Preface

**Section 3 Nanoimprint 167**

**VI** Contents

Hongbo Lan

**Lithography 197** Grégory Barbillon

**Substrate 211**

**Section 4 Fabrication of 3D Nano-Structure 209**

Chapter 7 **Soft UV Nanoimprint Lithography and Its Applications 169**

Chapter 8 **Sub-30 nm Plasmonic Nanostructures by Soft UV Nanoimprint**

Chapter 9 **The Fabrication of High Aspect Ratio Nanostructures on Quartz**

Chapter 10 **Fabrication of 3D Micro- and Nano-Structures by Prism-Assisted**

Khairudin Mohamed and Maan M. Alkaisi

**UV and Holographic Lithography 227** Guomin Jiang, Kai Shen and Michael R. Wang

Advanced lithography grows up to several fields such as nano-lithography, micro electromechanical system (MEMS) and nano-phonics, etc. Nano-lithography reaches to 20 nm size in advanced electron device. Consequently, we have to study and develop true single nano‐ meter size lithography. One of the solutions is to study a fusion of top down and bottom up technologies such as EB drawing and self-assembly with block copolymer. In MEMS and nano-photonics, 3 dimensional structures are needed to achieve some functions in the devi‐ ces for the applications. Their formation are done by several methods such as colloid lithog‐ raphy, stereo-lithography, dry etching, sputtering, deposition, etc. This book covers a wide area regarding nano-lithography, nano structure and 3-dimensional structure, and introdu‐ ces readers to the methods, methodology and its applications.

> **Prof. Sumio Hosaka** Graduate School of Engineering, Gunma University, Japan

**Section 1**

**Lithography for 3D Structure and Nano Scale**

**Lithography for 3D Structure and Nano Scale**

**Chapter 1**

**Colloidal Lithography**

Additional information is available at the end of the chapter

block copolymers, and self-assembly of proteins and nanoparticles.

The advent of nanoscience and nanotechnology has led to tremendous enthusiasm of re‐ searchers from different scientific disciplines such as physics, chemistry, and biology to engage with nanostructures with the intent of pursuing the innovative property derived from the nanometer dimension. In this context, fabrication of nanostructures accordingly becomes an increasing demand nowadays. Obviously, low-throughput and expensive maskless lithogra‐ phy is a less accessible choice for chemists, physicists, material scientists, and biologists. The success of extending mask-assisted lithography beyond microelectronics workshops is largely limited by the mask design and preparation. Recently a host of effort has been devoted to develop non-conventional lithographic techniques especially integrated with a bottom-up nanochemical procedure for surface patterning with low cost, flexible processing capability, and high throughput. However, most of the non-conventional lithographic techniques require an assistance of conventional lithographic techniques such as photolithography to design and make masks or masters. To develop ingenious, cheap, and non-lithographic ways to make masks or masters with high resolution (below 100 nm), a great deal of self-assembly nano‐ structures have been recruited for masking, including laterally structured Langmuir–Blodgett monolayers, liquid crystalline structures of surfactants, micro-phase separation structures of

Monodisperse colloidal particles with size ranging from tens of nanometers to tens of micro‐ meters can be easily synthesized via wet chemistry ways such as emulsion polymerization and sol-gel synthesis. Due to the size and shape monodispersity, they can self-assemble into a two dimensional (2D) and three dimensional (3D) extended periodic array, usually referred to as colloidal crystal. Colloidal crystals are usually characterized by a brilliant iridescence arising from the Bragg reflection of light by their periodic structures. Despite the beauty, the iridescent color has recently inspired the explosive study of fabrication of 3D colloidal crystals or inverse opals – 3D inverted replication of the crystals – for pursuing a complete energy bandgap to

> © 2013 Yu and Zhang; licensee InTech. This is an open access article 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.

© 2013 Yu and Zhang; licensee InTech. This is a paper 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.

Ye Yu and Gang Zhang

http://dx.doi.org/10.5772/56576

**1. Introduction**

## **Chapter 1**

## **Colloidal Lithography**

Ye Yu and Gang Zhang

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/56576

## **1. Introduction**

The advent of nanoscience and nanotechnology has led to tremendous enthusiasm of re‐ searchers from different scientific disciplines such as physics, chemistry, and biology to engage with nanostructures with the intent of pursuing the innovative property derived from the nanometer dimension. In this context, fabrication of nanostructures accordingly becomes an increasing demand nowadays. Obviously, low-throughput and expensive maskless lithogra‐ phy is a less accessible choice for chemists, physicists, material scientists, and biologists. The success of extending mask-assisted lithography beyond microelectronics workshops is largely limited by the mask design and preparation. Recently a host of effort has been devoted to develop non-conventional lithographic techniques especially integrated with a bottom-up nanochemical procedure for surface patterning with low cost, flexible processing capability, and high throughput. However, most of the non-conventional lithographic techniques require an assistance of conventional lithographic techniques such as photolithography to design and make masks or masters. To develop ingenious, cheap, and non-lithographic ways to make masks or masters with high resolution (below 100 nm), a great deal of self-assembly nano‐ structures have been recruited for masking, including laterally structured Langmuir–Blodgett monolayers, liquid crystalline structures of surfactants, micro-phase separation structures of block copolymers, and self-assembly of proteins and nanoparticles.

Monodisperse colloidal particles with size ranging from tens of nanometers to tens of micro‐ meters can be easily synthesized via wet chemistry ways such as emulsion polymerization and sol-gel synthesis. Due to the size and shape monodispersity, they can self-assemble into a two dimensional (2D) and three dimensional (3D) extended periodic array, usually referred to as colloidal crystal. Colloidal crystals are usually characterized by a brilliant iridescence arising from the Bragg reflection of light by their periodic structures. Despite the beauty, the iridescent color has recently inspired the explosive study of fabrication of 3D colloidal crystals or inverse opals – 3D inverted replication of the crystals – for pursuing a complete energy bandgap to

© 2013 Yu and Zhang; licensee InTech. This is an open access article 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. © 2013 Yu and Zhang; licensee InTech. This is a paper 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.

manipulate electromagnetic waves, similar to that to do to electrons in semiconductors. Before being used as photonic materials, both the ordered arrays of solid particles and those of the interstices between the particles of colloidal crystals have already been used as masks or templates for surface patterning via for instance etching or deposition of materials. This bottom-up masking methodology has recently gained increasing attention for surface pat‐ terning due to the processing simplicity, the low cost, the flexibility of extending on various substrates with different surface chemistry and even curvatures, the ease of scaling down the feature size below 100 nm. In the present chapter, we refer to as various surface patterning processes based on use of colloidal crystals as masks as a whole as colloidal lithography (CL), overview the processing principles, and survey the recent advances.

on a solid substrate via sedimentation by tilting the substrate about 9° and keeping the system

Colloidal Lithography

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**Figure 1.** A) Two spheres partially immersed in a liquid layer on a horizontal solid substrate. The deformation of the liquid meniscus gives rise to interparticle attraction. (B) Convective flux toward the ordered phase due to the water evaporation from the menisci between the particles in the 2D array. (C) Photographs of 2D-crystal growth. Reprinted

When a supporting substrate is held vertically in a suspension of colloidal particles, moving the front of the suspension flow either by the solvent evaporation or by withdrawing the substrate out of the suspension can pin colloidal particles on the substrates – nucleation – and the convective transfer of the particles from the bulk phase to the drying front – crystallization (Fig. 2) [9]. The thickness of colloidal crystals obtained via vertical deposition is dependent on the ratio of the thickness of the liquid films remaining of supporting substrates to the diameter of the colloidal particles [9]. When the ratio is far larger than 1, 3D colloidal crystals are obtained with high quality; the crystal thickness can be tuned by the particle concentration [10]. When the ratio is comparable to or smaller than 1, 2D colloidal crystals can be obtained [9]. Vertical deposition may allow formation of large-area crack-free colloidal crystals provided the suspensions of colloidal particles wet well supporting substrates, there is no interaction between the particles and the substrates, the suspensions are sufficiently stable and the solvent

temperature constantly using a Peltier cell [8].

with permission [7].

*2.1.2. Vertical deposition*

evaporation is well controlled [9].

## **2. Colloidal masks**

The success of using colloidal crystals as masks for surface patterning is determined by the capability of directing self-assembly of colloidal particles and manipulating the crystal packing structures. Provided their size and shape are monodisperse, colloidal particles can be readily to self-assemble into long-range ordered arrays with a hexagonal packing, driven simply by entropic depletion and gravity. Up to date a variety of colloidal crystallization techniques – with and without the aid of templates – have successfully been developed to implement colloidal crystallization in a controlled fashion [1-3]. Due to enormous numbers of publications on colloidal crystallization and immense diversity of crystallization techniques reported thus far and especially by taking into account that colloidal lithography relies on masking of single layers or double layers of colloidal crystals, this section is centered mainly on techniques for 2D colloidal crystallization developed thus far.

#### **2.1. Simple colloidal masks**

#### *2.1.1. Sedimentation*

Sedimentation is a natural way for colloidal crystallization. In dispersion colloidal particles tend to settle out of the fluid under gravity and to accumulate and precipitate on a wall, which can be described by Stokes' law. This sedimentation process can be used to grow colloidal crystals with high quality, and the crystal thickness can be tuned by the particle concentration. However, the sedimentation time is always up to several hundreds of hours; time-consuming is a big drawback of this technique [4-6].

At the beginning of 1990's Nagayama's group has commenced a systematic study of sedi‐ mentation of colloidal particles in the presence of strongly attractive capillary forces [7]. With the help of optical microscopy and using a Teflon ring to confine the dispersions of colloidal particles, they have directly observed the particle sedimentation dynamics on a solid substrate. Their observations suggest a two-stage mechanism for 2D colloidal crystallization: 1) nuclea‐ tion and 2) crystal growth (Fig. 1) [7]. Micheletto's group has fabricated 2D colloidal crystals on a solid substrate via sedimentation by tilting the substrate about 9° and keeping the system temperature constantly using a Peltier cell [8].

**Figure 1.** A) Two spheres partially immersed in a liquid layer on a horizontal solid substrate. The deformation of the liquid meniscus gives rise to interparticle attraction. (B) Convective flux toward the ordered phase due to the water evaporation from the menisci between the particles in the 2D array. (C) Photographs of 2D-crystal growth. Reprinted with permission [7].

#### *2.1.2. Vertical deposition*

manipulate electromagnetic waves, similar to that to do to electrons in semiconductors. Before being used as photonic materials, both the ordered arrays of solid particles and those of the interstices between the particles of colloidal crystals have already been used as masks or templates for surface patterning via for instance etching or deposition of materials. This bottom-up masking methodology has recently gained increasing attention for surface pat‐ terning due to the processing simplicity, the low cost, the flexibility of extending on various substrates with different surface chemistry and even curvatures, the ease of scaling down the feature size below 100 nm. In the present chapter, we refer to as various surface patterning processes based on use of colloidal crystals as masks as a whole as colloidal lithography (CL),

The success of using colloidal crystals as masks for surface patterning is determined by the capability of directing self-assembly of colloidal particles and manipulating the crystal packing structures. Provided their size and shape are monodisperse, colloidal particles can be readily to self-assemble into long-range ordered arrays with a hexagonal packing, driven simply by entropic depletion and gravity. Up to date a variety of colloidal crystallization techniques – with and without the aid of templates – have successfully been developed to implement colloidal crystallization in a controlled fashion [1-3]. Due to enormous numbers of publications on colloidal crystallization and immense diversity of crystallization techniques reported thus far and especially by taking into account that colloidal lithography relies on masking of single layers or double layers of colloidal crystals, this section is centered mainly on techniques for

Sedimentation is a natural way for colloidal crystallization. In dispersion colloidal particles tend to settle out of the fluid under gravity and to accumulate and precipitate on a wall, which can be described by Stokes' law. This sedimentation process can be used to grow colloidal crystals with high quality, and the crystal thickness can be tuned by the particle concentration. However, the sedimentation time is always up to several hundreds of hours; time-consuming

At the beginning of 1990's Nagayama's group has commenced a systematic study of sedi‐ mentation of colloidal particles in the presence of strongly attractive capillary forces [7]. With the help of optical microscopy and using a Teflon ring to confine the dispersions of colloidal particles, they have directly observed the particle sedimentation dynamics on a solid substrate. Their observations suggest a two-stage mechanism for 2D colloidal crystallization: 1) nuclea‐ tion and 2) crystal growth (Fig. 1) [7]. Micheletto's group has fabricated 2D colloidal crystals

overview the processing principles, and survey the recent advances.

**2. Colloidal masks**

4 Updates in Advanced Lithography

2D colloidal crystallization developed thus far.

is a big drawback of this technique [4-6].

**2.1. Simple colloidal masks**

*2.1.1. Sedimentation*

When a supporting substrate is held vertically in a suspension of colloidal particles, moving the front of the suspension flow either by the solvent evaporation or by withdrawing the substrate out of the suspension can pin colloidal particles on the substrates – nucleation – and the convective transfer of the particles from the bulk phase to the drying front – crystallization (Fig. 2) [9]. The thickness of colloidal crystals obtained via vertical deposition is dependent on the ratio of the thickness of the liquid films remaining of supporting substrates to the diameter of the colloidal particles [9]. When the ratio is far larger than 1, 3D colloidal crystals are obtained with high quality; the crystal thickness can be tuned by the particle concentration [10]. When the ratio is comparable to or smaller than 1, 2D colloidal crystals can be obtained [9]. Vertical deposition may allow formation of large-area crack-free colloidal crystals provided the suspensions of colloidal particles wet well supporting substrates, there is no interaction between the particles and the substrates, the suspensions are sufficiently stable and the solvent evaporation is well controlled [9].

**Figure 2.** A) Sketch of the particle and water fluxes in the vicinity of monolayer particle arrays growing on a substrate plate that is being withdrawn from a suspension. The inset shows the menisci shape between neighbouring particles. (B and C) A part of the leading edge of a growing monolayer particle array. The upper-half of the photographs shows the formations of (B) differently oriented small domains of ordered 814-nm particles and (C) a single domain of or‐ dered 953-nm particles. The lower-half shows particles dragged by the water flow toward the forming monolayer. Reprinted with permission [9].

crystal of the larger particles firstly formed on the substrate is used to template the growth of the 2D colloidal crystal of the smaller particles. By deliberately tuning the concentration of the small particle suspension, binary colloidal crystals with the stoichiometric ratios of large to

**Figure 3.** Library of surface micropatterns produced by accelerated evaporation co-assembly of binary dispersions of monodisperse microspheres with large size ratio and imaged with field emission scanning electron microscopy. Larger spheres of all binary dispersions were PS latex of size, *dL*= 1.28 μm, while varying their volume fraction (φ*L*), the volume fraction (φ*S*), and size (*dS*) of smaller spheres. Scale bars in (a-e) and (g-i) are 3 μm and that in (f) is 1 μm. Reprinted

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7

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Spin coating was the first technique for growth of 2D colloidal crystal masks for colloidal lithography due to the fact that it allows easy and quick formation of 2D crystals over large area [22]. The long range ordering degree of 2D colloidal crystals obtained via spin coating can be improved by increasing the wettability of the suspensions of colloidal particles on sup‐ porting substrates by for instance adding ethylene glycol into the suspension [23]. However, the spin coating is a process far more complicated than it appears and the underlying mech‐ anism remains in debate. Rehg and Higgins have conducted a theoretical analysis of the physics governing spin coating of a colloidal particle suspension on a planar substrate [24]. Jiang and Mcfarland have succeeded in fabrication of wafer scale long-rang ordered and nonclose-packed 2D and 3D colloidal crystals by spin coating of highly viscous triacrylate

small particle sizes of 1:2, 1:3, 1:4, or 1:5 have been constructed [20, 21].

*2.1.3. Spin coating*

with permission [19].

Dip-coating is a fast and dip-coater assisted variant of vertical deposition [11]. Besides, a number of techniques have been developed to improve the efficiency and quality of colloidal crystallization via vertical deposition, such as variable-flow deposition [12], isothermal heating evaporation-induced self-assembly [13], two-substrate deposition [14], reduction of the humidity fluctuation [15], adjustment of the meniscus shape [16], temperature-induced convective flow [17] and vertical deposition with a tilted angle [18]. The maximal size of colloidal particles used for vertical deposition is limited by the particles sedimentation of colloidal particles, for instance 400-500 nm for silica particles and 1 μm for polystyrene particles. To compete with sedimentation, Kitaev and Ozin have used low pressure to accelerate the solvent evaporation, and successful growth of large-area 2D binary colloidal crystals with the diameter ratios of the large particles to the small ones in the range of 0.175 to 0.225 (Fig. 3) [19].

Vertical deposition has recently been extended to stepwise growth of 2D colloidal crystals with large and small colloidal particles on a substrate [20, 21]. In their procedure, the 2D colloidal

**Figure 3.** Library of surface micropatterns produced by accelerated evaporation co-assembly of binary dispersions of monodisperse microspheres with large size ratio and imaged with field emission scanning electron microscopy. Larger spheres of all binary dispersions were PS latex of size, *dL*= 1.28 μm, while varying their volume fraction (φ*L*), the volume fraction (φ*S*), and size (*dS*) of smaller spheres. Scale bars in (a-e) and (g-i) are 3 μm and that in (f) is 1 μm. Reprinted with permission [19].

crystal of the larger particles firstly formed on the substrate is used to template the growth of the 2D colloidal crystal of the smaller particles. By deliberately tuning the concentration of the small particle suspension, binary colloidal crystals with the stoichiometric ratios of large to small particle sizes of 1:2, 1:3, 1:4, or 1:5 have been constructed [20, 21].

#### *2.1.3. Spin coating*

Dip-coating is a fast and dip-coater assisted variant of vertical deposition [11]. Besides, a number of techniques have been developed to improve the efficiency and quality of colloidal crystallization via vertical deposition, such as variable-flow deposition [12], isothermal heating evaporation-induced self-assembly [13], two-substrate deposition [14], reduction of the humidity fluctuation [15], adjustment of the meniscus shape [16], temperature-induced convective flow [17] and vertical deposition with a tilted angle [18]. The maximal size of colloidal particles used for vertical deposition is limited by the particles sedimentation of colloidal particles, for instance 400-500 nm for silica particles and 1 μm for polystyrene particles. To compete with sedimentation, Kitaev and Ozin have used low pressure to accelerate the solvent evaporation, and successful growth of large-area 2D binary colloidal crystals with the diameter ratios of the large particles to the small ones in the range of 0.175 to

**Figure 2.** A) Sketch of the particle and water fluxes in the vicinity of monolayer particle arrays growing on a substrate plate that is being withdrawn from a suspension. The inset shows the menisci shape between neighbouring particles. (B and C) A part of the leading edge of a growing monolayer particle array. The upper-half of the photographs shows the formations of (B) differently oriented small domains of ordered 814-nm particles and (C) a single domain of or‐ dered 953-nm particles. The lower-half shows particles dragged by the water flow toward the forming monolayer.

Vertical deposition has recently been extended to stepwise growth of 2D colloidal crystals with large and small colloidal particles on a substrate [20, 21]. In their procedure, the 2D colloidal

0.225 (Fig. 3) [19].

Reprinted with permission [9].

