carbon. Nature. 1953;**171**:756.

Chemical Vapor Deposition of Helical Carbon Nanofibers

http://dx.doi.org/10.5772/intechopen.81676

151

[2] Hofer LJE, Sterling E, McCartney JT. Structure of carbon deposited from carbon monoxide on iron, cobalt and nickel. The Journal of Physical Chemistry. 1955;**59**:1153-1155.

[3] Baker R. Nucleation and growth of carbon deposits from the nickel catalyzed decomposition of acetylene. Journal of Catalysis. 1972;**26**:51-62. DOI: 10.1016/0021-9517(72)90032-2

[4] Baker RTK, Harris PS, Terry S. Unique form of filamentous carbon. Nature. 1975;**253**:37-39.

[5] Boehm HP. Carbon from carbon monoxide disproportionation on nickel and iron catalysts: Morphological studies and possible growth mechanisms. Carbon. 1973;**11**:583-590.

[6] Kroto HW, Heath JR, O'Brien SC, Curl RF, Smalley RE. C60: Buckminsterfullerene. Nature.

[7] Iijima S. Helical microtubules of graphitic carbon. Nature. 1991;**354**:56-58. DOI: 10.1038/

[8] Iijima S, Yudasaka M, Yamada R, Bandow S, Suenaga K, Kokai F, et al. Nano-aggregates of single-walled graphitic carbon nano-horns. Chemical Physics Letters. 1999;**309**:165-

[9] Motojima S, Kawaguchi M, Nozaki K, Iwanaga H. Growth of regularly coiled carbon filaments by Ni catalyzed pyrolysis of acetylene, and their morphology and extension

[10] Ivanov V, Nagy JB, Lambin P, Lucas A, Zhang XB, Zhang XF, et al. The study of carbon nanotubules produced by catalytic method. Chemical Physics Letters. 1994;**223**:329-335.

[11] Kong J, Cassell AM, Dai H. Chemical vapor deposition of methane for single-walled carbon nanotubes. Chemical Physics Letters. 1998;**292**:567-574. DOI: 10.1016/S0009-2614

[12] Amelinckx S, Zhang XB, Bernaerts D, Zhang XF, Ivanov V, Nagy JB. A formation mechanism for catalytically grown helix-shaped graphite nanotubes. Science. 1994;**265**:635-639.

[13] Fonseca A, Hernadi K, Nagy Jb, Lambin P, Lucas AA. Model structure of perfectly graphitizable coiled carbon nanotubes. Carbon. 1995;**33**:1759-1775. DOI: 10.1016/0008-

[14] Zhao D-L, Shen Z-M. Preparation and microwave absorption properties of carbon nanocoils. Materials Letters. 2008;**62**:3704-3706. DOI: 10.1016/j.matlet.2008.04.032

characteristics. Applied Physics Letters. 1990;**56**:321-323. DOI: 10.1063/1.102816

## **4. Summary**

CNC, one type of HCNFs, is introduced. CNC has a coil diameter of 20–1000 nm, and a length of several tens of μm and has mainly been synthesized by a CVD method. Although it is very small, CNC is predicted to have a high mechanical strength and an electrical property and hence is expected to have a use in nanodevices such as electromagnetic wave absorbers and material in energy devices. The history, expected application, and synthesis method were described. The authors' recent studies on the improvement of the purity of CNCs by improving CVD conditions were summarized.

## **Acknowledgements**

This work was partly supported by the JSPS KAKENHI Grant Number 24360108 and 25630110; the Ministry of Education, Culture, Sports, Science and Technology (MEXT) KAKENHI Grant Number 24110708; and Analysis and Development System for Advanced Materials (ADAM), Research Institute for Sustainable Humanosphere (RISH), Kyoto University. I greatly appreciate the experimental contribution by Mr. Yuichi Ishii, Mr. Tatsuki Miki, and Mr. Koji Maruyama of Toyohashi University of Technology. This study was performed under the cooperative research with Tokai Carbon Co., Ltd.; Shonan Plastic Manufacturing Co., Ltd.; and Toho Gas Co., Ltd. I acknowledge Mr. Hitoshi Ue, Mr. Kazuki Shimizu, and Dr. Yoshito Umeda for their fruitful discussion.