6 Updates in Advanced Lithography

Spin coating was the first technique for growth of 2D colloidal crystal masks for colloidal lithography due to the fact that it allows easy and quick formation of 2D crystals over large area [22]. The long range ordering degree of 2D colloidal crystals obtained via spin coating can be improved by increasing the wettability of the suspensions of colloidal particles on sup‐ porting substrates by for instance adding ethylene glycol into the suspension [23]. However, the spin coating is a process far more complicated than it appears and the underlying mech‐ anism remains in debate. Rehg and Higgins have conducted a theoretical analysis of the physics governing spin coating of a colloidal particle suspension on a planar substrate [24]. Jiang and Mcfarland have succeeded in fabrication of wafer scale long-rang ordered and nonclose-packed 2D and 3D colloidal crystals by spin coating of highly viscous triacrylate

*2.1.4. Colloidal crystallization at interface*

a Langmuir trough [36].

**2.2. Complex colloidal masks**

*2.2.1. Deformed colloidal masks*

Using the water/air interface as a platform for molecular self-assembly has been exten‐ sively studied. Langmuir-Blodgett (LB) technique has been proved as a powerful and ver‐ satile way to organize amphiphilic molecules (referring molecules that are hydrophobic on one end and hydrophilic on the other end) to macroscopic monolayer films at the wa‐ ter/air interface and transfer the films to solid substrates in a controlled manner [28]. It is also well studied but less recognized that in a biphasic system, e.g. water/oil, colloidal particles behave rather similar to amphiphilic molecules; they thermo- dynamically prefer to attach to the interface [29]. Due to this analogy, the water/air interface has been extend‐ ed to support self-assembly of colloidal particles. Pieranski has conducted the first delib‐ erate microscopic observation of 2D colloidal crystallization at the water/air interface and hypothesized the repulsive interaction between the dipoles of colloidal particles trapped at the interface due to the asymmetric charge distribution on the particle surface drives the particles to self-assemble into an ordered array (Fig. 5) [30]. Park *et al.* have devel‐ oped heat-assisted interfacial colloidal crystallization, while the success of their technique relies on the convective flow generated during heating rather than the interface activity of colloidal particles [31]. Once 2D colloidal crystals are formed at the water/air interface, the LB technique has been used to transfer of them on different substrates [32-35]. Of sig‐ nificance is that the LB technique allows repetition of transfer of 2D colloidal crystals on a

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substrate into 3D colloidal crystals with precisely defined layer numbers [35].

Colloidal monolayers with high order and increased complexity beyond plain hexagonal packing geometries are useful for 2D templating of surface nanostructures and lithographic applications. Weiss and co-workers developed binary colloidal monolayers featuring a closepacked monolayer of large spheres with a superlattice of small particles in a single step using

As compared with the water/air interface, the water/oil interface is a much better platform to trap colloidal particles due to the relatively low interfacial tension [29]. Thus, water/oil interfaces have been used for growth of 2D colloidal crystals [37, 38], while transfer of the resulting 2D colloidal crystals to solid substrates remains problematic. Besides water/air interfaces, air/water/air interfaces have also been utilized for colloidal crystallization. Velikov and coworkers have studied of colloidal crystallization in thinning foam films [39]. Using air/ water/air interfaces for crystallization, Wang and co-workers have successfully obtained freestanding and crack-free colloidal crystal films with sizes over several square millimeters [40]. Instead of water/air interface, Zental and co-workers have used the interface between melted germanium and air for colloidal crystallization and obtained crack-free colloidal crystals [41].

In general, polymers undergo a second-order phase transition from hard glassy state to soft rubbery state above a glass transition temperature (Tg) due to the free-volume change between the polymer chains. Therefore, annealing slightly above Tg can cause deformation of spherical

**Figure 4.** Left panel: Illustration of the procedure used to fabricate binary colloidal crystals by stepwise spin coating. Right panel: SEM micrographs of the binary colloidal crystals produced by stepwise spin coating at a spin speed of 3000 rpm, in which 519 nm (a), 442 nm (b), and 222 nm silica spheres (c) were confined within the interstices be‐ tween hexagonal close packed 891 nm silica spheres. Reprinted with permission [27].

suspension of silica particles and subsequent polymerization of triacrylate, followed by partial removal of the polymer matrices [25, 26]. Wang and Möhwald have developed a stepwise spin coating protocol to consecutively deposit large and small colloidal particles into binary colloidal crystals, in which the interstitial arrays in the 2D colloidal crystal of the large particles are used to template the deposition of the small particles due to the spatial and depletion entrapment (Fig. 4) [27].

## *2.1.4. Colloidal crystallization at interface*

Using the water/air interface as a platform for molecular self-assembly has been exten‐ sively studied. Langmuir-Blodgett (LB) technique has been proved as a powerful and ver‐ satile way to organize amphiphilic molecules (referring molecules that are hydrophobic on one end and hydrophilic on the other end) to macroscopic monolayer films at the wa‐ ter/air interface and transfer the films to solid substrates in a controlled manner [28]. It is also well studied but less recognized that in a biphasic system, e.g. water/oil, colloidal particles behave rather similar to amphiphilic molecules; they thermo- dynamically prefer to attach to the interface [29]. Due to this analogy, the water/air interface has been extend‐ ed to support self-assembly of colloidal particles. Pieranski has conducted the first delib‐ erate microscopic observation of 2D colloidal crystallization at the water/air interface and hypothesized the repulsive interaction between the dipoles of colloidal particles trapped at the interface due to the asymmetric charge distribution on the particle surface drives the particles to self-assemble into an ordered array (Fig. 5) [30]. Park *et al.* have devel‐ oped heat-assisted interfacial colloidal crystallization, while the success of their technique relies on the convective flow generated during heating rather than the interface activity of colloidal particles [31]. Once 2D colloidal crystals are formed at the water/air interface, the LB technique has been used to transfer of them on different substrates [32-35]. Of sig‐ nificance is that the LB technique allows repetition of transfer of 2D colloidal crystals on a substrate into 3D colloidal crystals with precisely defined layer numbers [35].

Colloidal monolayers with high order and increased complexity beyond plain hexagonal packing geometries are useful for 2D templating of surface nanostructures and lithographic applications. Weiss and co-workers developed binary colloidal monolayers featuring a closepacked monolayer of large spheres with a superlattice of small particles in a single step using a Langmuir trough [36].

As compared with the water/air interface, the water/oil interface is a much better platform to trap colloidal particles due to the relatively low interfacial tension [29]. Thus, water/oil interfaces have been used for growth of 2D colloidal crystals [37, 38], while transfer of the resulting 2D colloidal crystals to solid substrates remains problematic. Besides water/air interfaces, air/water/air interfaces have also been utilized for colloidal crystallization. Velikov and coworkers have studied of colloidal crystallization in thinning foam films [39]. Using air/ water/air interfaces for crystallization, Wang and co-workers have successfully obtained freestanding and crack-free colloidal crystal films with sizes over several square millimeters [40]. Instead of water/air interface, Zental and co-workers have used the interface between melted germanium and air for colloidal crystallization and obtained crack-free colloidal crystals [41].

#### **2.2. Complex colloidal masks**

suspension of silica particles and subsequent polymerization of triacrylate, followed by partial removal of the polymer matrices [25, 26]. Wang and Möhwald have developed a stepwise spin coating protocol to consecutively deposit large and small colloidal particles into binary colloidal crystals, in which the interstitial arrays in the 2D colloidal crystal of the large particles are used to template the deposition of the small particles due to the spatial and depletion

tween hexagonal close packed 891 nm silica spheres. Reprinted with permission [27].

**Figure 4.** Left panel: Illustration of the procedure used to fabricate binary colloidal crystals by stepwise spin coating. Right panel: SEM micrographs of the binary colloidal crystals produced by stepwise spin coating at a spin speed of 3000 rpm, in which 519 nm (a), 442 nm (b), and 222 nm silica spheres (c) were confined within the interstices be‐

entrapment (Fig. 4) [27].

8 Updates in Advanced Lithography

#### *2.2.1. Deformed colloidal masks*

In general, polymers undergo a second-order phase transition from hard glassy state to soft rubbery state above a glass transition temperature (Tg) due to the free-volume change between the polymer chains. Therefore, annealing slightly above Tg can cause deformation of spherical

Giersig and coworkers have recently developed a new annealing approach – using microwave pulse to heat polystyrene (PS) microspheres in a mixture of good and poor solvents for PS, which allows not only reduction of the sizes of the interstices of 2D PS colloidal crystals but also deformation of their geometry from triangular to rodlike, while preserving the interpar‐ ticle spacing and packing order of the original crystals (Fig. 6) [43]. Recently, Yang *et al.* have demonstrated a photolithographic process to produce hierarchical arrays of nanopores or nanobowls with using colloidal crystals of photoresist particles [44]. In the case of inorganic particles, deformation is hard to achieve by thermal annealing. Polman and coworkers have successfully deformed silica@Au core-shell microspheres to oblate ellipsoids by using high energy ion irradiation due to the fact that the ion-induced deformation of the silica core is counteracted by the mechanical constraint of the gold shell [45]. Vossen and coworkers have recently reported that silica particles undergo anisotropic deformation under ion bombard‐

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Many specific colloidal masks have been made using methods mentioned above, usually utilizing one or two kinds of spherical colloidal particles as building blocks. Colloidal particles with anisotropic interactions are expected to enable a wide range of materials with novel optical and mechanical properties. While the self-assembly of spherical particles into periodic structures is relatively robust and well-characterized, the phase space describing the selfassembly of anisotropic particles is vast and has been only partially explored. It includes phases that are impossible for spherical particles to form, including gyroids, simple cubic lattices, and

Eric R. Dufresne and co-workers demonstrated the use of an external electric field to align and assemble the dumbbells to make a birefringent suspension with structural color. In this way, dumbbells combine the structural color of photonic crystals with the field addressability of liquid crystals. In addition, if the solvent is removed in the presence of an electric field, the particles self-assemble into a novel, dense crystalline packing hundreds of particles thick,

When a 2D colloidal crystal is formed on a solid substrate, the interstices between the solid particles can used as masks for reactive ions to create patterned bumps or pores on the substrate. In the beginning of 1980's Deckmann and Dunsmuir have pioneered the work of etching of a colloidal crystal into a textured surface using a reactive ion beam (RIE) [48]. Since then, reactive ion etching (RIE) has been widely used to interdependently reduce the particle sizes and thus widen the interstitial space in 2D colloidal crystal masks and eventually turn close-packing structures of the crystals to non-close packing one (vide infra). RIE in 3D colloidal crystals is an anisotropic process as the upper layers act as shadow masks for etching

ment due to expansion in the plane perpendicular to the ion beam [46].

*2.2.2. Colloidal masks derived from modified colloidal particles*

plastic crystals.

which was shown in Fig. 7 [47].

**3. Colloidal lithography**

**3.1. Controllable etching**

**Figure 5.** Upper panel: Schematic of the model of interaction of colloidal particles at the water (W)/air (A) interface. Lower panel: Photographs of polystyrene spheres (black dots) trapped at water/air interface. (a) Crystalline structure; (b) disordered structure. Reprinted with permission [30].

**Figure 6.** Precise control of the degree of annealing is achieved via adjustment of the number of microwave expo‐ sures: A 540 nm PS latex mask annealed in 25 mL of water/EtOH/acetone mixture by A) 1, B) 2, C) 4, D) 6, E) 7, and F) 10 microwave pulses. Scale bar: 500 nm. Reprinted with permission [43].

polymeric beads. It is demonstrated that microwave radiation can much more precisely control the deformation of spherical polymer particles by adjusting the microwave intensity than heating in oven [42].

Giersig and coworkers have recently developed a new annealing approach – using microwave pulse to heat polystyrene (PS) microspheres in a mixture of good and poor solvents for PS, which allows not only reduction of the sizes of the interstices of 2D PS colloidal crystals but also deformation of their geometry from triangular to rodlike, while preserving the interpar‐ ticle spacing and packing order of the original crystals (Fig. 6) [43]. Recently, Yang *et al.* have demonstrated a photolithographic process to produce hierarchical arrays of nanopores or nanobowls with using colloidal crystals of photoresist particles [44]. In the case of inorganic particles, deformation is hard to achieve by thermal annealing. Polman and coworkers have successfully deformed silica@Au core-shell microspheres to oblate ellipsoids by using high energy ion irradiation due to the fact that the ion-induced deformation of the silica core is counteracted by the mechanical constraint of the gold shell [45]. Vossen and coworkers have recently reported that silica particles undergo anisotropic deformation under ion bombard‐ ment due to expansion in the plane perpendicular to the ion beam [46].

## *2.2.2. Colloidal masks derived from modified colloidal particles*

Many specific colloidal masks have been made using methods mentioned above, usually utilizing one or two kinds of spherical colloidal particles as building blocks. Colloidal particles with anisotropic interactions are expected to enable a wide range of materials with novel optical and mechanical properties. While the self-assembly of spherical particles into periodic structures is relatively robust and well-characterized, the phase space describing the selfassembly of anisotropic particles is vast and has been only partially explored. It includes phases that are impossible for spherical particles to form, including gyroids, simple cubic lattices, and plastic crystals.

Eric R. Dufresne and co-workers demonstrated the use of an external electric field to align and assemble the dumbbells to make a birefringent suspension with structural color. In this way, dumbbells combine the structural color of photonic crystals with the field addressability of liquid crystals. In addition, if the solvent is removed in the presence of an electric field, the particles self-assemble into a novel, dense crystalline packing hundreds of particles thick, which was shown in Fig. 7 [47].

## **3. Colloidal lithography**

### **3.1. Controllable etching**

polymeric beads. It is demonstrated that microwave radiation can much more precisely control the deformation of spherical polymer particles by adjusting the microwave intensity than

**Figure 6.** Precise control of the degree of annealing is achieved via adjustment of the number of microwave expo‐ sures: A 540 nm PS latex mask annealed in 25 mL of water/EtOH/acetone mixture by A) 1, B) 2, C) 4, D) 6, E) 7, and F)

**Figure 5.** Upper panel: Schematic of the model of interaction of colloidal particles at the water (W)/air (A) interface. Lower panel: Photographs of polystyrene spheres (black dots) trapped at water/air interface. (a) Crystalline structure;

(b) disordered structure. Reprinted with permission [30].

10 Updates in Advanced Lithography

10 microwave pulses. Scale bar: 500 nm. Reprinted with permission [43].

heating in oven [42].

When a 2D colloidal crystal is formed on a solid substrate, the interstices between the solid particles can used as masks for reactive ions to create patterned bumps or pores on the substrate. In the beginning of 1980's Deckmann and Dunsmuir have pioneered the work of etching of a colloidal crystal into a textured surface using a reactive ion beam (RIE) [48]. Since then, reactive ion etching (RIE) has been widely used to interdependently reduce the particle sizes and thus widen the interstitial space in 2D colloidal crystal masks and eventually turn close-packing structures of the crystals to non-close packing one (vide infra). RIE in 3D colloidal crystals is an anisotropic process as the upper layers act as shadow masks for etching

ally used polymer masks such as photoresists removed by organic developers, colloidal masks can be removed easily by ultrasonication and thus cause less damage to nanostructured substrates obtained via RIE. Ordered arrays of polyacrylic acid domes have been fabricated by using 2D PS colloidal crystals as masks for O2 RIE of the polymeric films; the removal of the PS masks has no damage to the surface chemistry and the structure of the resulting polymeric domes, thus enabling conjugation of proteins [55]. 2D PS colloidal crystals have been also used as masks for dry etching of SiO2 slides to create periodic arrays of nanoplates, which can be transferred onto polymer films by imprinting [56]. Using colloidal crystals as masks for catalytic etching, Zhu *et al.* have fabricated large-scale periodic arrays of silicon nanowires and the diameters and heights of the nanowires and the center-to-center distances between the nanowires can be accurately controlled [57]. Using colloidal crystals as masks to create arrays of nanopores on supporting solid substrates via RIE, followed by consecutively deposition of gold films and removal of the colloidal masks, Ong *et al.* have fabricated 2D ordered arrays of gold nanoparticles nested in the nanopores of the templated substrate [58]. One potential extension of having gold nanoparticles confined in nanopores is to use them as

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catalysts for growth of nanowires of other materials such as ZnO nanowires.

**Figure 8.** Modification of a mask using RIE for the fabrication of binary and ternary particle arrays with nonspherical building blocks. (a) and (b) Triangle arrays using binary and ternary colloidal spheres with an hcp arrangement. (c) and (d) Polygonal structures produced from colloidal layers with the (111) plane and the (100) plane of the *fcc* structure,

respectively. Reprinted with permission [49].

**Figure 7.** Crystal structure of suspension dried in an AC electric field. (a) SEM image of crystal formed by drying a sus‐ pension of dumbbells in the presence of an electric field. The field of view is 27 μm across. (b) SEM image highlighting crystal structure. Two adjoining hexagons formed by the dumbbell lobes are highlighted by the yellow hexagons. The field of view is 3.6 μm across. (c) Model of the crystal structure suggested by SEM images. Two adjoining hexagons are highlighted and correspond to the highlighted facet in (b). (d) Packing fraction versus aspect ratio for crystalline struc‐ tures (line) and random, jammed packings (circles) generated from numerical simulations described in the Methods section. Reprinted with permission [47].

the lower layer particles. This anisotropic RIE can turn spherical particles to non-spherical particles, and the particle shapes and the hierarchical nanostructures obtained so strongly depends on the stacking sequence of the colloidal crystals, the crystal orientation relative to the substrate, the number of colloidal layers, and the RIE conditions (Fig. 8) [49]. Of most significance is that the anisotropic RIE paves a new way to machine the surfaces of colloidal particles. Such as nanopores arranged in threefold or fourfold symmetry, depending on the crystalline orientation of the original colloidal crystals, were machined on PS particles [50-52].

Forests of silicon pillars with diameters of sub-500 nm and an aspect ratio of up to 10 were have been fabricated by firstly conduct O2 RIE to turn close-packed PS particle monolayers to non-close packed on and subsequently conduct a "Bosch" process to etch the supporting silicon wafers [53]. Sow *et al.* have demonstrated the characteristic features of a RIE silicon substrate using a PS colloidal crystal mask and produced a double dome structure by simultaneous etching of the mask and the regions beneath the particles [54]. As compared with convention‐ ally used polymer masks such as photoresists removed by organic developers, colloidal masks can be removed easily by ultrasonication and thus cause less damage to nanostructured substrates obtained via RIE. Ordered arrays of polyacrylic acid domes have been fabricated by using 2D PS colloidal crystals as masks for O2 RIE of the polymeric films; the removal of the PS masks has no damage to the surface chemistry and the structure of the resulting polymeric domes, thus enabling conjugation of proteins [55]. 2D PS colloidal crystals have been also used as masks for dry etching of SiO2 slides to create periodic arrays of nanoplates, which can be transferred onto polymer films by imprinting [56]. Using colloidal crystals as masks for catalytic etching, Zhu *et al.* have fabricated large-scale periodic arrays of silicon nanowires and the diameters and heights of the nanowires and the center-to-center distances between the nanowires can be accurately controlled [57]. Using colloidal crystals as masks to create arrays of nanopores on supporting solid substrates via RIE, followed by consecutively deposition of gold films and removal of the colloidal masks, Ong *et al.* have fabricated 2D ordered arrays of gold nanoparticles nested in the nanopores of the templated substrate [58]. One potential extension of having gold nanoparticles confined in nanopores is to use them as catalysts for growth of nanowires of other materials such as ZnO nanowires.

the lower layer particles. This anisotropic RIE can turn spherical particles to non-spherical particles, and the particle shapes and the hierarchical nanostructures obtained so strongly depends on the stacking sequence of the colloidal crystals, the crystal orientation relative to the substrate, the number of colloidal layers, and the RIE conditions (Fig. 8) [49]. Of most significance is that the anisotropic RIE paves a new way to machine the surfaces of colloidal particles. Such as nanopores arranged in threefold or fourfold symmetry, depending on the crystalline orientation of the original colloidal crystals, were machined on PS particles [50-52].

section. Reprinted with permission [47].

12 Updates in Advanced Lithography

**Figure 7.** Crystal structure of suspension dried in an AC electric field. (a) SEM image of crystal formed by drying a sus‐ pension of dumbbells in the presence of an electric field. The field of view is 27 μm across. (b) SEM image highlighting crystal structure. Two adjoining hexagons formed by the dumbbell lobes are highlighted by the yellow hexagons. The field of view is 3.6 μm across. (c) Model of the crystal structure suggested by SEM images. Two adjoining hexagons are highlighted and correspond to the highlighted facet in (b). (d) Packing fraction versus aspect ratio for crystalline struc‐ tures (line) and random, jammed packings (circles) generated from numerical simulations described in the Methods

Forests of silicon pillars with diameters of sub-500 nm and an aspect ratio of up to 10 were have been fabricated by firstly conduct O2 RIE to turn close-packed PS particle monolayers to non-close packed on and subsequently conduct a "Bosch" process to etch the supporting silicon wafers [53]. Sow *et al.* have demonstrated the characteristic features of a RIE silicon substrate using a PS colloidal crystal mask and produced a double dome structure by simultaneous etching of the mask and the regions beneath the particles [54]. As compared with convention‐

**Figure 8.** Modification of a mask using RIE for the fabrication of binary and ternary particle arrays with nonspherical building blocks. (a) and (b) Triangle arrays using binary and ternary colloidal spheres with an hcp arrangement. (c) and (d) Polygonal structures produced from colloidal layers with the (111) plane and the (100) plane of the *fcc* structure, respectively. Reprinted with permission [49].

## **3.2. Controllable deposition**

#### *3.2.1. Colloidal masks-assisted chemical deposition*

Combining microcontact printing with colloidal crystal masking, Xia *et al.* have developed a simple method – edge spreading lithography (ESL) – to generate mesoscopic structures on substrates [59]. As the name suggests, ESL utilizes the edges of masks – the perimeters of the footprint of particles on substrates – to define the features of resultant structures. The ESL procedure begins with formation of 2D colloidal crystals of silica beads on the surfaces of gold or silver thin films [59]. As shown in Fig. 9(left panel), typically, a planar polydimethylsiloxane (PDMS) stamp bearing a thin film of the ethanol solution of an alkanethiol was placed on a 2D silica colloidal crystal.

The thiol molecules were released from the stamp to the silica particle during contact and subsequently transferred to the substrate along the surfaces of silica particles, leading to a selfassembled monolayer (SAM) circling the footprint of each silica particle. The area of the thiol SAM could expand laterally via reactive spreading as long as the thiols were continuously supplies. Upon removal of the stamp and lift-off of the beads, the ring pattern was developed by wet etching with aqueous Fe3+/thiourea using the patterned SAM as a resist [59]. Of importance is that ESL allows generation of the concentric rings of different alkanethiol SAMs by successive printing different thiol inks, and the removal of silica particle templates and selective etching yield concentric gold rings and the width of the rings were determined by the printing period (Fig. 9, right panel) [60].

Shin *et al.* have developed another way to integrate colloidal masking and contact print‐ ing, referred to contact area lithography (CAL), to directly generate periodic surface chemical patterns at the sub-100 nm scale [61, 62]. Different from ESL, CAL relies on selfassembly of octadecyltrichlorosilane (OTS). After a 2D colloidal crystal of silica was formed on silicon wafer, the SAM of OTS was homogeneously grown both on silica parti‐ cles and the supporting silicon wafer via a sol-gel process. The removal of silica particles left behind a periodically arranged array of the openings in the OTS SAM with the same symmetry as that of 2D colloidal crystals. The openings were subsequently used as masks for growth of the ordered arrays of nanoparticles of such as titania or for selectively etch‐ ing of the ordered arrays of silica cavities on the silicon wafer. In the case of growth of ti‐ tania, nucleation is rather site-selective due to the significant difference in the surface energy between the growing and surrounding surfaces [63].

process for increasing the structural complexity of 2D colloidal crystal masks [48]. Since then the group of Van Duyne has devoted numerous efforts to develop patterning techni‐ ques using colloidal crystals as masks for metallic vapor deposition [23, 64-66]. In the con‐ text of nanoscience, they changed the name "Natural Lithography" to "Nanosphere Lithography" (NSL). Most important is that they have intensively investigated the plas‐ mon resonance properties of metallic patterns obtained via NSL and their correlation with the feature morphology with the intent of developing high sensitive biosensors based on

**Figure 9.** Left panel: Schematic illustration of the two-stepped ESL procedure used for side-by-side patterning of SHA and ECT monolayer rings on a gold substrate. Right panel. LFM images of concentric rings of carboxy- (bright), hy‐ droxy-(gray), and methyl-terminated (dark) thiolate monolayers on gold. a) The rings were fabricated under the fol‐ lowing conditions: 1 min for SHA, 1.5 min for HDDT, and 3 min for ECT. b) An increase in the printing times for HDDT and ECT to 3.5 and 4 min, respectively, resulted in wider rings for these two monolayers. c, d) The position of each monolayer in the concentric structure could be varied by changing the printing order. The pattern in (c) was generat‐ ed by printing HDDT for 1.5 min, followed by printing of SHA and ECT for 3 min each. The sample shown in (d) was prepared by printing both ECT and HDDT for 1 min, and SHA for 2 min. All scale bars correspond to 500 nm. Reprinted

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In a NSL procedure, a 2D colloidal crystal is used as masks for material deposition. The materials for physical deposition can be freely chosen without any limitations; commonly used are various metals such as gold and silver. The projection of the interstices between ordered

surface enhanced Raman spectroscopy (SERS) [67].

with permission [60].

#### *3.2.2. Colloidal masks-assisted physical deposition*

In 1981 Fischer and Zingsheim have used 2D colloidal crystals as masks for contact imag‐ ing with visible light [22]. A year later Deckman and Dunsmuir have demonstrated the feasibility of using 2D colloidal crystals as masks for both physical deposition of materials and in turn patterning the surfaces of supporting substrates [63]. They have coined the term "Natural Lithography" to describe this process as "naturally" assembled single lay‐ ers of latex particles were used as masks rather than lithographic masks. Later on they have expanded the capability of "Natural Lithography" and especially developed the RIE

**3.2. Controllable deposition**

14 Updates in Advanced Lithography

silica colloidal crystal.

*3.2.1. Colloidal masks-assisted chemical deposition*

the printing period (Fig. 9, right panel) [60].

energy between the growing and surrounding surfaces [63].

*3.2.2. Colloidal masks-assisted physical deposition*

Combining microcontact printing with colloidal crystal masking, Xia *et al.* have developed a simple method – edge spreading lithography (ESL) – to generate mesoscopic structures on substrates [59]. As the name suggests, ESL utilizes the edges of masks – the perimeters of the footprint of particles on substrates – to define the features of resultant structures. The ESL procedure begins with formation of 2D colloidal crystals of silica beads on the surfaces of gold or silver thin films [59]. As shown in Fig. 9(left panel), typically, a planar polydimethylsiloxane (PDMS) stamp bearing a thin film of the ethanol solution of an alkanethiol was placed on a 2D

The thiol molecules were released from the stamp to the silica particle during contact and subsequently transferred to the substrate along the surfaces of silica particles, leading to a selfassembled monolayer (SAM) circling the footprint of each silica particle. The area of the thiol SAM could expand laterally via reactive spreading as long as the thiols were continuously supplies. Upon removal of the stamp and lift-off of the beads, the ring pattern was developed by wet etching with aqueous Fe3+/thiourea using the patterned SAM as a resist [59]. Of importance is that ESL allows generation of the concentric rings of different alkanethiol SAMs by successive printing different thiol inks, and the removal of silica particle templates and selective etching yield concentric gold rings and the width of the rings were determined by

Shin *et al.* have developed another way to integrate colloidal masking and contact print‐ ing, referred to contact area lithography (CAL), to directly generate periodic surface chemical patterns at the sub-100 nm scale [61, 62]. Different from ESL, CAL relies on selfassembly of octadecyltrichlorosilane (OTS). After a 2D colloidal crystal of silica was formed on silicon wafer, the SAM of OTS was homogeneously grown both on silica parti‐ cles and the supporting silicon wafer via a sol-gel process. The removal of silica particles left behind a periodically arranged array of the openings in the OTS SAM with the same symmetry as that of 2D colloidal crystals. The openings were subsequently used as masks for growth of the ordered arrays of nanoparticles of such as titania or for selectively etch‐ ing of the ordered arrays of silica cavities on the silicon wafer. In the case of growth of ti‐ tania, nucleation is rather site-selective due to the significant difference in the surface

In 1981 Fischer and Zingsheim have used 2D colloidal crystals as masks for contact imag‐ ing with visible light [22]. A year later Deckman and Dunsmuir have demonstrated the feasibility of using 2D colloidal crystals as masks for both physical deposition of materials and in turn patterning the surfaces of supporting substrates [63]. They have coined the term "Natural Lithography" to describe this process as "naturally" assembled single lay‐ ers of latex particles were used as masks rather than lithographic masks. Later on they have expanded the capability of "Natural Lithography" and especially developed the RIE

**Figure 9.** Left panel: Schematic illustration of the two-stepped ESL procedure used for side-by-side patterning of SHA and ECT monolayer rings on a gold substrate. Right panel. LFM images of concentric rings of carboxy- (bright), hy‐ droxy-(gray), and methyl-terminated (dark) thiolate monolayers on gold. a) The rings were fabricated under the fol‐ lowing conditions: 1 min for SHA, 1.5 min for HDDT, and 3 min for ECT. b) An increase in the printing times for HDDT and ECT to 3.5 and 4 min, respectively, resulted in wider rings for these two monolayers. c, d) The position of each monolayer in the concentric structure could be varied by changing the printing order. The pattern in (c) was generat‐ ed by printing HDDT for 1.5 min, followed by printing of SHA and ECT for 3 min each. The sample shown in (d) was prepared by printing both ECT and HDDT for 1 min, and SHA for 2 min. All scale bars correspond to 500 nm. Reprinted with permission [60].

process for increasing the structural complexity of 2D colloidal crystal masks [48]. Since then the group of Van Duyne has devoted numerous efforts to develop patterning techni‐ ques using colloidal crystals as masks for metallic vapor deposition [23, 64-66]. In the con‐ text of nanoscience, they changed the name "Natural Lithography" to "Nanosphere Lithography" (NSL). Most important is that they have intensively investigated the plas‐ mon resonance properties of metallic patterns obtained via NSL and their correlation with the feature morphology with the intent of developing high sensitive biosensors based on surface enhanced Raman spectroscopy (SERS) [67].

In a NSL procedure, a 2D colloidal crystal is used as masks for material deposition. The materials for physical deposition can be freely chosen without any limitations; commonly used are various metals such as gold and silver. The projection of the interstices between ordered

their triangular shape (Fig. 11). By rotating substrates, Giersig and coworkers have recently found that AR-NSL can generate much more complicate metallic nanostructures and they referred to this process as shadow NSL [43, 68, 69]. Zhang and Wang have recently demon‐ strated the feasibility of consecutively depositing two different metals, such as gold and silver, at two different incidence angles, to construct ordered binary arrays of gold and silver nanoparticles [70].Due to the rotation of the colloidal mask, shadow NSL relies in a process resolved by the azimuth angle (ϕ) of the incidence deposition beam rather than the incidence

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**Figure 11.** Field emission SEM images of AR NSL fabricated - gold nanodot arrays and images with simulated geome‐ try superimposed, respectively. (A1, A2) θ = 108 º, ϕ = 288 º, (B1, B2) θ = 208 º, ϕ = 28 º, (C1, C2) θ = 268 º, ϕ = 168 º, and (D1, D2) θ = 408 º, ϕ = 28 º. All samples are Cr deposited onto Si (111) substrates. Images were collected at 40k

The elegant extension of AR-NSL is to stepwise conduct physical vapor deposition of identical or different materials at the different incidence angles. The group of Van Duyne has succeeded in growth of surface patterning features composed of two triangular nanodots either overlap‐ ped or separated by two deposition steps at θ = 0º and θ > 0º, respectively [65]. Giersig *et al.* have also developed a stepwise shadow NSL protocol to deposit different materials at different incidence angles when the colloidal masks were rotating and they have succeeded in encap‐

Prior to physical vapor deposition, colloidal crystal masks can undergo RIE to reduce the sizes of the particles and widen the interstitial spaces, thus increasing the dimension of triangular nanodots obtained via NSL. Increasing the RIE time can turn close-packed colloidal crystal masks to non-close packed ones, which leads to thin films with hexagonally arranged pores [71, 72]. Wang *et al.* have recently integrated AR-NSL with the use of RIE-modified colloidal crystals as masks to diversify the structural complexity of the patterning feature derived from NSL from triangular (or deformed) nanodots to nanorods and nanowires [73]. Laterally arranging different nanowires into a periodic array with a defined alignment is hard to

magnification. θ is the incidence angle and ϕ the azimuth angle. Reprinted with permission [66].

sulation of the metallic structures to prevent them from oxidation [69].

angle (θ).

**Figure 10.** Schematic diagrams of single-layer (SL) and double-layer (DL) nanosphere masks and the corresponding periodic particle array (PPA) surfaces. (A) *a*(111) SL mask, dotted line represents the unit cell, *a* refers to the first layer nanosphere; (B) SL PPA, 2 particles per unit cell; (C) 1.7× 1.7 μm constant height AFM image of a SL PPA with M = Ag, S = mica, *D* = 264 nm, *dm* = 22 nm, *rd* = 0.2 nm s-1. (D) *a*(111)*p*(1× 1)-*b* DL mask, dotted line represents the unit cell, *b*refers to the second layer nanosphere; (E) DL PPA, 1 particle per unit cell; (F) 2.0× 2.0 μm constant height AFM image of a DL PPA with M = Ag, S = mica, *D =* 264 nm, *dm =* 22 nm, *rd =* 0.2 nm s-1. Scale bar: 300 nm. Reprinted with permis‐ sion [23].

close-packed particles defines the shape of the nanodots deposited on substrates; the dots usually show a quasi-triangular shape and are arranged in a space group P6mm array due to the hexagonal packing of the colloidal crystal mask (Fig. 10a-c). Van Duyne *et al.* have extended colloidal crystal masking from single layer of hexagonally close packed particles to double layers [23]. Since the overlapping of the interstices between the upper and lower layers leads to a hexagonal array of quasi-hexagonal projection on a substrate, using double layer colloidal crystals as masks yields a hexagonal array of quasi-hexagonal nanodots (Fig. 10d-e).

In a general NSL procedure, the substrate to be patterned is positioned normal to the direction of material deposition. The in-plane shape of the nanodots and the spacing of the nearestneighboring dots derived from NSL are dictated by the projection of the interstices of single or double layers of colloidal crystals on substrates. They can be tuned by varying the projection geometry of the interstices on substrates by titling the masks with respect to the incidence of the vapor beam for instance. This has inspired development of angle-resolved NSL (AR-NSL), pioneered by the group of Van Duyne [66].

In a AR-NSL process, the incidence angle of the propagation vector of the material deposition beam with respect to the normal direction of the colloidal mask (θ) and/or the azimuth angle of the propagation vector with respect to the nearest neighboring particles in the colloidal masks (ϕ) – the mask registry with respect to the vector of the material deposition beam – have been employed to reduce the size of the nanodots obtained and, at the same time, elongate their triangular shape (Fig. 11). By rotating substrates, Giersig and coworkers have recently found that AR-NSL can generate much more complicate metallic nanostructures and they referred to this process as shadow NSL [43, 68, 69]. Zhang and Wang have recently demon‐ strated the feasibility of consecutively depositing two different metals, such as gold and silver, at two different incidence angles, to construct ordered binary arrays of gold and silver nanoparticles [70].Due to the rotation of the colloidal mask, shadow NSL relies in a process resolved by the azimuth angle (ϕ) of the incidence deposition beam rather than the incidence angle (θ).

**Figure 11.** Field emission SEM images of AR NSL fabricated - gold nanodot arrays and images with simulated geome‐ try superimposed, respectively. (A1, A2) θ = 108 º, ϕ = 288 º, (B1, B2) θ = 208 º, ϕ = 28 º, (C1, C2) θ = 268 º, ϕ = 168 º, and (D1, D2) θ = 408 º, ϕ = 28 º. All samples are Cr deposited onto Si (111) substrates. Images were collected at 40k magnification. θ is the incidence angle and ϕ the azimuth angle. Reprinted with permission [66].

close-packed particles defines the shape of the nanodots deposited on substrates; the dots usually show a quasi-triangular shape and are arranged in a space group P6mm array due to the hexagonal packing of the colloidal crystal mask (Fig. 10a-c). Van Duyne *et al.* have extended colloidal crystal masking from single layer of hexagonally close packed particles to double layers [23]. Since the overlapping of the interstices between the upper and lower layers leads to a hexagonal array of quasi-hexagonal projection on a substrate, using double layer colloidal

**Figure 10.** Schematic diagrams of single-layer (SL) and double-layer (DL) nanosphere masks and the corresponding periodic particle array (PPA) surfaces. (A) *a*(111) SL mask, dotted line represents the unit cell, *a* refers to the first layer nanosphere; (B) SL PPA, 2 particles per unit cell; (C) 1.7× 1.7 μm constant height AFM image of a SL PPA with M = Ag, S = mica, *D* = 264 nm, *dm* = 22 nm, *rd* = 0.2 nm s-1. (D) *a*(111)*p*(1× 1)-*b* DL mask, dotted line represents the unit cell, *b*refers to the second layer nanosphere; (E) DL PPA, 1 particle per unit cell; (F) 2.0× 2.0 μm constant height AFM image of a DL PPA with M = Ag, S = mica, *D =* 264 nm, *dm =* 22 nm, *rd =* 0.2 nm s-1. Scale bar: 300 nm. Reprinted with permis‐

In a general NSL procedure, the substrate to be patterned is positioned normal to the direction of material deposition. The in-plane shape of the nanodots and the spacing of the nearestneighboring dots derived from NSL are dictated by the projection of the interstices of single or double layers of colloidal crystals on substrates. They can be tuned by varying the projection geometry of the interstices on substrates by titling the masks with respect to the incidence of the vapor beam for instance. This has inspired development of angle-resolved NSL (AR-NSL),

In a AR-NSL process, the incidence angle of the propagation vector of the material deposition beam with respect to the normal direction of the colloidal mask (θ) and/or the azimuth angle of the propagation vector with respect to the nearest neighboring particles in the colloidal masks (ϕ) – the mask registry with respect to the vector of the material deposition beam – have been employed to reduce the size of the nanodots obtained and, at the same time, elongate

crystals as masks yields a hexagonal array of quasi-hexagonal nanodots (Fig. 10d-e).

pioneered by the group of Van Duyne [66].

sion [23].