## **Conflict of interest**

The author declares that he has no conflict of interest.

## **Author details**

Yoshiyuki Suda

Address all correspondence to: suda@ee.tut.ac.jp

Toyohashi University of Technology, Toyohashi, Japan

## **References**

upper and lower CNC layers. As the experimental results show, the thickness of the upper CNC layer was always thicker than the thickness of the lower CNC layer. This also explains

limited. Based on the above experimental results, the authors concluded that the optimum

CNC, one type of HCNFs, is introduced. CNC has a coil diameter of 20–1000 nm, and a length of several tens of μm and has mainly been synthesized by a CVD method. Although it is very small, CNC is predicted to have a high mechanical strength and an electrical property and hence is expected to have a use in nanodevices such as electromagnetic wave absorbers and material in energy devices. The history, expected application, and synthesis method were described. The authors' recent studies on the improvement of the purity of CNCs by improv-

This work was partly supported by the JSPS KAKENHI Grant Number 24360108 and 25630110; the Ministry of Education, Culture, Sports, Science and Technology (MEXT) KAKENHI Grant Number 24110708; and Analysis and Development System for Advanced Materials (ADAM), Research Institute for Sustainable Humanosphere (RISH), Kyoto University. I greatly appreciate the experimental contribution by Mr. Yuichi Ishii, Mr. Tatsuki Miki, and Mr. Koji Maruyama of Toyohashi University of Technology. This study was performed under the cooperative research with Tokai Carbon Co., Ltd.; Shonan Plastic Manufacturing Co., Ltd.; and Toho Gas Co., Ltd. I acknowledge Mr. Hitoshi Ue, Mr. Kazuki Shimizu, and Dr. Yoshito

layer is too thick, the amount of C<sup>2</sup>

catalyst structure for growing CNC in high yield and high purity is the SnO<sup>2</sup>

/Fe<sup>2</sup> O3

H2

supplied to Fe<sup>2</sup>

catalyst structure. This indicates

O3

is appreciably

/Fe<sup>2</sup> O3 /SnO<sup>2</sup>

the reason for that CNC hardly grows on the SnO<sup>2</sup>

ing CVD conditions were summarized.

Umeda for their fruitful discussion.

The author declares that he has no conflict of interest.

Address all correspondence to: suda@ee.tut.ac.jp

Toyohashi University of Technology, Toyohashi, Japan

**Conflict of interest**

**Author details**

Yoshiyuki Suda

**Acknowledgements**

that when the SnO<sup>2</sup>

150 Chemical Vapor Deposition for Nanotechnology

catalyst structure.

**4. Summary**


[15] Liu L, He P, Zhou K, Chen T. Microwave absorption properties of carbon fibers with carbon coils of different morphologies (double microcoils and single nanocoils) grown on them. Journal of Materials Science. 2014;**49**:4379-4386. DOI: 10.1007/s10853-014-8137-z

[28] Hu G, Nitze F, Sharifi T, Barzegar HR, Wågberg T. Self-assembled palladium nanocrystals on helical carbon nanofibers as enhanced electrocatalysts for electro-oxidation of small molecules. Journal of Materials Chemistry. 2012;**22**:8541. DOI: 10.1039/c2jm16075g

Chemical Vapor Deposition of Helical Carbon Nanofibers

http://dx.doi.org/10.5772/intechopen.81676

153

[29] Nitze F, Mazurkiewicz M, Malolepszy A, Mikolajczuk A, Kędzierzawski P, Tai C-W, et al. Synthesis of palladium nanoparticles decorated helical carbon nanofiber as highly active anodic catalyst for direct formic acid fuel cells. Electrochimica Acta. 2012;**63**:323-328.