16 Updates in Advanced Lithography

The elegant extension of AR-NSL is to stepwise conduct physical vapor deposition of identical or different materials at the different incidence angles. The group of Van Duyne has succeeded in growth of surface patterning features composed of two triangular nanodots either overlap‐ ped or separated by two deposition steps at θ = 0º and θ > 0º, respectively [65]. Giersig *et al.* have also developed a stepwise shadow NSL protocol to deposit different materials at different incidence angles when the colloidal masks were rotating and they have succeeded in encap‐ sulation of the metallic structures to prevent them from oxidation [69].

Prior to physical vapor deposition, colloidal crystal masks can undergo RIE to reduce the sizes of the particles and widen the interstitial spaces, thus increasing the dimension of triangular nanodots obtained via NSL. Increasing the RIE time can turn close-packed colloidal crystal masks to non-close packed ones, which leads to thin films with hexagonally arranged pores [71, 72]. Wang *et al.* have recently integrated AR-NSL with the use of RIE-modified colloidal crystals as masks to diversify the structural complexity of the patterning feature derived from NSL from triangular (or deformed) nanodots to nanorods and nanowires [73]. Laterally arranging different nanowires into a periodic array with a defined alignment is hard to implement by otherwise means, either conventional lithographic techniques or self-assembly techniques.

includes a number of the advantages of NSL, such as large area coverage, high fabrication speed, independent control over feature size and spacing, and processing simplicity. It can be

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Various colloidal spheres, organic and inorganic, can be produced that are exceedingly monodisperse in terms of size and shape. Nevertheless, their surfaces still remain chemically homogeneous or heterogeneous. Controlling the surface properties of colloidal particles is one of the oldest and, at the same time, the most vital topics in colloid science and physical chemistry. Patchy particles, i.e., particles with more than one patch or patches that are less than 50% of the total particle surface, should present the next generation of particles for assembly [77-79]. However, patterning the surface of colloidal particles with sizes of micrometers or

When 2D colloidal crystals are used as masks for physical vapor deposition, it is expected that only the upper surfaces of the colloidal particles, exposing directly to the vapor beam, will be coated with new materials, which leads to two spatially well-separated halves on the colloidal particles, coated and non-coated, with two distinct surface chemical functionalities [80, 81]. Such particles are usually referred to as Janus particles. By embedding a monomer of closepacked colloidal particles in a photoresist layer, Bao *et al.* have succeeded in tuning the surface areas of the colloidal particles exposing to the vapor beam during material deposition via etching the photoresist layer with O2 plasma, thus leading to a good control of the domain sizes deposited on the particles [82]. When a monolayer of close-packed colloidal particles is constructed at the water/air interface or the wax/liquid interfaces, selective modification can

Wang *et al.* have pioneered the study of using the upper single layers of colloidal crystals as masks for the lower layer particles during physical vapor deposition [85]. Of importance is that the methodology reported by Wang *et al.* – using colloidal crystals for self-masking – is independent of the curvature and chemical composition of the surfaces (Fig. 13). By using O2 plasma to etch colloidal crystal templates, mainly the top layer, and conducting physical vapor deposition at the non-zero incidence angle, Wang *et al.* have also demonstrated that the size and shape of the patterns obtained on the second layer particles show a pronounced depend‐ ence on the plasma etching time and the incidence angle [86]. Pawar and Kretzschmar have recently extended the concept of using colloidal crystal for self-masking for glancing angle

Wang *et al.* have recently employed the upper double layers as masks for patterning the particles in the third layers via physical deposition [88]. As a consequence, they have succeeded in stereo-decoration of colloidal particles with two, three, four, or five nanodots. The number of dots per sphere is dependent on the crystalline structure of the colloidal crystal masks, the plasma etching time, and the incidence angle. The nanodots decorated on particles are arranged in a linear, trigonal, tetrahedral, or right-pyramidal fashion, which provides the nanoscale analogues of sp, sp2, sp3 sphybridized atomic orbitals of carbon. The Au nanodots obtained on microspheres, therefore, can be recruited as the bonding site to dictate the integration of the spheres, thus paving a new way of colloidal self-assembly – colloidal valent chemistry of spheres [89] – to create hierarchical and complicated "supraparticles" [77].

applied to a wide range of materials, including Au, Ag, Pd, Pt, SiO2 etc.

submicrometers is a formidable challenge due to lack of the proper mask.

be implemented in either of the two phases, leading to Janus particles [83, 84].

deposition [87].

**Figure 12.** SEM pictures of hexagonally arranged Au nano-shuttlecocks obtained by using bilayers of hexagonal closepacked 925 nm PS spheres, etched by O2-plasma for 10 (a), 20 (b), 25 (c), and 30 min (d), as masks for Au vapor deposi‐ tion. The incidence angle of Au vapor flow was set as 15°. The scale bars are 500 nm. Reprinted with permission [74].

Wang *et al.* have recently extended the RIE process for modification of double layers of colloidal crystals for AR-NSL [74]. Using O2 plasma etched bilayers of hexagonally packed particles as masks for gold deposition, highly ordered binary arrays of gold nanoparticles with varied shapes, for instance, with a shuttlecock-like shape composed of a small crescent-shaped nanoparticle and a big fan-shaped one, have been fabricated (Fig. 12). As compared to that of the corresponding bulk materials, the melting point of nanoparticles is much lower and especially more sensitive to the surface tension. Since the large curvature causes a high surface tension, the annealing of non-round nanoparticles may give rise to a retraction of their apexes, eventually generating a round shape [75]. Wang *et al.* have successfully transformed the shape of Au nanoparticles obtained from crescent-like or fan-shaped to round with a rather narrow distribution in terms of size and shape [74].

Dmitriev *et al.* have extended colloidal crystal masking from the use for material deposition to that for controlled etching and developed an interesting variant of NSL – hole-mask colloidal lithography (HCL) [76]. Different from conventional NSL, the essential new feature of HCL is that the substrate and the colloidal crystal mask are interspaced by a sacrificial layer. After physical vapor deposition, the removal of the colloidal mask leads to a thin film mask with nanoholes, which is known as "hole-mask". The hole-mask is subsequently used for vapor deposition and/or etching steps to further define a patterning feature on the substrate. HCL includes a number of the advantages of NSL, such as large area coverage, high fabrication speed, independent control over feature size and spacing, and processing simplicity. It can be applied to a wide range of materials, including Au, Ag, Pd, Pt, SiO2 etc.

implement by otherwise means, either conventional lithographic techniques or self-assembly

**Figure 12.** SEM pictures of hexagonally arranged Au nano-shuttlecocks obtained by using bilayers of hexagonal closepacked 925 nm PS spheres, etched by O2-plasma for 10 (a), 20 (b), 25 (c), and 30 min (d), as masks for Au vapor deposi‐ tion. The incidence angle of Au vapor flow was set as 15°. The scale bars are 500 nm. Reprinted with permission [74].

Wang *et al.* have recently extended the RIE process for modification of double layers of colloidal crystals for AR-NSL [74]. Using O2 plasma etched bilayers of hexagonally packed particles as masks for gold deposition, highly ordered binary arrays of gold nanoparticles with varied shapes, for instance, with a shuttlecock-like shape composed of a small crescent-shaped nanoparticle and a big fan-shaped one, have been fabricated (Fig. 12). As compared to that of the corresponding bulk materials, the melting point of nanoparticles is much lower and especially more sensitive to the surface tension. Since the large curvature causes a high surface tension, the annealing of non-round nanoparticles may give rise to a retraction of their apexes, eventually generating a round shape [75]. Wang *et al.* have successfully transformed the shape of Au nanoparticles obtained from crescent-like or fan-shaped to round with a rather narrow

Dmitriev *et al.* have extended colloidal crystal masking from the use for material deposition to that for controlled etching and developed an interesting variant of NSL – hole-mask colloidal lithography (HCL) [76]. Different from conventional NSL, the essential new feature of HCL is that the substrate and the colloidal crystal mask are interspaced by a sacrificial layer. After physical vapor deposition, the removal of the colloidal mask leads to a thin film mask with nanoholes, which is known as "hole-mask". The hole-mask is subsequently used for vapor deposition and/or etching steps to further define a patterning feature on the substrate. HCL

distribution in terms of size and shape [74].

techniques.

18 Updates in Advanced Lithography

Various colloidal spheres, organic and inorganic, can be produced that are exceedingly monodisperse in terms of size and shape. Nevertheless, their surfaces still remain chemically homogeneous or heterogeneous. Controlling the surface properties of colloidal particles is one of the oldest and, at the same time, the most vital topics in colloid science and physical chemistry. Patchy particles, i.e., particles with more than one patch or patches that are less than 50% of the total particle surface, should present the next generation of particles for assembly [77-79]. However, patterning the surface of colloidal particles with sizes of micrometers or submicrometers is a formidable challenge due to lack of the proper mask.

When 2D colloidal crystals are used as masks for physical vapor deposition, it is expected that only the upper surfaces of the colloidal particles, exposing directly to the vapor beam, will be coated with new materials, which leads to two spatially well-separated halves on the colloidal particles, coated and non-coated, with two distinct surface chemical functionalities [80, 81]. Such particles are usually referred to as Janus particles. By embedding a monomer of closepacked colloidal particles in a photoresist layer, Bao *et al.* have succeeded in tuning the surface areas of the colloidal particles exposing to the vapor beam during material deposition via etching the photoresist layer with O2 plasma, thus leading to a good control of the domain sizes deposited on the particles [82]. When a monolayer of close-packed colloidal particles is constructed at the water/air interface or the wax/liquid interfaces, selective modification can be implemented in either of the two phases, leading to Janus particles [83, 84].

Wang *et al.* have pioneered the study of using the upper single layers of colloidal crystals as masks for the lower layer particles during physical vapor deposition [85]. Of importance is that the methodology reported by Wang *et al.* – using colloidal crystals for self-masking – is independent of the curvature and chemical composition of the surfaces (Fig. 13). By using O2 plasma to etch colloidal crystal templates, mainly the top layer, and conducting physical vapor deposition at the non-zero incidence angle, Wang *et al.* have also demonstrated that the size and shape of the patterns obtained on the second layer particles show a pronounced depend‐ ence on the plasma etching time and the incidence angle [86]. Pawar and Kretzschmar have recently extended the concept of using colloidal crystal for self-masking for glancing angle deposition [87].

Wang *et al.* have recently employed the upper double layers as masks for patterning the particles in the third layers via physical deposition [88]. As a consequence, they have succeeded in stereo-decoration of colloidal particles with two, three, four, or five nanodots. The number of dots per sphere is dependent on the crystalline structure of the colloidal crystal masks, the plasma etching time, and the incidence angle. The nanodots decorated on particles are arranged in a linear, trigonal, tetrahedral, or right-pyramidal fashion, which provides the nanoscale analogues of sp, sp2, sp3 sphybridized atomic orbitals of carbon. The Au nanodots obtained on microspheres, therefore, can be recruited as the bonding site to dictate the integration of the spheres, thus paving a new way of colloidal self-assembly – colloidal valent chemistry of spheres [89] – to create hierarchical and complicated "supraparticles" [77].

The second extension is to use NSL-derived surface patterns as etching masks to create surface topography. Chen *et al.* have fabricated silicon nanopillar arrays with diameters as small as 40 nm and aspect ratios as high as seven [93]. The size and shape of the nanopillars can be controlled by the size and shape of the sputtered aluminum masks, which are again determined by the feature size of the colloidal mask and the number of the colloidal layers. Nanopillars with different shapes can also be fabricated by adjusting the RIE conditions such as the gas species, bias voltage, and exposure duration for an aluminum mask with a given shape. Asprepared nanopillar arrays can be utilized for imprinting a layer of PMMA above its glass transition temperature [94]. Similarly, Weekes *et al.* have fabricated ordered arrays of cobalt nanodots for patterned magnetic media [95]. By introducing intermediary layers of SiO2 between colloidal crystal masks and substrates, this etching strategy can be applied to a wide range of materials without much concern for the surface hydrophilicity of the targeted substrates. Using the similar protocol, large-area ordered arrays of 512 nm pitch hole, with

Colloidal Lithography

21

http://dx.doi.org/10.5772/56576

vertical and smooth sidewalls, has been successfully formed on GaAs substrates [96].

**Figure 14.** Steps of Si nanowire fabrication. NSL: (a) deposition of a mask of polystyrene particles (b) deposition of gold by thermal evaporation, (c) removal of the spheres, (d) thermal annealing and cleaning step to remove the oxide layer, and (e) Si deposition and growth of nanowires by MBE. (Right) corresponding SEM micrographs of wafers at

different steps. Reprinted with permission [100].

**Figure 13.** Left panel: Schematic illustration of the procedure to create colloidal spheres with Au-patterned surfaces by the combination of Au vapor deposition and using the top mono- or bilayers of colloidal crystals with (111) facets parallel to the substrates as masks. Right panel: Low (a) and high magnification scanning electron microscope (SEM) picture of 925 nm polystyrene spheres with Au patterned surfaces generated by templating the top monolayers of colloidal crystals with (111) (b) (100) (c) and (110) (d) facets parallel to the substrates. Reprinted with permission [85]

#### *3.2.3. Extension of colloidal lithography*

One extension of NSL is to use the surface patterns obtained as templates to grow nanostruc‐ tures of a variety of materials via bottom-up self-assembly. Mulvaney's group has grown monolayer and multilayer films of semiconductor quantum dots on surface patterns derived from NSL, leading to nanostructured luminescent thin films [90, 91]. Valsesia *et al.* have used ordered arrays of polyacrylic acid domes derived via NSL to selectively couple with bovine serum albumin [55]. Using NSL-derived surface patterns as templates to grow proteins, Sutherland *et al* have found that the surface topography enhances the binding selectivity of fibrinogens to platelets [92].

The second extension is to use NSL-derived surface patterns as etching masks to create surface topography. Chen *et al.* have fabricated silicon nanopillar arrays with diameters as small as 40 nm and aspect ratios as high as seven [93]. The size and shape of the nanopillars can be controlled by the size and shape of the sputtered aluminum masks, which are again determined by the feature size of the colloidal mask and the number of the colloidal layers. Nanopillars with different shapes can also be fabricated by adjusting the RIE conditions such as the gas species, bias voltage, and exposure duration for an aluminum mask with a given shape. Asprepared nanopillar arrays can be utilized for imprinting a layer of PMMA above its glass transition temperature [94]. Similarly, Weekes *et al.* have fabricated ordered arrays of cobalt nanodots for patterned magnetic media [95]. By introducing intermediary layers of SiO2 between colloidal crystal masks and substrates, this etching strategy can be applied to a wide range of materials without much concern for the surface hydrophilicity of the targeted substrates. Using the similar protocol, large-area ordered arrays of 512 nm pitch hole, with vertical and smooth sidewalls, has been successfully formed on GaAs substrates [96].

**Figure 13.** Left panel: Schematic illustration of the procedure to create colloidal spheres with Au-patterned surfaces by the combination of Au vapor deposition and using the top mono- or bilayers of colloidal crystals with (111) facets parallel to the substrates as masks. Right panel: Low (a) and high magnification scanning electron microscope (SEM) picture of 925 nm polystyrene spheres with Au patterned surfaces generated by templating the top monolayers of colloidal crystals with (111) (b) (100) (c) and (110) (d) facets parallel to the substrates. Reprinted with permission [85]

One extension of NSL is to use the surface patterns obtained as templates to grow nanostruc‐ tures of a variety of materials via bottom-up self-assembly. Mulvaney's group has grown monolayer and multilayer films of semiconductor quantum dots on surface patterns derived from NSL, leading to nanostructured luminescent thin films [90, 91]. Valsesia *et al.* have used ordered arrays of polyacrylic acid domes derived via NSL to selectively couple with bovine serum albumin [55]. Using NSL-derived surface patterns as templates to grow proteins, Sutherland *et al* have found that the surface topography enhances the binding selectivity of

*3.2.3. Extension of colloidal lithography*

20 Updates in Advanced Lithography

fibrinogens to platelets [92].

**Figure 14.** Steps of Si nanowire fabrication. NSL: (a) deposition of a mask of polystyrene particles (b) deposition of gold by thermal evaporation, (c) removal of the spheres, (d) thermal annealing and cleaning step to remove the oxide layer, and (e) Si deposition and growth of nanowires by MBE. (Right) corresponding SEM micrographs of wafers at different steps. Reprinted with permission [100].

The third extension is to use NSL-derived surface patterns to template or catalyze the growth of other functional materials. Zhou *et al.* have successfully used the ordered arrays of gold nanodots derived from NSL as seeds to highly aligned single-walled carbon nanotubes laid on quartz and sapphire substrates [97]. This method has great potential to produce carbon nanotube arrays with simultaneous control over the nanotube orientation, position, density, diameter, and even chirality, which may work as building blocks for future nanoelectronics and ultra-high-speed electronics [98]. Wang *et al.* have used gold nanodot arrays as seeds for hexagonally arranged arrays of zinc oxide nanorods aligned perpendicular to the substrates [99]. Similarly Fuhrmann *et al.* have obtained ordered arrays of Si nanorods by using the gold nanodots as seeds for molecular beam epitaxy (Fig. 14) [100]. Similarly, discretely ordered arrays of organic light-emitting nanodiode (OLED) have been fabricated based on NSLderived surface patterns [101].

## **4. Applications**

## **4.1. Optical properties**

Surface patterns derived from CL, especially NSL, are usually made up of metals such as gold and silver. Noble metal nanostructure arrays have pronounced surface plasmon resonance, which results from incident electromagnetic radiation exciting coherent oscillations of conduction electrons near a metal-dielectric interface [102]. Giessen and co-workers intro‐ duced an angle-controlled colloidal lithography as a fast and low-cost fabrication technique for large-area periodic plasmonic oligomers with complex shape and tunable geometry parameters, and investigated the optical properties and found highly modulated plasmon modes in oligomers with triangular building blocks. Fundamental modes, higher-order modes, as well as Fano resonances due to coupling between bright and dark modes within the same complex structure are present, depending on polarization and structure geometry. This process is well-suited for mass fabrication of novel large-area plasmonic sensing devices and nanoantennas (Fig. 15) [103].

**Figure 15.** Fabrication of oligomers with triangular building blocks. (a) Schematic diagram of the evaporation setup. (b)Simplified geometrical model for symmetric pentamer fabrication. Top left: top view. Bottom: side view at cross sec‐ tion line as indicated in top view. Top right: geometrical parameters. (c) Scanning electron microscopy (SEM) images of the large are asymmetric pentamers. The gap size is as small as 20 nm. (d) Artistic view of our fabrication scheme, us‐ ing a real SEM image of a symmetric pentamer. The transmittance spectrum of the sample using a large-area optical

Colloidal Lithography

23

http://dx.doi.org/10.5772/56576

Light trapping across a wide band of frequencies is important for applications such as solar cells and photo detectors. Yao Y. and Yao J. *et al.* demonstrated a new approach based on colloidal lithography to light management by forming whispering-gallery resonant modes inside a spherical nanoshell structure. A broadband absorption enhancement across a large range of incident angles was observed. The absorption of a single layer of 50-nm-thick spherical nanoshells is equivalent to a 1-μm-thick planar nc-Si film. This light-trapping structure could enable the manufacturing of high-throughput ultra-thin film absorbers in a variety of material systems that demand shorter deposition time, less material usage and transferability to flexible

The wettability of solid surfaces is a significant property depending on both chemical compo‐ sitions and the surface structure. A great number of ordered arrays generated through simple

beam diameter. Reprinted by permission of [103].

substrates. [109]

**4.2. Wettability**

One of the straightforward technical applications of CL is to use as highly sensitive biosensors relied on the localized-surface plasmon resonance (LSPR) of metallic nanostructures [67]. The LSPR of metallic nanostructures composed of gold rings [104] and disks [105], obtained via NSL has been studied. It is found that the LSPR can be tuned by varying either the diameter of the disks at a constant disk height or the ring thickness. The shape-dependent red shift originates from the electromagnetic coupling between the inner and outer ring surfaces, which leads to energy shifts and splitting of degenerate modes [106]. NSL has been also used to create nanocaps and nanocups; their LSPR behavior has been studied [107]. Lee *et al.* have generated gold crescent moon structures with a sub-10 nm sharp edge via NSL, which exhibit a very strong SERS [108].

**Figure 15.** Fabrication of oligomers with triangular building blocks. (a) Schematic diagram of the evaporation setup. (b)Simplified geometrical model for symmetric pentamer fabrication. Top left: top view. Bottom: side view at cross sec‐ tion line as indicated in top view. Top right: geometrical parameters. (c) Scanning electron microscopy (SEM) images of the large are asymmetric pentamers. The gap size is as small as 20 nm. (d) Artistic view of our fabrication scheme, us‐ ing a real SEM image of a symmetric pentamer. The transmittance spectrum of the sample using a large-area optical beam diameter. Reprinted by permission of [103].

Light trapping across a wide band of frequencies is important for applications such as solar cells and photo detectors. Yao Y. and Yao J. *et al.* demonstrated a new approach based on colloidal lithography to light management by forming whispering-gallery resonant modes inside a spherical nanoshell structure. A broadband absorption enhancement across a large range of incident angles was observed. The absorption of a single layer of 50-nm-thick spherical nanoshells is equivalent to a 1-μm-thick planar nc-Si film. This light-trapping structure could enable the manufacturing of high-throughput ultra-thin film absorbers in a variety of material systems that demand shorter deposition time, less material usage and transferability to flexible substrates. [109]

### **4.2. Wettability**

The third extension is to use NSL-derived surface patterns to template or catalyze the growth of other functional materials. Zhou *et al.* have successfully used the ordered arrays of gold nanodots derived from NSL as seeds to highly aligned single-walled carbon nanotubes laid on quartz and sapphire substrates [97]. This method has great potential to produce carbon nanotube arrays with simultaneous control over the nanotube orientation, position, density, diameter, and even chirality, which may work as building blocks for future nanoelectronics and ultra-high-speed electronics [98]. Wang *et al.* have used gold nanodot arrays as seeds for hexagonally arranged arrays of zinc oxide nanorods aligned perpendicular to the substrates [99]. Similarly Fuhrmann *et al.* have obtained ordered arrays of Si nanorods by using the gold nanodots as seeds for molecular beam epitaxy (Fig. 14) [100]. Similarly, discretely ordered arrays of organic light-emitting nanodiode (OLED) have been fabricated based on NSL-

Surface patterns derived from CL, especially NSL, are usually made up of metals such as gold and silver. Noble metal nanostructure arrays have pronounced surface plasmon resonance, which results from incident electromagnetic radiation exciting coherent oscillations of conduction electrons near a metal-dielectric interface [102]. Giessen and co-workers intro‐ duced an angle-controlled colloidal lithography as a fast and low-cost fabrication technique for large-area periodic plasmonic oligomers with complex shape and tunable geometry parameters, and investigated the optical properties and found highly modulated plasmon modes in oligomers with triangular building blocks. Fundamental modes, higher-order modes, as well as Fano resonances due to coupling between bright and dark modes within the same complex structure are present, depending on polarization and structure geometry. This process is well-suited for mass fabrication of novel large-area plasmonic sensing devices and

One of the straightforward technical applications of CL is to use as highly sensitive biosensors relied on the localized-surface plasmon resonance (LSPR) of metallic nanostructures [67]. The LSPR of metallic nanostructures composed of gold rings [104] and disks [105], obtained via NSL has been studied. It is found that the LSPR can be tuned by varying either the diameter of the disks at a constant disk height or the ring thickness. The shape-dependent red shift originates from the electromagnetic coupling between the inner and outer ring surfaces, which leads to energy shifts and splitting of degenerate modes [106]. NSL has been also used to create nanocaps and nanocups; their LSPR behavior has been studied [107]. Lee *et al.* have generated gold crescent moon structures with a sub-10 nm sharp edge via NSL, which exhibit a very

derived surface patterns [101].

**4. Applications**

**4.1. Optical properties**

22 Updates in Advanced Lithography

nanoantennas (Fig. 15) [103].

strong SERS [108].

The wettability of solid surfaces is a significant property depending on both chemical compo‐ sitions and the surface structure. A great number of ordered arrays generated through simple or modified colloidal lithography could induce different wettabilities. Recently, Koshizaki and co-workers fabricated vertically ordered Co3O4 hierarchical nanorod arrays using pulsed laser deposition (PLD) onto colloidal crystal masks followed by an annealing process, and the asprepared Co3O4 nanorod arrays demonstrated stable superhydrophilicity without UV irradiation even after half a year owing to the improved roughness of the hierarchical structure and the abundant OH− groups induced by the PLD and annealing processes.

microspheres on a targeted substrate. The feature size can easily shrink below 100 nm by reducing the diameter of the microspheres used according to the simple correlation between the interstice size and the sphere diameter. The feature shape can be easily diversified by the crystalline structure of a colloidal crystal mask, the time of anisotropic etching of the mask, the incidence angle of vapor beam and the mask registry (the azimuth angle of vapor beam). Currently, CL allows fabrication of very complicated 2D and 3D nanostructured features, such as multiplex nanostructures with a clear-cut lateral and vertical heterogeneity. A number of new nanostructures are hard to be implemented, or cannot be in some cases, by conventional lithographic techniques. As such, CL provides a nanochemical and complementary tool of conventional and fully top-down lithographic techniques, and thus holds immense promise

Colloidal Lithography

25

http://dx.doi.org/10.5772/56576

However, CL is still in a very early stage of development. Despite the great progress in colloidal crystallization it still remains a formidable challenge to create a defect-free single crystal with a defined crystalline face. The presence of defects dramatically reduces the patterning precision of CL. For instance, the random orientation of polycrystalline crystalline domains in a colloidal mask is a disaster for collimating the mask registry. In this aspect, template-assisted epitaxy for colloidal crystallization is promising as it allows growth of colloidal crystals with defined packing structure and orientation. Since a patterned substrate is necessitated for the colloidal epitaxy, its applicability for patterning is limited. How to transfer a colloidal crystal derived from this colloidal epitaxy onto different substrates without deterioration of the crystal quality should be an ensuing task for CL. Besides, fabrication of large area monolayer of periodically close-packed microspheres with sizes smaller than 100 nm remains highly challenging, which brings a technical problem to reduce the feature size below 10 nm via CL. In a CL patterning process, furthermore, the feature size and the interspace size between the features cannot be separately manipulate, as they both are directly proportional with the sphere size in a colloidal

State Key Lab of Supramolecular Structure and Materials, College of Chemistry, Jilin Uni‐

[1] D. Wang, H. Möhwald, Template-directed colloidal self-assembly – the route to 'top-

down' nanochemical engineering, *J. Mater. Chem.* 2004, *14,* 459-468.

mask, which largely limit the patterning capability of CL.

\*Address all correspondence to: gang@jlu.edu.cn

in surface patterning.

**Author details**

Ye Yu and Gang Zhang\*

versity, Changchun, China

**References**

### **4.3. Other applications**

Besides the exploitation of CL and the patterns obtained thereof in LSPR-assisted sensing, the magnetic properties of CL-derived nanostructures gain increasing attention. In general, nanoscale magnetic materials often exhibit superparamagnetic behavior. Moreover, an ordered nanostructure of magnetic materials is required for investigation of the mesoscopic effects induced by the confinement of magnetic materials in nanoscale domains [111]. Since magnetic properties are strongly dependent on the domain size and the distance between domains, Weekes *et al.* have created ordered arrays of isolated magnetic nanodots via NSL [95]. The coercivity and switching width of the isolated nanodot arrays are enhanced as compared to those of continuous magnetic films. Well-organized arrays of magnetic nanorings over a large area have been prepared via NSL and they show a stable vortex state due to the absence of a destabilizing vortex core, which should hold promise in vertical magnetic random access memories [112, 113]. Albrecht *et al.* have found that Co/Pd multilayers on a colloid surface exhibited a pronounced magnetic anisotropy [114].

What's more, a core question in materials science is how to encode non-trivial organized structures within simple building blocks. A recent report from this laboratory described methods for functionalizing latex spheres to make them hydrophobic at their poles, leading to the directed self-assembly of a kagome lattice pattern in which each sphere was coordinated with four neighbors, two at each pole. Granick and co-workers developed methods for functionalizing micrometer-sized colloidal spheres with three or more zones of chemical functionality through colloidal lithography, literally combining double-sided angle-resolved physical deposition and controllable chemical etching. These synthesis methods allowed targeting of various lattice structures whose bonding between neighboring particles in liquid suspension was visualized in situ by optical microscopy [115,116].

## **5. Summary**

The recent development of CL, especially the integration of etching the colloidal mask, altering the incidence angle, and stepwise and regularly changing the mask registry, leads to a powerful nanochemical patterning tool with low cost in capital and operation, high throughput, and ease to be adopted on various planar and curved surfaces and even on microparticles. Different from conventional mask-assisted lithographic processes in which the mask design and production usually remain a challenge for scaling down the feature size and diversifying the feature shape, CL embodies a simple way for masking – self-assembly of monodisperse microspheres on a targeted substrate. The feature size can easily shrink below 100 nm by reducing the diameter of the microspheres used according to the simple correlation between the interstice size and the sphere diameter. The feature shape can be easily diversified by the crystalline structure of a colloidal crystal mask, the time of anisotropic etching of the mask, the incidence angle of vapor beam and the mask registry (the azimuth angle of vapor beam). Currently, CL allows fabrication of very complicated 2D and 3D nanostructured features, such as multiplex nanostructures with a clear-cut lateral and vertical heterogeneity. A number of new nanostructures are hard to be implemented, or cannot be in some cases, by conventional lithographic techniques. As such, CL provides a nanochemical and complementary tool of conventional and fully top-down lithographic techniques, and thus holds immense promise in surface patterning.

However, CL is still in a very early stage of development. Despite the great progress in colloidal crystallization it still remains a formidable challenge to create a defect-free single crystal with a defined crystalline face. The presence of defects dramatically reduces the patterning precision of CL. For instance, the random orientation of polycrystalline crystalline domains in a colloidal mask is a disaster for collimating the mask registry. In this aspect, template-assisted epitaxy for colloidal crystallization is promising as it allows growth of colloidal crystals with defined packing structure and orientation. Since a patterned substrate is necessitated for the colloidal epitaxy, its applicability for patterning is limited. How to transfer a colloidal crystal derived from this colloidal epitaxy onto different substrates without deterioration of the crystal quality should be an ensuing task for CL. Besides, fabrication of large area monolayer of periodically close-packed microspheres with sizes smaller than 100 nm remains highly challenging, which brings a technical problem to reduce the feature size below 10 nm via CL. In a CL patterning process, furthermore, the feature size and the interspace size between the features cannot be separately manipulate, as they both are directly proportional with the sphere size in a colloidal mask, which largely limit the patterning capability of CL.

## **Author details**

or modified colloidal lithography could induce different wettabilities. Recently, Koshizaki and co-workers fabricated vertically ordered Co3O4 hierarchical nanorod arrays using pulsed laser deposition (PLD) onto colloidal crystal masks followed by an annealing process, and the asprepared Co3O4 nanorod arrays demonstrated stable superhydrophilicity without UV irradiation even after half a year owing to the improved roughness of the hierarchical structure

Besides the exploitation of CL and the patterns obtained thereof in LSPR-assisted sensing, the magnetic properties of CL-derived nanostructures gain increasing attention. In general, nanoscale magnetic materials often exhibit superparamagnetic behavior. Moreover, an ordered nanostructure of magnetic materials is required for investigation of the mesoscopic effects induced by the confinement of magnetic materials in nanoscale domains [111]. Since magnetic properties are strongly dependent on the domain size and the distance between domains, Weekes *et al.* have created ordered arrays of isolated magnetic nanodots via NSL [95]. The coercivity and switching width of the isolated nanodot arrays are enhanced as compared to those of continuous magnetic films. Well-organized arrays of magnetic nanorings over a large area have been prepared via NSL and they show a stable vortex state due to the absence of a destabilizing vortex core, which should hold promise in vertical magnetic random access memories [112, 113]. Albrecht *et al.* have found that Co/Pd multilayers on a colloid surface

What's more, a core question in materials science is how to encode non-trivial organized structures within simple building blocks. A recent report from this laboratory described methods for functionalizing latex spheres to make them hydrophobic at their poles, leading to the directed self-assembly of a kagome lattice pattern in which each sphere was coordinated with four neighbors, two at each pole. Granick and co-workers developed methods for functionalizing micrometer-sized colloidal spheres with three or more zones of chemical functionality through colloidal lithography, literally combining double-sided angle-resolved physical deposition and controllable chemical etching. These synthesis methods allowed targeting of various lattice structures whose bonding between neighboring particles in liquid

The recent development of CL, especially the integration of etching the colloidal mask, altering the incidence angle, and stepwise and regularly changing the mask registry, leads to a powerful nanochemical patterning tool with low cost in capital and operation, high throughput, and ease to be adopted on various planar and curved surfaces and even on microparticles. Different from conventional mask-assisted lithographic processes in which the mask design and production usually remain a challenge for scaling down the feature size and diversifying the feature shape, CL embodies a simple way for masking – self-assembly of monodisperse

and the abundant OH− groups induced by the PLD and annealing processes.

**4.3. Other applications**

24 Updates in Advanced Lithography

**5. Summary**

exhibited a pronounced magnetic anisotropy [114].

suspension was visualized in situ by optical microscopy [115,116].

Ye Yu and Gang Zhang\*

\*Address all correspondence to: gang@jlu.edu.cn

State Key Lab of Supramolecular Structure and Materials, College of Chemistry, Jilin Uni‐ versity, Changchun, China

## **References**

[1] D. Wang, H. Möhwald, Template-directed colloidal self-assembly – the route to 'topdown' nanochemical engineering, *J. Mater. Chem.* 2004, *14,* 459-468.

[2] Y. Xia, B. Gates, Y. Yin, Y. Lu, Monodisperse colloidal particles: old materials with new applications, *Adv. Mater.* 2000, *12,* 693-713.

[17] Z. Cheng, W. B. Russel, P. M. Chaikin, Controlled growth of hard-sphere colloidal

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27

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**Chapter 2**

**Recent Advances in Two-Photon Stereolithography**

Recent developments in nanoscience and nanotechnology were strongly supported by significant advances in nanofabrication. The growing demand for the fabrication of nano‐ structured materials has become increasingly important because of the ever-decreasing dimensions of various devices, including those used in electronics, optics, photonics, biology, electrochemistry, and electromechanics (Henzie et al., 2004; Fan et al., 2006). In particular, a societal revolution is expected with the miniaturization of mechanical, chemical or biological systems known as microlectromechanical systems (MEMS) (Lee et al. 2012), or micrototal

Among all fabrication processes, photolithography has been strongly developed since few decades to fulfil to the needs of the microelectronics industry. Researches in this area were essentially motivated by finding ways to provide new solutions to pursue the trend towards a constant decrease of the size of the transistors as stated in the "Moore's law" (Moore, 1965). To reach these objectives, lithographic fabrication methods have been widely diversified leading to DUV lithography (Ridaoui et al., 2010), X-ray lithography (Im et al., 2009) and ebeam lithography (Gonsalves et al. 2009) to quote a few. Although some of these techniques exhibit resolution of less than 10 nm, these methods are inherently 2D. Unlike conventional microelectronics components, many MEMS or μTAS devices (motors, pumps, valves…) require 3D fabrication capability to insure the same function as the corresponding macroscopic device. Thus, lithographic fabrication of 3D microstructures has emerged and has been divided in two categories depending if they give access to restricted or arbitrary 3D pattern fabrication. On the first hand, specific structures as periodic patterns have been made using self-assembly

> © 2013 Spangenberg et al.; licensee InTech. This is an open access article 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.

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.

© 2013 Spangenberg et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons

analysis systems (μTAS) (Reyes et al. 2002, Dittrich et al. 2006, West et al. 2008).

Arnaud Spangenberg, Nelly Hobeika,

Prem Prabhakaran, Patrice Baldeck and

Additional information is available at the end of the chapter

Olivier Soppera

**1. Introduction**

http://dx.doi.org/10.5772/56165

Fabrice Stehlin, Jean-Pierre Malval, Fernand Wieder,

## **Recent Advances in Two-Photon Stereolithography**

Arnaud Spangenberg, Nelly Hobeika, Fabrice Stehlin, Jean-Pierre Malval, Fernand Wieder, Prem Prabhakaran, Patrice Baldeck and Olivier Soppera

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/56165

## **1. Introduction**

Recent developments in nanoscience and nanotechnology were strongly supported by significant advances in nanofabrication. The growing demand for the fabrication of nano‐ structured materials has become increasingly important because of the ever-decreasing dimensions of various devices, including those used in electronics, optics, photonics, biology, electrochemistry, and electromechanics (Henzie et al., 2004; Fan et al., 2006). In particular, a societal revolution is expected with the miniaturization of mechanical, chemical or biological systems known as microlectromechanical systems (MEMS) (Lee et al. 2012), or micrototal analysis systems (μTAS) (Reyes et al. 2002, Dittrich et al. 2006, West et al. 2008).

Among all fabrication processes, photolithography has been strongly developed since few decades to fulfil to the needs of the microelectronics industry. Researches in this area were essentially motivated by finding ways to provide new solutions to pursue the trend towards a constant decrease of the size of the transistors as stated in the "Moore's law" (Moore, 1965). To reach these objectives, lithographic fabrication methods have been widely diversified leading to DUV lithography (Ridaoui et al., 2010), X-ray lithography (Im et al., 2009) and ebeam lithography (Gonsalves et al. 2009) to quote a few. Although some of these techniques exhibit resolution of less than 10 nm, these methods are inherently 2D. Unlike conventional microelectronics components, many MEMS or μTAS devices (motors, pumps, valves…) require 3D fabrication capability to insure the same function as the corresponding macroscopic device. Thus, lithographic fabrication of 3D microstructures has emerged and has been divided in two categories depending if they give access to restricted or arbitrary 3D pattern fabrication. On the first hand, specific structures as periodic patterns have been made using self-assembly

© 2013 Spangenberg et al.; licensee InTech. This is an open access article 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. © 2013 Spangenberg et al.; licensee InTech. This is a paper 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.

(Shevchenko et al. 2002), layer-by-layer assembly (Kovacs et al. 1998), soft lithography (Quake et al. 2000), and holographic photopolymerization (Campbell et al. 2000). However by these techniques, no free-moving or complex microstructures have been achieved. On the other hand, arbitrary 3D patterns have been realized by using the so-called direct write technologies which gathers ink-based writing (Lewis et al. 2004), microstereolithography (Maruo et al. 2002) and two-photon stereolithography (TPS) (Kawata et al. 2001; Maruo et al. 1997). Though examples of submicrometer resolution have been demonstrated for ink-based writing (Lewis et al. 2004) and microstereolithography (Maruo et al. 2002), these techniques are mainly used for micro or macrofabrication.

photon absorption (TPA). This work was directly inspired of the first demonstration of localized excitation in two-photon fluorescence microscopy by Denk et al. two years before

illustrated by Maruo et al. in 1997 through the fabrication of a 7 μm-diameter and 50 μm-long spiral coil (Maruo et al. 1997). Finally, in 2001, micromachines and microbull with feature sizes close to the diffraction limit were realized (Kawata et al. 2001). By using non-linear effects of TPS, subdiffraction-limit spatial resolution of 120 nanometers has been successfully achieved.