[30] Suda Y, Shimizu Y, Ozaki M, Tanoue H, Takikawa H, Ue H, et al. Electrochemical properties of fuel cell catalysts loaded on carbon nanomaterials with different geometries. Materials Today Communications. 2015;**3**:96-103. DOI: 10.1016/j.mtcomm.2015.02.003

[31] Shimizu Y, Suda Y, Takikawa H, Ue H, Shimizu K, Umeda Y. Effective utilization of carbon nanocoil-supported PtRu anode catalyst by applying anode microporous layer for improved direct methanol fuel cell performance. Electrochemistry. 2015;**83**:381-385.

[32] Tang S, Huangfu H, Dai Z, Sui L, Zhu Z. Preparation of Fe-N-carbon nanocoils as catalyst for oxygen reduction reaction. International Journal of Electrochemical Science.

[33] Rakhi RB, Cha D, Chen W, Alshareef HN. Electrochemical energy storage devices using electrodes incorporating carbon nanocoils and metal oxides nanoparticles. The Journal

[34] Rakhi RB, Chen W, Alshareef HN. Conducting polymer/carbon nanocoil composite electrodes for efficient supercapacitors. Journal of Materials Chemistry. 2012;**22**:5177. DOI:

[35] Choi WH, Choi MJ, Bang JH. Nitrogen-doped carbon nanocoil array integrated on carbon nanofiber paper for supercapacitor electrodes. ACS Applied Materials & Interfaces.

[36] Izumi H, Okabe Y, Suda Y, Takikawa H, Tanoue H, Ue H, et al. Manufacturing of electric double-layer capacitors using carbon nanocoils and evaluation of their specific capacitance at a high scan rate. IEEJ Transactions on Fundamentals and Materials. 2013;**133**:660-667.

[37] Yu L, Wan G, Qin Y, Wang G. Atomic layer deposition assisted fabrication of high-purity carbon nanocoil for electrochemical energy storage. Electrochimica Acta. 2018;**268**:283-294.

[38] Tibbetts GG. Carbon fibers produced by pyrolysis of natural gas in stainless steel tubes.

[39] Yokota M, Hosokawa Y, Shinohara Y, Kawabata T, Takimoto K, Suda Y, et al. Splitting and flattening of helical carbon nanofibers by acid treatment. Journal of Nanoscience

[40] Pan L, Zhang M, Nakayama Y. Growth mechanism of carbon nanocoils. Journal of

Applied Physics Letters. 1983;**42**:666-668. DOI: 10.1063/1.94066

Applied Physics. 2002;**91**:10058. DOI: 10.1063/1.1471575

and Nanotechnology. 2010;**10**:3910-3914. DOI: 10.1166/jnn.2010.1964

2015;**10**:12. http://www.electrochemsci.org/papers/vol10/100907180.pdf

of Physical Chemistry C. 2011;**115**:14392-14399. DOI: 10.1021/jp202519e

DOI: 10.1016/j.electacta.2011.12.104

DOI: 10.5796/electrochemistry.83.381

2015;**7**:19370-19381. DOI: 10.1021/acsami.5b05527

10.1039/c2jm15740c

DOI: 10.1541/ieejfms.133.660

DOI: 10.1016/j.electacta.2018.02.114


[28] Hu G, Nitze F, Sharifi T, Barzegar HR, Wågberg T. Self-assembled palladium nanocrystals on helical carbon nanofibers as enhanced electrocatalysts for electro-oxidation of small molecules. Journal of Materials Chemistry. 2012;**22**:8541. DOI: 10.1039/c2jm16075g

[15] Liu L, He P, Zhou K, Chen T. Microwave absorption properties of carbon fibers with carbon coils of different morphologies (double microcoils and single nanocoils) grown on them. Journal of Materials Science. 2014;**49**:4379-4386. DOI: 10.1007/s10853-014-8137-z

[16] Eguchi U, Takikawa H, Suda Y. Electromagnetic wave absorption characteristics of multiwalled carbon nanocoils. Japanese Journal of Applied Physics. 2014;**53**:045102. DOI:

[17] Jian X, Chen X, Zhou Z, Li G, Jiang M, Xu X, et al. Remarkable improvement in microwave absorption by cloaking a micro-scaled tetrapod hollow with helical carbon nanofibers. Physical Chemistry Chemical Physics. 2015;**17**:3024-3031. DOI: 10.1039/C4CP04849K [18] Li L, Lu L, Qi S. Preparation, characterization and microwave absorption properties of porous nickel ferrite hollow nanospheres/helical carbon nanotubes/polypyrrole nanowires composites. Journal of Materials Science: Materials in Electronics. 2018;**29**:8513-8522.

[19] Jia X, Hu G, Nitze F, Barzegar HR, Sharifi T, Tai C-W, et al. Synthesis of palladium/helical carbon nanofiber hybrid nanostructures and their application for hydrogen peroxide and glucose detection. ACS Applied Materials & Interfaces. 2013;**5**:12017-12022. DOI:

[20] Su C-C, Huang C-L, Chang S-H. Fabrication of aligned carbon nanocoil thermal sensor with a high temperature coefficient of electrical resistance at 25-100 °C. IEEE Transactions

[21] Li C, Pan L, Deng C, Wang P, Huang Y, Nasir H. A flexible, ultra-sensitive strain sensor based on carbon nanocoil network fabricated by an electrophoretic method. Nanoscale.

[22] Deng C, Pan L, Zhang D, Li C, Nasir H. A super stretchable and sensitive strain sensor based on a carbon nanocoil network fabricated by a simple peeling-off approach.

[23] Li C, Pan L, Deng C, Cong T, Yin P, Wu Z. A highly sensitive and wide-range pressure sensor based on a carbon nanocoil network fabricated by an electrophoretic method.

[24] Chen X, Zhang S, Dikin DA, Ding W, Ruoff RS, Pan L, et al. Mechanics of a carbon

[25] Wiselin J, Suseela SB, Jalaja BV, Ramani SDSP, Prasad R, Devaraj S, et al. A Low cost carbon nanofiber based spiral inductor: inference and implementation. Advances in

[26] Park K-W, Sung Y-E, Han S, Yun Y, Hyeon T. Origin of the enhanced catalytic activity of carbon nanocoil-supported PtRu alloy electrocatalysts. The Journal of Physical

[27] Leela Mohana Reddy A, Jafri RI, Jha N, Ramaprabhu S, Ajayan PM. Carbon nanocoils for multi-functional energy applications. Journal of Materials Chemistry. 2011;**21**:16103.

Materials Science and Engineering. 2014;**2014**:1-8. DOI: 10.1155/2014/384917

Journal of Materials Chemistry C. 2017;**5**:11892-11900. DOI: 10.1039/c7tc04166g

on Nanotechnology. 2015;**14**:794-797. DOI: 10.1109/TNANO.2015.2426611

10.7567/JJAP.53.045102

152 Chemical Vapor Deposition for Nanotechnology

DOI: 10.1007/s10854-018-8865-y

2017;**9**:9872-9878. DOI: 10.1039/C7NR01945A

Nanoscale. 2017;**9**:16404-16411. DOI: 10.1039/C7NR05486F

nanocoil. Nano Letters. 2003;**3**:1299-1304. DOI: 10.1021/nl034367o

Chemistry B. 2004;**108**:939-944. DOI: 10.1021/jp0368031

DOI: 10.1039/c1jm12580j

10.1021/am4037383


[41] Li D, Pan L. Necessity of base fixation for helical growth of carbon nanocoils. Journal of

[42] Hirahara K, Nakayama Y. The effect of a tin oxide buffer layer for the high yield synthesis of carbon nanocoils. Carbon. 2013;**56**:264-270. DOI: 10.1016/j.carbon.2013.01.007

[43] Suda Y, Ishii Y, Miki T, Maruyama K, Tanoue H, Takikawa H, et al. Improvement of carbon nanocoil purity achieved by supplying catalyst molecules from the vapor phase in chemical vapor deposition. Journal of Materials Research. 2014;**29**:2179-2187. DOI:

[44] Chang N-K, Chang S-H. High-yield synthesis of carbon nanocoils on stainless steel.