Contrary to conventional stereolithography techniques where polymerization is induced by absorption of a single photon, TPS is based on two photon absorption (TPA) process. TPA and more generally multiphoton absorption (MPA) process have been first predicted in 1931 by Marie Goeppert-Mayer (Goeppert-Mayer, 1931) and then verified experimentally thirty years later (Kaiser et al. 1961), thanks to the advent of laser. Finally, two-photon photopolymeriza‐ tion was experimentally reported for the first time in 1965 as the first example of multiphoton excitation-induced photochemical reactions (Pao et al. 1965). However, it's only with the commercialization of tunable solide ultrashort pulse laser like Ti:Sapphire laser in the 1990s that application of TPA is widespread in various domains like biology imaging (two photon

**Figure 1.** A. Mechanism of TPA when simultaneous excitation occurs. B and C. Illustration of two methods for increas‐ ing the probability that TPA occurs: density of photon is increased by B. spatial compression using objectives with high

Two different mechanisms have been described for TPA: the sequential excitation and the simultaneous two-photon excitation. In the frame of TPS, only the second one is involved (Figure 1A). In this case, a virtual intermediate state is created by interaction of the material with the first absorbed photon. In order to reach the first real excited state, a second photon

1 TPS is also called two-photon polymerization (TPP), multiphoton absorption polymerization (MAP), 3-dimensional

as a real 3D fabrication technique was

Recent Advances in Two-Photon Stereolithography

http://dx.doi.org/10.5772/56165

37

(Denk et al. 1990). However, the feasibility of TPS1

fluorescence microscopy) or microfabrication (TPS).

numerical aperture, C. temporal compression using ultrafast lasers.

Direct Laser Writing (3D DLW) or 3-dimensional lithography.

**2.2. Two-photon absorption**

In this context, two-photon stereolithography which is an advanced version of microstereoli‐ thography appears of high interest since it offers intrinsically sub-100 nm resolution. Addi‐ tionally to its unique ability of writing arbitrary structures with sub-100 nm features without use of any mask, TPS is also an attractive fabrication process due to the versatility of materials used including polymers, biopolymers, ceramics, metals, and hybrid materials.

The aim of this chapter is to review some recent works about two-photon stereolithography and its applications. In the first part, a brief introduction to TPS and the fundamental concepts will allow illustrating its interest and its current development. The second part will be dedicated to the most relevant materials developed for TPS regarding to the applications targeted. Furthermore, some typical applications where 3 dimensionalities play a crucial role will be highlighted. Finally, the last part will describe the recent advances in TPS both from the writing speed and the resolution (Li et al. 2009) in order to compete with other nanofabri‐ cation techniques. As the result, the contribution of this chapter is to propose a comprehensive overview of fundamental issues in TPS as well as its current and future promising potential.

## **2. Two-photon stereolithography**

### **2.1. Introduction**

One of the first attempts to fabricate 3D structures arised from IBM in 1969 (Cerrina, 1997). By combining electrodeposition and X-ray lithography, high-aspect ratio metal structures were obtained. Further works on X-ray lithography gave rise to the well-known process LIGA in the early 1980s (Becker et al. 1984). Despite the maturity of the technique and demonstration of some 3D complex structures, their application has not been widespread due to the availa‐ bility of synchrotron radiation sources and X-ray masks.

Historically speaking, TPS began with the 3D microfabrication process using photopolymers developed by Kodama in 1981 (Kodama, 1981). Further developments lead to the birth of stereolithography, then microstereolithography to achieve resolution down to 1 μm. Even if in some cases, submicrometer resolution has been demonstrated (Maruo et al. 2002), it is still challenging to obtain microstructures with nano or submicron features due to the layer-bylayer nature of this technique. To overcome this drawback, Wu et al. (Wu et al. 1992) proposed the two-photon lithography concept which is based on the nonlinear optical process of two photon absorption (TPA). This work was directly inspired of the first demonstration of localized excitation in two-photon fluorescence microscopy by Denk et al. two years before (Denk et al. 1990). However, the feasibility of TPS1 as a real 3D fabrication technique was illustrated by Maruo et al. in 1997 through the fabrication of a 7 μm-diameter and 50 μm-long spiral coil (Maruo et al. 1997). Finally, in 2001, micromachines and microbull with feature sizes close to the diffraction limit were realized (Kawata et al. 2001). By using non-linear effects of TPS, subdiffraction-limit spatial resolution of 120 nanometers has been successfully achieved.

## **2.2. Two-photon absorption**

(Shevchenko et al. 2002), layer-by-layer assembly (Kovacs et al. 1998), soft lithography (Quake et al. 2000), and holographic photopolymerization (Campbell et al. 2000). However by these techniques, no free-moving or complex microstructures have been achieved. On the other hand, arbitrary 3D patterns have been realized by using the so-called direct write technologies which gathers ink-based writing (Lewis et al. 2004), microstereolithography (Maruo et al. 2002) and two-photon stereolithography (TPS) (Kawata et al. 2001; Maruo et al. 1997). Though examples of submicrometer resolution have been demonstrated for ink-based writing (Lewis et al. 2004) and microstereolithography (Maruo et al. 2002), these techniques are mainly used

In this context, two-photon stereolithography which is an advanced version of microstereoli‐ thography appears of high interest since it offers intrinsically sub-100 nm resolution. Addi‐ tionally to its unique ability of writing arbitrary structures with sub-100 nm features without use of any mask, TPS is also an attractive fabrication process due to the versatility of materials

The aim of this chapter is to review some recent works about two-photon stereolithography and its applications. In the first part, a brief introduction to TPS and the fundamental concepts will allow illustrating its interest and its current development. The second part will be dedicated to the most relevant materials developed for TPS regarding to the applications targeted. Furthermore, some typical applications where 3 dimensionalities play a crucial role will be highlighted. Finally, the last part will describe the recent advances in TPS both from the writing speed and the resolution (Li et al. 2009) in order to compete with other nanofabri‐ cation techniques. As the result, the contribution of this chapter is to propose a comprehensive overview of fundamental issues in TPS as well as its current and future promising potential.

One of the first attempts to fabricate 3D structures arised from IBM in 1969 (Cerrina, 1997). By combining electrodeposition and X-ray lithography, high-aspect ratio metal structures were obtained. Further works on X-ray lithography gave rise to the well-known process LIGA in the early 1980s (Becker et al. 1984). Despite the maturity of the technique and demonstration of some 3D complex structures, their application has not been widespread due to the availa‐

Historically speaking, TPS began with the 3D microfabrication process using photopolymers developed by Kodama in 1981 (Kodama, 1981). Further developments lead to the birth of stereolithography, then microstereolithography to achieve resolution down to 1 μm. Even if in some cases, submicrometer resolution has been demonstrated (Maruo et al. 2002), it is still challenging to obtain microstructures with nano or submicron features due to the layer-bylayer nature of this technique. To overcome this drawback, Wu et al. (Wu et al. 1992) proposed the two-photon lithography concept which is based on the nonlinear optical process of two

used including polymers, biopolymers, ceramics, metals, and hybrid materials.

for micro or macrofabrication.

36 Updates in Advanced Lithography

**2. Two-photon stereolithography**

bility of synchrotron radiation sources and X-ray masks.

**2.1. Introduction**

Contrary to conventional stereolithography techniques where polymerization is induced by absorption of a single photon, TPS is based on two photon absorption (TPA) process. TPA and more generally multiphoton absorption (MPA) process have been first predicted in 1931 by Marie Goeppert-Mayer (Goeppert-Mayer, 1931) and then verified experimentally thirty years later (Kaiser et al. 1961), thanks to the advent of laser. Finally, two-photon photopolymeriza‐ tion was experimentally reported for the first time in 1965 as the first example of multiphoton excitation-induced photochemical reactions (Pao et al. 1965). However, it's only with the commercialization of tunable solide ultrashort pulse laser like Ti:Sapphire laser in the 1990s that application of TPA is widespread in various domains like biology imaging (two photon fluorescence microscopy) or microfabrication (TPS).

**Figure 1.** A. Mechanism of TPA when simultaneous excitation occurs. B and C. Illustration of two methods for increas‐ ing the probability that TPA occurs: density of photon is increased by B. spatial compression using objectives with high numerical aperture, C. temporal compression using ultrafast lasers.

Two different mechanisms have been described for TPA: the sequential excitation and the simultaneous two-photon excitation. In the frame of TPS, only the second one is involved (Figure 1A). In this case, a virtual intermediate state is created by interaction of the material with the first absorbed photon. In order to reach the first real excited state, a second photon

<sup>1</sup> TPS is also called two-photon polymerization (TPP), multiphoton absorption polymerization (MAP), 3-dimensional Direct Laser Writing (3D DLW) or 3-dimensional lithography.

has to be absorbed during the short lifetime (around 10-15 s) of this virtual state. To increase the probability of such a non-linear absorption process, high density of photons is requested. Consequently, in main applications (including TPS) where TPA is involved, objectives with high numerical aperture (NA) and ultra short pulse laser are employed for increasing spatial and temporal density of photons, respectively (Figure 1B and C). The main interest of TPA compared to single photon absorption is that excitation is localized within the focal volume of a laser beam. Consequently, it gives access to 3D microfabrication since the polymerization threshold is not reached out of the focal volume. Typically, volume less than 1 μm3 can be addressed. In parallel to the technical developments for TPA, molecular engineering has been strongly developed to design molecules or molecular architectures with large TPA cross section. An exhaustive review on this point can be found in reference (He et al. 2008) and few typical examples will be given in the next section.

## **2.3. Experimental set-up for TPS**

The typical TPS setup is composed of three main parts: (i) the excitation source, (ii) the computer-aided design (CAD) system and (iii) the scan method. The excitation source with high intensity is important to favor TPA process. Even if Ti:Sapphire laser operating at 800 nm are often used, Baldeck and coworkers have demonstrated that TPS could be performed successfully by using a cheap Nd-YAG microlaser operating at 532 nm (Wang et al. 2002). The CAD system has to be chosen carefully since trajectories can influence the writing time and more important the mechanical resistance of the final structure. Finally, the scan method will have a crucial impact of the throughput of the writing process. The first possibility is to use Galvano mirrors for horizontal scanning coupled to piezoelectronic stage for vertical scanning which presents the advantage to scan with high speed. However, the total horizontal range accessible by this optical system is limited to few ten of micrometers due to spherical aberration when using objectives with high numerical aperture. The most popular solution consists to scan in x, y and z direction by using a piezoelectric stage. While the scan speed is low compared to previous option, few hundred of micrometer can be scanned.

**3. Materials for two-photon stereolithography**

**Figure 2.** Schematic typical experimental TPS set-up.

the irradiation wavelength and nature of the monomer.

scheme.

of free radical photoinitiators.

Basically, the formulation destined to TPS is composed of 2 main components: a photoinitiator system and a monomer. The two-photon photoinitiator is a species, or a combination of chemicals able to absorb efficiently two photons to generate excited states from which reactive species can be created. One of the most important parameter is the two-photon absorption cross section that directly characterizes the capacity of the photoinitiator to absorb two photons. The reactive species (radicals or ions) must be able to initiate polymerization of the monomers that constitute the building blocks of the final material. After initiation, propagation and termination reactions take place as observed in the sequences of classical polymerization

Recent Advances in Two-Photon Stereolithography

http://dx.doi.org/10.5772/56165

39

In principle, any one-photon photopolymerizable system can be adapted to TPS, provided that a suitable TPS photoinitiator can be added to the monomer system. Most of the published works deal with free radical photopolymers. The main reason is relative to a wider availability

Additionally to the photoinitiator and monomers, other chemicals can be added in TPS systems like inhibitors (to control the polymerization threshold and thus spatial extend of polymeri‐ zation - some examples will be given in the next section), and additives to bring specific properties (fluorophores, metal nanoparticles, quantum dots, etc...). The choice of the mono‐ mer is in relation with the final application whereas the choice of the photoinitiator integrates

As depicted in Figure 2, a typical set-up is composed of a mode-locked Ti:Sapphire laser as excitation source which presents duration pulse of ten hundred of femtosecond at 800 nm, repetition rate around 80 MHz and average output power of 1-3 W. The intensity of the laser is controlled by an optical or mechanical shutter. Before the introduction of the beam into the microscope, it is expanded so as to overfill the back aperture of the objective. By tightly focusing the pulsed laser beam (ns to fs pulses) into a multi-photon absorbing material, it is possible to trigger a photoreaction (e.g. photopolymerization) inside a volume below the dimension of the voxel. Complex structures (such as in Figures 3, 4, 6 and 8) can then be generated by moving in the laser focus in the 3 dimensions inside the monomer substrate. Usually, samples are placed on a 3D piezoelectric stage, and then move above the fixed laser beam by CAD. Upon the irradiation, only areas exposed at the focal point are polymerized further than the poly‐ merization threshold, leading to the desired structures after washing away the unsolidified photoresist. Finally, the back reflection is collected by an additional port and send to a CCD to monitor the fabrication in real time.

**Figure 2.** Schematic typical experimental TPS set-up.

has to be absorbed during the short lifetime (around 10-15 s) of this virtual state. To increase the probability of such a non-linear absorption process, high density of photons is requested. Consequently, in main applications (including TPS) where TPA is involved, objectives with high numerical aperture (NA) and ultra short pulse laser are employed for increasing spatial and temporal density of photons, respectively (Figure 1B and C). The main interest of TPA compared to single photon absorption is that excitation is localized within the focal volume of a laser beam. Consequently, it gives access to 3D microfabrication since the polymerization

threshold is not reached out of the focal volume. Typically, volume less than 1 μm3

typical examples will be given in the next section.

to previous option, few hundred of micrometer can be scanned.

**2.3. Experimental set-up for TPS**

38 Updates in Advanced Lithography

to monitor the fabrication in real time.

addressed. In parallel to the technical developments for TPA, molecular engineering has been strongly developed to design molecules or molecular architectures with large TPA cross section. An exhaustive review on this point can be found in reference (He et al. 2008) and few

The typical TPS setup is composed of three main parts: (i) the excitation source, (ii) the computer-aided design (CAD) system and (iii) the scan method. The excitation source with high intensity is important to favor TPA process. Even if Ti:Sapphire laser operating at 800 nm are often used, Baldeck and coworkers have demonstrated that TPS could be performed successfully by using a cheap Nd-YAG microlaser operating at 532 nm (Wang et al. 2002). The CAD system has to be chosen carefully since trajectories can influence the writing time and more important the mechanical resistance of the final structure. Finally, the scan method will have a crucial impact of the throughput of the writing process. The first possibility is to use Galvano mirrors for horizontal scanning coupled to piezoelectronic stage for vertical scanning which presents the advantage to scan with high speed. However, the total horizontal range accessible by this optical system is limited to few ten of micrometers due to spherical aberration when using objectives with high numerical aperture. The most popular solution consists to scan in x, y and z direction by using a piezoelectric stage. While the scan speed is low compared

As depicted in Figure 2, a typical set-up is composed of a mode-locked Ti:Sapphire laser as excitation source which presents duration pulse of ten hundred of femtosecond at 800 nm, repetition rate around 80 MHz and average output power of 1-3 W. The intensity of the laser is controlled by an optical or mechanical shutter. Before the introduction of the beam into the microscope, it is expanded so as to overfill the back aperture of the objective. By tightly focusing the pulsed laser beam (ns to fs pulses) into a multi-photon absorbing material, it is possible to trigger a photoreaction (e.g. photopolymerization) inside a volume below the dimension of the voxel. Complex structures (such as in Figures 3, 4, 6 and 8) can then be generated by moving in the laser focus in the 3 dimensions inside the monomer substrate. Usually, samples are placed on a 3D piezoelectric stage, and then move above the fixed laser beam by CAD. Upon the irradiation, only areas exposed at the focal point are polymerized further than the poly‐ merization threshold, leading to the desired structures after washing away the unsolidified photoresist. Finally, the back reflection is collected by an additional port and send to a CCD

can be

## **3. Materials for two-photon stereolithography**

Basically, the formulation destined to TPS is composed of 2 main components: a photoinitiator system and a monomer. The two-photon photoinitiator is a species, or a combination of chemicals able to absorb efficiently two photons to generate excited states from which reactive species can be created. One of the most important parameter is the two-photon absorption cross section that directly characterizes the capacity of the photoinitiator to absorb two photons. The reactive species (radicals or ions) must be able to initiate polymerization of the monomers that constitute the building blocks of the final material. After initiation, propagation and termination reactions take place as observed in the sequences of classical polymerization scheme.

In principle, any one-photon photopolymerizable system can be adapted to TPS, provided that a suitable TPS photoinitiator can be added to the monomer system. Most of the published works deal with free radical photopolymers. The main reason is relative to a wider availability of free radical photoinitiators.

Additionally to the photoinitiator and monomers, other chemicals can be added in TPS systems like inhibitors (to control the polymerization threshold and thus spatial extend of polymeri‐ zation - some examples will be given in the next section), and additives to bring specific properties (fluorophores, metal nanoparticles, quantum dots, etc...). The choice of the mono‐ mer is in relation with the final application whereas the choice of the photoinitiator integrates the irradiation wavelength and nature of the monomer.

anthracene-thioxanthone was shown to be five times lower with respect to that of thioxan‐

Recent Advances in Two-Photon Stereolithography

http://dx.doi.org/10.5772/56165

41

Additional examples of polyacrylate structures realized by TPS can be found in figure 4

Cationic photoinitiated polymerization of epoxides, vinyl ethers and methylenedioxolanes has received increasing attention, owing in large part to the oxygen insensitivity of the cationic process (Belfield et al. 1997a and 1997b). Moreover, cationic photoresist appears as an inter‐ esting choice from application point of view since UV negative tone photoresists have

However, the difficulty to design efficient TPA photoacid generators has limited the devel‐ opment of TPA cationic photoresists. For this reasons, many efforts have been devoted to increase the sensitivity of such systems. First approach was based on sensitizers such as coumarin (Li et al. 2001), phenothiazine (Billone et al. 2009), or thioxanthone (Steidl et al. 2009) associated to a commercial PAG such as onium salts. Second approach relies on a molecular association of the acid generator functionality into the structure of the two-photon active chromophore. In the latest case, the reactivity of the PAG is no longer limited by diffusion and thus a significant improvement of the photopolymerization efficiency was

Among other application, the epoxy-based photoresists are extremely interesting when complex structures with high aspect ratio are needed. Indeed, thanks to their good mechanical properties, they have been successfully used for application in microfluidic (Maruo et al.

Despite their advantages, polymers have intrinsic limitations for some applications. For instance, their mechanical properties at high temperature or in contact with solvents degrade rapidly. They also present low refractive index that limits their use in optical applications. Their toxicity may prevent them from use in contact with living organisms. For these reasons, alternative strategies have been developed to combine the advantages of 3D structuration by

The sol-gel route is interesting in the frame of micro-nano-fabrication since it allows the fabrication of inorganic or hybrid organic-inorganic materials at relatively low temperature. The first strategy followed for combining lithography and sol-gel materials consisted in developing hybrid precursors that can undergo both sol-gel hydrolysis-condensation reaction and photoinduced crosslinking (Blanc et al. 1999; Soppera et al. 2001). These materials, also called Ormorcer® or Ormorsil® have been adapted to TPA by use of suitable photoinitiators and interesting applications in the frame of optics (Ovsianikov et al. 2008) or biology (Klein et al. 2011, see Figure 3B, C) were demonstrated. These materials were mostly used in optics since

demonstrated their interest for microelectronics, optics, microfluidic or MEMS.

demonstrated (Zhou et al. 2002; Yanez et al. 2009; Xia et al. 2012).

2006) or MEMS (Bückmann et al. 2012).

**3.3. Advanced functional materials**

TPA and functional materials.

thone.

section 4.1.

**3.2. Cationic photoresist**

**Figure 3.** structures realized by TPS with A. polyacrylate derivatives (λexc: 532 nm; average power : 20 µW; writing speed: 45 µm.s-1 ; Malval et al. 2011), B. and C. sol-gel materials for biocompatible 3D scaffold (λexc: 808 nm; average power : 20 mW; writing speed: 200 µm.s-1 Klein et al. 2011), D. trypsine derivative for biocatalysis degradation (λexc: 532 nm; average power : 1 mW; writing speed: nc, Iosin et al. 2012). Reproduced from the respective references.

As illustrated in Figure 3, different examples of 3D structures have been realized with various materials. Interest of such structure will be discussed in each corresponding material subsection. The next paragraphs are aimed at giving some examples of systems and associated applications.

#### **3.1. Free-radical photoresists**

Among different monomers available, acrylate monomers have been the most widely used for TPS. The reason for this success is a wide range of commercially available acrylate monomers with tailored properties: chain length, number of reactive function, viscosity, polarity to quote a few. Moreover, acrylate monomers exhibit a high reactivity and good mechanical properties that allow complex 3D structures being created.

In parallel, a wide choice of free-radical TPP photoinitiators has been developed. Highly efficient two-photon absorbing systems such as 4,4'-dialkylamino trans-stilbene (Cumpston et al. 1999) and other bis-donor bis(styryl)benzene or bis(phenyl)polyene (Lu et al. 2004; Zhang et al. 2005; Rumi et al. 2000) were employed for two-photon initiated free radical polymeriza‐ tion. Other conjugated photoinitiators as fluorenes (Belfield et al. 2000; Martineau et al. 2002, Jin et al. 2008) and ketocoumarins (Li et al. 2007) derivatives also demonstrated remarkable TPS properties. Finally, another promissing strategy is the direct photogeneration of highly reactive radicals such as α-aminoalkyl ones.

In particular, Malval et al. demonstrated an elegant strategy to improve the efficiency of thioxanthone-based systems (Malval et al. 2011, Figure 3A). New hybrid anthracene-thioxan‐ thone system assembled into a chevron-shaped molecular architecture was proposed. A strong increase in the two-photon absorption cross section by more than a factor of 30 as compared to thioxanthone was observed. As a consequence, anthracene-thioxanthone constitutes a suitable two-photon initiating chromophore with a much higher efficiency as thioxanthone used as reference. At λexc = 710 nm for instance, the two-photon polymerization threshold of anthracene-thioxanthone was shown to be five times lower with respect to that of thioxan‐ thone.

Additional examples of polyacrylate structures realized by TPS can be found in figure 4 section 4.1.

## **3.2. Cationic photoresist**

**Figure 3.** structures realized by TPS with A. polyacrylate derivatives (λexc: 532 nm; average power : 20 µW; writing speed: 45 µm.s-1 ; Malval et al. 2011), B. and C. sol-gel materials for biocompatible 3D scaffold (λexc: 808 nm; average power : 20 mW; writing speed: 200 µm.s-1 Klein et al. 2011), D. trypsine derivative for biocatalysis degradation (λexc: 532 nm; average power : 1 mW; writing speed: nc, Iosin et al. 2012). Reproduced from the respective references.

As illustrated in Figure 3, different examples of 3D structures have been realized with various materials. Interest of such structure will be discussed in each corresponding material subsection. The next paragraphs are aimed at giving some examples of systems and associated

Among different monomers available, acrylate monomers have been the most widely used for TPS. The reason for this success is a wide range of commercially available acrylate monomers with tailored properties: chain length, number of reactive function, viscosity, polarity to quote a few. Moreover, acrylate monomers exhibit a high reactivity and good mechanical properties

In parallel, a wide choice of free-radical TPP photoinitiators has been developed. Highly efficient two-photon absorbing systems such as 4,4'-dialkylamino trans-stilbene (Cumpston et al. 1999) and other bis-donor bis(styryl)benzene or bis(phenyl)polyene (Lu et al. 2004; Zhang et al. 2005; Rumi et al. 2000) were employed for two-photon initiated free radical polymeriza‐ tion. Other conjugated photoinitiators as fluorenes (Belfield et al. 2000; Martineau et al. 2002, Jin et al. 2008) and ketocoumarins (Li et al. 2007) derivatives also demonstrated remarkable TPS properties. Finally, another promissing strategy is the direct photogeneration of highly

In particular, Malval et al. demonstrated an elegant strategy to improve the efficiency of thioxanthone-based systems (Malval et al. 2011, Figure 3A). New hybrid anthracene-thioxan‐ thone system assembled into a chevron-shaped molecular architecture was proposed. A strong increase in the two-photon absorption cross section by more than a factor of 30 as compared to thioxanthone was observed. As a consequence, anthracene-thioxanthone constitutes a suitable two-photon initiating chromophore with a much higher efficiency as thioxanthone used as reference. At λexc = 710 nm for instance, the two-photon polymerization threshold of

applications.

**3.1. Free-radical photoresists**

40 Updates in Advanced Lithography

that allow complex 3D structures being created.

reactive radicals such as α-aminoalkyl ones.

Cationic photoinitiated polymerization of epoxides, vinyl ethers and methylenedioxolanes has received increasing attention, owing in large part to the oxygen insensitivity of the cationic process (Belfield et al. 1997a and 1997b). Moreover, cationic photoresist appears as an inter‐ esting choice from application point of view since UV negative tone photoresists have demonstrated their interest for microelectronics, optics, microfluidic or MEMS.

However, the difficulty to design efficient TPA photoacid generators has limited the devel‐ opment of TPA cationic photoresists. For this reasons, many efforts have been devoted to increase the sensitivity of such systems. First approach was based on sensitizers such as coumarin (Li et al. 2001), phenothiazine (Billone et al. 2009), or thioxanthone (Steidl et al. 2009) associated to a commercial PAG such as onium salts. Second approach relies on a molecular association of the acid generator functionality into the structure of the two-photon active chromophore. In the latest case, the reactivity of the PAG is no longer limited by diffusion and thus a significant improvement of the photopolymerization efficiency was demonstrated (Zhou et al. 2002; Yanez et al. 2009; Xia et al. 2012).

Among other application, the epoxy-based photoresists are extremely interesting when complex structures with high aspect ratio are needed. Indeed, thanks to their good mechanical properties, they have been successfully used for application in microfluidic (Maruo et al. 2006) or MEMS (Bückmann et al. 2012).

## **3.3. Advanced functional materials**

Despite their advantages, polymers have intrinsic limitations for some applications. For instance, their mechanical properties at high temperature or in contact with solvents degrade rapidly. They also present low refractive index that limits their use in optical applications. Their toxicity may prevent them from use in contact with living organisms. For these reasons, alternative strategies have been developed to combine the advantages of 3D structuration by TPA and functional materials.

The sol-gel route is interesting in the frame of micro-nano-fabrication since it allows the fabrication of inorganic or hybrid organic-inorganic materials at relatively low temperature. The first strategy followed for combining lithography and sol-gel materials consisted in developing hybrid precursors that can undergo both sol-gel hydrolysis-condensation reaction and photoinduced crosslinking (Blanc et al. 1999; Soppera et al. 2001). These materials, also called Ormorcer® or Ormorsil® have been adapted to TPA by use of suitable photoinitiators and interesting applications in the frame of optics (Ovsianikov et al. 2008) or biology (Klein et al. 2011, see Figure 3B, C) were demonstrated. These materials were mostly used in optics since the refractive index of the material can be tuned by adding metal alcoxides. However, in these hybrid materials, the proportion of organic moieties in the crosslinked material is still impor‐ tant, so many efforts have been dedicated to the formulation of fully inorganic materials (Passinger et al. 2007).

several commercial set-ups have emerged on the market since 5 years. However TPS suffers from two main drawbacks for a more largely widespread in other scientific area or in indus‐ tries. The first roadblock concerns the low-throughput of the process. Indeed, TPS is based on a serial process (i.e. point-by-point writing) which is a serious problem when mass production is needed. Moreover, compared to low-throughput techniques like e-beam lithography,

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43

In this section, we will discuss about different approaches to address these specific points and highlight some recent developments which answer mostly to these drawbacks and promise a

Because of the rapid improvement of TPS resolution in the past decade, a special attention has to be care on the way to define and measure it. In the most of works, "resolution" corresponds to the lateral and/or axial features size of single voxel or single line. Different methods such as ascending scan method (Sun et al. 2002) or suspending bridge method (DeVoe et al. 2003) have been proposed in order to improve the accuracy of the measurements. However, these two methods suffer from drawbacks which are the difficulty to avoid the truncation effect and the unknown influence of material's shrinkage. Therefore, though less information is provided,

Nevertheless, with the emergence of several STED-like lithographies, more precise definition of resolution has become necessary in order to compare their abilities. Although extension of the famous Abbe's criterion introduced in conventional or two-photon absorption microscopy can be extended to TPS to describe the optical limitation of the lithography system, ultimate resolution for a given optical lithography system has to be determined by considering the role of the photopolymer. Indeed, in the frame of the writing of two close lines, due to consumption of photoinitiators and diffusion of various species (photoinitiator, scavengers), the writing process of the second line can be strongly affected. This effect is sometimes referred as the resin's memory. Thus strong dependence with respect to the initial concentrations of photoi‐ nitiator as well as the viscosity of the matrix is expected. Up to date, no mathematical model includes all the parameters. Therefore, as suggested by Fischer et al. (Fischer et al. 2012), a better solution for determine both axial and lateral resolution would be the fabrication of a 3D periodic unit as a crystal photonic for a given photopolymerizable system. It has to be mentioned that typical ratio between axial over lateral resolution in TPS is ranging from 2 to 5 depending of optical conditions. In the next part, one has to keep in mind that the resolution

Since the microbull with 120 nm features size realized by Kawata and coworkers (Kawata et al. 2001), various approaches have been attempted to improve the resolution of TPS (Figure 4).The first approach which is still used nowadays relies on the design of high-efficiency photoinitiators. By this method, linewidths of 80 nm have been measured (Xing et al. 2007).

most of the groups define resolution as the width of a single line on the surface.

or feature size is given for both an optical and chemical system.

*4.1.1. Overview of the different strategies*

resolution achievable by TPS is still 1 or 2 order of magnitude lower.

brilliant future to TPS.

**4.1. Resolution**

Another important class of inorganic materials is metals. Metals nanoparticles, nanostructures or thin layers are indeed very interesting for electrical connections in devices and also for their plasmonic properties. Recent works have been reported on the fabrication of 3D metallic structures by combining TPS and silver evaporation (Rill et al. 2008). However, by this process, full metal coverage is challenging and induces a supplementary step. Therefore, several groups have developed more direct strategies based only on TPS. For instance, Prasad et al. have fabricated submicrometric plasmonic structures which exhibit interesting conductive proper‐ ties (Shukla et al. 2011). More recently, Spangenberg et al. also demonstrated that a silver complexe can be used as TPA photoinitiating system and as precursors for nanoparticles fabrications, leading in a single step to a polymer-metal nanoparticles nanocomposite (Spangenberg et al. 2012). Such routes open the doors towards microstructures with conduc‐ tive properties or magnetic properties that can be useful for MEMS actuation.

One other very important and growing field of applications for TPA materials is relative to biological applications. TPA microstructuring has been extensively used the last years to propose micro and nanostructured surfaces with tailored chemical composition to be used as model substrates to investigate the development of biofilms. The unique advantage of TPS is to propose real 3D structures that mimic with more accuracy the local environment of cells or bacteria than planar surfaces. In this context, polymer matrixes have been widely used but also, sol-gel materials were proposed since they are biocompatbile materials that can be used as inert topological matrixes, as illustrated in Figure 3B and 3C (Klein et al. 2011). Additionally, biological materials like trypsin or collagen precursors were developed to propose a direct writing route for 3D biocompatible structures. Besides the interest of allowing a direct writing of complex structures, an advantage of TPA microfabrication is to be adapted for integration of microstructures in closed environment. For example, Iosin et al. demonstrated the possi‐ bility of integrating trypsin's micropillars in a microfluidic system (Figure 3D). Trypsin is an enzyme used for catalyze the degradation of specific peptide. Interestingly, by following the variation of fluorescence intensity resulting from the peptide clivage, the authors have shown that trypsin structures kept its enzyme catalysis activity.

Although numerous materials have been designed to fulfill the requirements of various applications, there is still an important demand for optimizing current systems. Besides, with the emergence of STED–like lithography (for STimulated Emission Depletion, see next section), designs of new photosystems will be crucial for the development of such technique.

## **4. Current challenges in TPS**

As shown through several examples, TPS is a powerful and attractive technique for present and futures applications. Due to the need of a well-controlled 3D nanofabrication technique, several commercial set-ups have emerged on the market since 5 years. However TPS suffers from two main drawbacks for a more largely widespread in other scientific area or in indus‐ tries. The first roadblock concerns the low-throughput of the process. Indeed, TPS is based on a serial process (i.e. point-by-point writing) which is a serious problem when mass production is needed. Moreover, compared to low-throughput techniques like e-beam lithography, resolution achievable by TPS is still 1 or 2 order of magnitude lower.

In this section, we will discuss about different approaches to address these specific points and highlight some recent developments which answer mostly to these drawbacks and promise a brilliant future to TPS.

### **4.1. Resolution**

the refractive index of the material can be tuned by adding metal alcoxides. However, in these hybrid materials, the proportion of organic moieties in the crosslinked material is still impor‐ tant, so many efforts have been dedicated to the formulation of fully inorganic materials

Another important class of inorganic materials is metals. Metals nanoparticles, nanostructures or thin layers are indeed very interesting for electrical connections in devices and also for their plasmonic properties. Recent works have been reported on the fabrication of 3D metallic structures by combining TPS and silver evaporation (Rill et al. 2008). However, by this process, full metal coverage is challenging and induces a supplementary step. Therefore, several groups have developed more direct strategies based only on TPS. For instance, Prasad et al. have fabricated submicrometric plasmonic structures which exhibit interesting conductive proper‐ ties (Shukla et al. 2011). More recently, Spangenberg et al. also demonstrated that a silver complexe can be used as TPA photoinitiating system and as precursors for nanoparticles fabrications, leading in a single step to a polymer-metal nanoparticles nanocomposite (Spangenberg et al. 2012). Such routes open the doors towards microstructures with conduc‐

One other very important and growing field of applications for TPA materials is relative to biological applications. TPA microstructuring has been extensively used the last years to propose micro and nanostructured surfaces with tailored chemical composition to be used as model substrates to investigate the development of biofilms. The unique advantage of TPS is to propose real 3D structures that mimic with more accuracy the local environment of cells or bacteria than planar surfaces. In this context, polymer matrixes have been widely used but also, sol-gel materials were proposed since they are biocompatbile materials that can be used as inert topological matrixes, as illustrated in Figure 3B and 3C (Klein et al. 2011). Additionally, biological materials like trypsin or collagen precursors were developed to propose a direct writing route for 3D biocompatible structures. Besides the interest of allowing a direct writing of complex structures, an advantage of TPA microfabrication is to be adapted for integration of microstructures in closed environment. For example, Iosin et al. demonstrated the possi‐ bility of integrating trypsin's micropillars in a microfluidic system (Figure 3D). Trypsin is an enzyme used for catalyze the degradation of specific peptide. Interestingly, by following the variation of fluorescence intensity resulting from the peptide clivage, the authors have shown

Although numerous materials have been designed to fulfill the requirements of various applications, there is still an important demand for optimizing current systems. Besides, with the emergence of STED–like lithography (for STimulated Emission Depletion, see next section),

As shown through several examples, TPS is a powerful and attractive technique for present and futures applications. Due to the need of a well-controlled 3D nanofabrication technique,

designs of new photosystems will be crucial for the development of such technique.

tive properties or magnetic properties that can be useful for MEMS actuation.

that trypsin structures kept its enzyme catalysis activity.

**4. Current challenges in TPS**

(Passinger et al. 2007).

42 Updates in Advanced Lithography

Because of the rapid improvement of TPS resolution in the past decade, a special attention has to be care on the way to define and measure it. In the most of works, "resolution" corresponds to the lateral and/or axial features size of single voxel or single line. Different methods such as ascending scan method (Sun et al. 2002) or suspending bridge method (DeVoe et al. 2003) have been proposed in order to improve the accuracy of the measurements. However, these two methods suffer from drawbacks which are the difficulty to avoid the truncation effect and the unknown influence of material's shrinkage. Therefore, though less information is provided, most of the groups define resolution as the width of a single line on the surface.

Nevertheless, with the emergence of several STED-like lithographies, more precise definition of resolution has become necessary in order to compare their abilities. Although extension of the famous Abbe's criterion introduced in conventional or two-photon absorption microscopy can be extended to TPS to describe the optical limitation of the lithography system, ultimate resolution for a given optical lithography system has to be determined by considering the role of the photopolymer. Indeed, in the frame of the writing of two close lines, due to consumption of photoinitiators and diffusion of various species (photoinitiator, scavengers), the writing process of the second line can be strongly affected. This effect is sometimes referred as the resin's memory. Thus strong dependence with respect to the initial concentrations of photoi‐ nitiator as well as the viscosity of the matrix is expected. Up to date, no mathematical model includes all the parameters. Therefore, as suggested by Fischer et al. (Fischer et al. 2012), a better solution for determine both axial and lateral resolution would be the fabrication of a 3D periodic unit as a crystal photonic for a given photopolymerizable system. It has to be mentioned that typical ratio between axial over lateral resolution in TPS is ranging from 2 to 5 depending of optical conditions. In the next part, one has to keep in mind that the resolution or feature size is given for both an optical and chemical system.