[45] Bi H, Kou K-C, Ostrikov KK, Wang Z-C. High-yield atmospheric-pressure CVD of highly-uniform carbon nanocoils using Co-P catalyst nanoparticles prepared by electroless plating. Journal of Alloys and Compounds. 2009;**484**:860-863. DOI: 10.1016/j.

[46] Qi X, Zhong W, Deng Y, Au C, Du Y. Synthesis of helical carbon nanotubes, wormlike carbon nanotubes and nanocoils at 450°C and their magnetic properties. Carbon.

[47] Li D, Pan L, Qian J, Liu D. Highly efficient synthesis of carbon nanocoils by catalyst particles prepared by a sol-gel method. Carbon. 2010;**48**:170-175. DOI: 10.1016/j.carbon.

[48] Wang W, Yang K, Gaillard J, Bandaru PR, Rao AM. Rational synthesis of helically coiled carbon nanowires and nanotubes through the use of tin and indium catalysts. Advanced

[49] Cervantes-Sodi F, Vilatela JJ, Jiménez-Rodríguez JA, Reyes-Gutiérrez LG, Rosas-Meléndez S, Íñiguez-Rábago A, et al. Carbon nanotube bundles self-assembled in double helix microstructures. Carbon. 2012;**50**:3688-3693. DOI: 10.1016/j.carbon.2012.03.042 [50] Waring CE, Horton WS. The kinetics of the thermal decomposition of gaseous tetramethyltin. Journal of the American Chemical Society. 1945;**67**:540-547. DOI: 10.1021/

[51] Dyagileva LM, Mar'in VP, Tsyganova EI, Razuvaev GA. Reactivity of the first transition row metallocenes in thermal decomposition reaction. The Journal of Organic Chemistry.

[52] Suda Y, Maruyama K, Iida T, Takikawa H, Ue H, Shimizu K, et al. High-yield synthesis of helical carbon nanofibers using iron oxide fine powder as a catalyst. Crystals. 2015;**5**:47-60.

[53] Sugioka Y, Suda Y, Tanoue H, Takikawa H, Ue H, Shimizu K, et al. Effects of dielectric barrier discharge treatment conditions on the uprightness of carbon nanofibers. IEEE Transactions on Plasma Science. 2012;**40**:1794-1800. DOI: 10.1109/TPS.2012.2199769

Materials Research. 2012;**27**:431-439. DOI: 10.1557/jmr.2011.401

Carbon. 2008;**46**:1106-1109. DOI: 10.1016/j.carbon.2008.03.020

2010;**48**:365-376. DOI: 10.1016/j.carbon.2009.09.038

Materials. 2008;**20**:179-182. DOI: 10.1002/adma.200701143

1979;**175**:63-72. DOI: 10.1016/S0022-328X(00)82299-8

10.1557/jmr.2014.247

154 Chemical Vapor Deposition for Nanotechnology

jallcom.2009.05.064

2009.08.045

ja01220a014

DOI: 10.3390/cryst5010047

## *Edited by Pietro Mandracci*

Chemical vapor deposition (CVD) techniques have played a major role in the development of modern technology, and the rise of nanotechnology has further increased their importance, thanks to techniques such as atomic layer deposition (ALD) and vapor liquid solid growth, which are able to control the growth process at the nanoscale. This book aims to contribute to the knowledge of recent developments in CVD technology and its applications. To this aim, important process innovations, such as spatial ALD, direct liquid injection CVD, and electron cyclotron resonance CVD, are presented. Moreover, some of the most recent applications of CVD techniques for the growth of nanomaterials, including graphene, nanofibers, and diamond-like carbon, are described in the book.

Published in London, UK © 2019 IntechOpen © taffpix / iStock

Chemical Vapor Deposition for Nanotechnology

Chemical Vapor Deposition

for Nanotechnology