#### *4.1.1. Overview of the different strategies*

Since the microbull with 120 nm features size realized by Kawata and coworkers (Kawata et al. 2001), various approaches have been attempted to improve the resolution of TPS (Figure 4).The first approach which is still used nowadays relies on the design of high-efficiency photoinitiators. By this method, linewidths of 80 nm have been measured (Xing et al. 2007). Another approach based on the use of a shorter wavelength has allowed writing of 3D structure with 60 nm feature size (Haske et al. 2007). Indeed, as dictated by the extended to TPS Abbe's criterion, lateral resolution is proportional to the wavelength. However, the wavelength can not be reduced indefinitely since the material may absorb linearly at shorter wavelength and consequently lead to the lost of the intrinsic resolution of TPS. Finally, more recent and impressive feature size was obtained by an enhanced version of TPS inspired by STED microscopy (Li et al. 2009). The principle of this technique will be described in detail in the next section. With 800 nm excitation wavelength, voxel of 40 nm height have been achieved, that represents λ/20. This spectacular result has to be compared with the voxel of 600 nm height obtained by using conventional TPS where excitation wavelength is set at 800 nm which corresponds to λ/1.33. Even if no experimental evidence has been shown for lateral resolution by this technique, λ/20 is also clearly achievable. Further insight of this new technique is addressed in the next subsection.

cally (Hell et al. 1994) and experimentally (Klar et al. 1999) birth, STED delivers nowadays routinely images of biological samples with a resolution down to 10-20 nm. Due to its great achievements in life-science, STED and more globally super resolution microscopy have been recognized as the "method of the year" in 2008 in Nature Methods. Finally, world record lateral resolution down to 5.6 nm using visible light has been reported by Hell's group (Rittweger et al. 2009). In STED, a first short laser pulse is used to bring fluorescent molecules in their excited state. In order to de-excite these chromophores through stimulated emission, a second laser pulse (usually at longer wavelength to avoid one photon absorption) has to occur after vibrational relaxation of the excited electronic state but before fluorescence occurs i.e. few ps to few ns later than the first laser pulse. The efficiency of the deactivation strongly depends on the intensity and the wavelength of the depletion pulse, as well as the time delay of depletion pulse versus the excitation pulse. The precise localization of fluorescence arises from the spatial phase shaping of the depletion beam. The latter causes de-excitation to occur everywhere except in a region at the center of the original focal volume. The idea to translate these groundbreaking concepts to optical lithography has been evoked in 2003 (Hell et al. 2003), but first demonstration applied to TPS has been published only 6 years later (Li et al. 2009). Nowadays, in the frame of STED-like optical lithography, 3 different depletion mechanisms have been reported in the literature. In all cases, two laser beams are used, one for excitation and a second one for deactivation as illustrated in Figure 5. Whereas the excitation beam allows the formation of species (i.e radicals) which initiate the polymerization, the phase shaped deactivation beam allow photophysically or photochemically inhibition of the reticulation around the central excited zone. Depending of the phase mask used, the voxel can be reduced along the axial direction (bottle-beam shape, see Figure 5B) or along the lateral direction (donut

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**Figure 5.** A. Schematic experimental set-up for STED-like optical lithographies. B. False-color, multiphoton-absorp‐ tion–induced luminescence images of the cubes of the PSFs of the excitation beam, the phase shaped deactivation

beam, and both beams together. Adapted from reference (Li et al. 2009).

shape).

**Figure 4.** Improving spatial resolution of two-photon microfabrication by different strategies during the past decade. a) the famous microbull exhibiting 120 nm features size due to intrinsic properties of TPS (λ = 780 nm, λ/6.5; Kawata et al. 2001), b) Photonic crystal with 60 nm features size due to the use of shorter wavelength (λ = 532 nm, λ/8; Haske et al. 2007), c) 80 nm linewidth by using a high efficient photoinitiator (λ = 800 nm, λ/10; Xing et al. 2007), d) and e) microtower fabricated by using a conventional TPS and a STED-like TPS, respectively. In d and e insets, AFM measure‐ ments of voxel size are shown. e) inset is the current smallest axial resolution (40 nm) for a voxel thanks to a STED-like method (λ = 800 nm, λ/20; Li et al. 2009). Reproduced from the respective references.

#### *4.1.2. STED-like lithography*

In the recent past, the diffraction resolution barrier of fluorescence light microscopy has been radically overcome by stimulated emission depletion (STED) microscopy. Since its theoreti‐ cally (Hell et al. 1994) and experimentally (Klar et al. 1999) birth, STED delivers nowadays routinely images of biological samples with a resolution down to 10-20 nm. Due to its great achievements in life-science, STED and more globally super resolution microscopy have been recognized as the "method of the year" in 2008 in Nature Methods. Finally, world record lateral resolution down to 5.6 nm using visible light has been reported by Hell's group (Rittweger et al. 2009). In STED, a first short laser pulse is used to bring fluorescent molecules in their excited state. In order to de-excite these chromophores through stimulated emission, a second laser pulse (usually at longer wavelength to avoid one photon absorption) has to occur after vibrational relaxation of the excited electronic state but before fluorescence occurs i.e. few ps to few ns later than the first laser pulse. The efficiency of the deactivation strongly depends on the intensity and the wavelength of the depletion pulse, as well as the time delay of depletion pulse versus the excitation pulse. The precise localization of fluorescence arises from the spatial phase shaping of the depletion beam. The latter causes de-excitation to occur everywhere except in a region at the center of the original focal volume. The idea to translate these groundbreaking concepts to optical lithography has been evoked in 2003 (Hell et al. 2003), but first demonstration applied to TPS has been published only 6 years later (Li et al. 2009). Nowadays, in the frame of STED-like optical lithography, 3 different depletion mechanisms have been reported in the literature. In all cases, two laser beams are used, one for excitation and a second one for deactivation as illustrated in Figure 5. Whereas the excitation beam allows the formation of species (i.e radicals) which initiate the polymerization, the phase shaped deactivation beam allow photophysically or photochemically inhibition of the reticulation around the central excited zone. Depending of the phase mask used, the voxel can be reduced along the axial direction (bottle-beam shape, see Figure 5B) or along the lateral direction (donut shape).

Another approach based on the use of a shorter wavelength has allowed writing of 3D structure with 60 nm feature size (Haske et al. 2007). Indeed, as dictated by the extended to TPS Abbe's criterion, lateral resolution is proportional to the wavelength. However, the wavelength can not be reduced indefinitely since the material may absorb linearly at shorter wavelength and consequently lead to the lost of the intrinsic resolution of TPS. Finally, more recent and impressive feature size was obtained by an enhanced version of TPS inspired by STED microscopy (Li et al. 2009). The principle of this technique will be described in detail in the next section. With 800 nm excitation wavelength, voxel of 40 nm height have been achieved, that represents λ/20. This spectacular result has to be compared with the voxel of 600 nm height obtained by using conventional TPS where excitation wavelength is set at 800 nm which corresponds to λ/1.33. Even if no experimental evidence has been shown for lateral resolution by this technique, λ/20 is also clearly achievable. Further insight of this new technique is

**Figure 4.** Improving spatial resolution of two-photon microfabrication by different strategies during the past decade. a) the famous microbull exhibiting 120 nm features size due to intrinsic properties of TPS (λ = 780 nm, λ/6.5; Kawata et al. 2001), b) Photonic crystal with 60 nm features size due to the use of shorter wavelength (λ = 532 nm, λ/8; Haske et al. 2007), c) 80 nm linewidth by using a high efficient photoinitiator (λ = 800 nm, λ/10; Xing et al. 2007), d) and e) microtower fabricated by using a conventional TPS and a STED-like TPS, respectively. In d and e insets, AFM measure‐ ments of voxel size are shown. e) inset is the current smallest axial resolution (40 nm) for a voxel thanks to a STED-like

In the recent past, the diffraction resolution barrier of fluorescence light microscopy has been radically overcome by stimulated emission depletion (STED) microscopy. Since its theoreti‐

method (λ = 800 nm, λ/20; Li et al. 2009). Reproduced from the respective references.

addressed in the next subsection.

44 Updates in Advanced Lithography

*4.1.2. STED-like lithography*

**Figure 5.** A. Schematic experimental set-up for STED-like optical lithographies. B. False-color, multiphoton-absorp‐ tion–induced luminescence images of the cubes of the PSFs of the excitation beam, the phase shaped deactivation beam, and both beams together. Adapted from reference (Li et al. 2009).

#### *4.1.2.1. Two-color photoinitiation/inhibition lithography*

Among the three STED-like optical lithography methods, the so called two-color photoinitia‐ tion / inhibition lithography (2PII) is the only one based on a *photochemical* deactivation of photoinitiator (Scott et al. 2009, McLeod et al. 2010). In this case, by using continuous laser, the excitation is performed by a single photon absorption process at 473 nm and deactivation occurs also by a linear process at a distinct wavelength (364 nm). Upon deactivation beam, two weakly reactive radicals are produced which can interact with the initiating radical to stop the polymerization. By using a donut mode for shaping the deactivation beam, reduction of the lateral extent of single voxel until 65 nm is reached. More recently, with a similar set-up, another group has claimed having designed a more efficiency photopolymerizable system which has been illustrated by the fabrication of 40 nm dots (Cao et al. 2011). While the idea to use inexpensive continuous laser sources is attractive, this process has been demonstrated only for fabrication of 2 dimensional structures. Although manipulation of the photoinhibiting wavelength into a "bottle beam" profile would induce confinement along the third axis, when focusing deeper into the photoresist volume, both excitation and deactivation might be attenuated by the linear absorption of the material. Besides, consumption of photoinitiator and/or photoinhibitor along the pathway of the beam could lead to a time and space depend‐ ence of concentration of photoactivable species during the 3D fabrication. 2PII based on twophoton absorption process for both excitation and deactivation could avoid this problem, but up to now, no such experiment has been reported in the literature.

sensitive that the excitation beam can induce itself the deactivation. Interestingly, for a RAPID compatible photoinitiator, the linewidth increases at faster scan speed. In the op‐ posite, in conventional photoinitiator, faster scan and consequently weaker exposure dose yields to decrease of the linewidth. This opposite effect for RAPID photoinitiator is ex‐ plained by the depletion effect done by the excitation beam. Indeed, slow down the scan speed allow to excited photoinitiator to be deactivate. Smartly, the authors have taken benefit of this unexpected dependence towards scan speed to propose system insensitive to abrupt change of trajectory (Figure 6). In this case, photopolymerizable system com‐ bines both conventional and RAPID photoinitiators. Finally, according to conclusions of Wegener and coworkers (Fischer et al. 2012), because the depletion time-constant is be‐ tween 15 and 350 ms in case of RAPID lithography, writing speed is comprised between 30 μm.s-1 to 150 μm.s-1. While this speed corresponds to typical speeds used in academic

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field, this slow speed might be an obstacle for its use in industry (see section 4.2).

**Figure 6.** A, B Large and close view of fabrication of sinusoidal structures with a conventional photoinitiator, respec‐ tively. C, D Large and close view of fabrication of sinusoidal structures with a RAPID photoinitiator, respectively. E, F Large and close view of fabrication of sinusoidal structures with a mixture of conventional and RAPID photoinitiators,

For efficient STED, molecules have to present large oscillator strength between the ground state S0 and the first excited sate S1 to favor later depletion. Because of their use in fluorescence microscopy, such type of molecules has also to exhibit strong fluorescence quantum yield. But this relatively long lifetime excited state (usually few ns) allows the depletion to take place.

In contrary, common photoinitiator exhibit low oscillator strength and are designed to present efficient intersystem crossing (ISC) yielding to reactive species which can give rise to radical and further to polymerization. Moreover, excited state lifetime of photoinitiator is usually found to be around 100 ps which would result in the use of high power pulses shorter than 100 ps to induce depletion. Unfortunately, with this large pulse energy, depletion could be competed by multiphoton absorption leading to undesired polymerization. Compare to previous STED-like lithography technique, STED lithography requires the use of two distinct-

respectively. Reproduced from reference (Stocker et al. 2010).

*4.1.2.3. STED lithography*

#### *4.1.2.2. RAPID lithography*

The first attempt to translate the spectacular optical resolution from STED microscopy to lithography has to be attributed to Fourkas's group (Li et al. 2009). Contrary to 2PII, deacti‐ vation is based on a *photophysical* process. On preliminary experiments, Fourkas and coworkers have used the conventional STED configuration. In this case, the excitation beam (800 nm) was overlaid by a second beam red-shifted with respect to the excitation beam wavelength. Then, in order to enhance the efficiency of stimulated emission, the depletion pulse was stretched to duration of 50 ps. to guarantee enough intensity of the depletion beam. Deactivation i.e. inhibition of the polymerization was observed for a wide range of wavelength (760 to 840 nm). However, whereas tuning the pulse delay from 0 to 13 ns between the two beams should affect the efficiency of the inhibition, no significant effect had been observed. Consequently the deactivation was not assigned to stimulated emission like in STED, but the authors have ascribed this effect to the depletion of an intermediate state with longer lifetime. Therefore they have decided to name this method RAPID for Resolution Augmentation through Photo-Induced Deactivation. Further experiments are under investigation to determine the nature of the intermediate specie.

Thanks to the longer lifetime of the intermediate, a second configuration has been suc‐ cessfully used: depletion effect has been performed with continuous laser which cancel the need to control the delay between excitation (800 nm) and depletion (800 nm) beams. By using the later configuration and a bottle beam profile for the depletion beam, voxels of 40 nm height have been achieved. In the frame of this study, the depletion effect is so sensitive that the excitation beam can induce itself the deactivation. Interestingly, for a RAPID compatible photoinitiator, the linewidth increases at faster scan speed. In the op‐ posite, in conventional photoinitiator, faster scan and consequently weaker exposure dose yields to decrease of the linewidth. This opposite effect for RAPID photoinitiator is ex‐ plained by the depletion effect done by the excitation beam. Indeed, slow down the scan speed allow to excited photoinitiator to be deactivate. Smartly, the authors have taken benefit of this unexpected dependence towards scan speed to propose system insensitive to abrupt change of trajectory (Figure 6). In this case, photopolymerizable system com‐ bines both conventional and RAPID photoinitiators. Finally, according to conclusions of Wegener and coworkers (Fischer et al. 2012), because the depletion time-constant is be‐ tween 15 and 350 ms in case of RAPID lithography, writing speed is comprised between 30 μm.s-1 to 150 μm.s-1. While this speed corresponds to typical speeds used in academic field, this slow speed might be an obstacle for its use in industry (see section 4.2).

**Figure 6.** A, B Large and close view of fabrication of sinusoidal structures with a conventional photoinitiator, respec‐ tively. C, D Large and close view of fabrication of sinusoidal structures with a RAPID photoinitiator, respectively. E, F Large and close view of fabrication of sinusoidal structures with a mixture of conventional and RAPID photoinitiators, respectively. Reproduced from reference (Stocker et al. 2010).

### *4.1.2.3. STED lithography*

*4.1.2.1. Two-color photoinitiation/inhibition lithography*

46 Updates in Advanced Lithography

up to now, no such experiment has been reported in the literature.

*4.1.2.2. RAPID lithography*

the intermediate specie.

Among the three STED-like optical lithography methods, the so called two-color photoinitia‐ tion / inhibition lithography (2PII) is the only one based on a *photochemical* deactivation of photoinitiator (Scott et al. 2009, McLeod et al. 2010). In this case, by using continuous laser, the excitation is performed by a single photon absorption process at 473 nm and deactivation occurs also by a linear process at a distinct wavelength (364 nm). Upon deactivation beam, two weakly reactive radicals are produced which can interact with the initiating radical to stop the polymerization. By using a donut mode for shaping the deactivation beam, reduction of the lateral extent of single voxel until 65 nm is reached. More recently, with a similar set-up, another group has claimed having designed a more efficiency photopolymerizable system which has been illustrated by the fabrication of 40 nm dots (Cao et al. 2011). While the idea to use inexpensive continuous laser sources is attractive, this process has been demonstrated only for fabrication of 2 dimensional structures. Although manipulation of the photoinhibiting wavelength into a "bottle beam" profile would induce confinement along the third axis, when focusing deeper into the photoresist volume, both excitation and deactivation might be attenuated by the linear absorption of the material. Besides, consumption of photoinitiator and/or photoinhibitor along the pathway of the beam could lead to a time and space depend‐ ence of concentration of photoactivable species during the 3D fabrication. 2PII based on twophoton absorption process for both excitation and deactivation could avoid this problem, but

The first attempt to translate the spectacular optical resolution from STED microscopy to lithography has to be attributed to Fourkas's group (Li et al. 2009). Contrary to 2PII, deacti‐ vation is based on a *photophysical* process. On preliminary experiments, Fourkas and coworkers have used the conventional STED configuration. In this case, the excitation beam (800 nm) was overlaid by a second beam red-shifted with respect to the excitation beam wavelength. Then, in order to enhance the efficiency of stimulated emission, the depletion pulse was stretched to duration of 50 ps. to guarantee enough intensity of the depletion beam. Deactivation i.e. inhibition of the polymerization was observed for a wide range of wavelength (760 to 840 nm). However, whereas tuning the pulse delay from 0 to 13 ns between the two beams should affect the efficiency of the inhibition, no significant effect had been observed. Consequently the deactivation was not assigned to stimulated emission like in STED, but the authors have ascribed this effect to the depletion of an intermediate state with longer lifetime. Therefore they have decided to name this method RAPID for Resolution Augmentation through Photo-Induced Deactivation. Further experiments are under investigation to determine the nature of

Thanks to the longer lifetime of the intermediate, a second configuration has been suc‐ cessfully used: depletion effect has been performed with continuous laser which cancel the need to control the delay between excitation (800 nm) and depletion (800 nm) beams. By using the later configuration and a bottle beam profile for the depletion beam, voxels of 40 nm height have been achieved. In the frame of this study, the depletion effect is so For efficient STED, molecules have to present large oscillator strength between the ground state S0 and the first excited sate S1 to favor later depletion. Because of their use in fluorescence microscopy, such type of molecules has also to exhibit strong fluorescence quantum yield. But this relatively long lifetime excited state (usually few ns) allows the depletion to take place.

In contrary, common photoinitiator exhibit low oscillator strength and are designed to present efficient intersystem crossing (ISC) yielding to reactive species which can give rise to radical and further to polymerization. Moreover, excited state lifetime of photoinitiator is usually found to be around 100 ps which would result in the use of high power pulses shorter than 100 ps to induce depletion. Unfortunately, with this large pulse energy, depletion could be competed by multiphoton absorption leading to undesired polymerization. Compare to previous STED-like lithography technique, STED lithography requires the use of two distinctwavelength short pulse lasers for both excitation and deactivation. In addition, pulse delay between the two beams has to be controlled carefully. While this configuration seems more constraining than 2PII and RAPID, higher scan speed (around 5 m.s-1) is expected (Fischer et al. 2012).

STED lithography experiment has been attempted with isopropylthioxantone (ITX) as a photoinitiator. But further experiments such as pump-probe experiment have shown that the STED mechanism was not the main depletion pathway (Wolf et al. 2011) as claimed in previous work (Fischer et al. 2010). Based on the pump-probe experiment in ethanol (Wolf et al. 2011), a better suitable candidate appears to be the dye (7-diethylamino-3-thenoylcoumarin) (DETC) since stimulated emission was clearly demonstrated.

However, because the S1 lifetime of a molecule usually depends on the solvent, a detailed and adapted pump probe study of DETC in the monomer has been realized (Fischer et al; 2012). While it has been shown that stimulated emission was not the only possible pathway, it was the first clear evidence of the possibility to perform true STED lithography. Fast and slow components of depletion were observed to exhibit opposite wavelength dependencies which indicate the existence of two distinct depletion mechanisms (Figure 7A). The fast component was ascribed to stimulated emission depletion, since its spectral dependence fits nicely the spectrum of the stimulated-emission (SE) cross-section. The slow component was not assigned in the frame of this study and further studies have to be accomplished to unravel this point. It has to be noted that at longer wavelength the relative strength of the fast component is weak regarding those of slow component.

the gain in lateral resolution is less remarkable. This is explained by the use of a bottleneck beam shaping for the depletion beam. While combination of bottleneck and donut phase masks could be used to shape the beam and so to improve simultaneously lateral and axial resolution, it may be interested for specific applications to use only bottleneck beam since it can induce a

**Figure 7.** A. Spectral sensitivity of the different processes for 10 mW depletion power. Due to pronounced single pho‐ ton absorption, depletion is not possible in grey area. B. schematic illustration of the different pathway involved in the

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49

Owing to its current and unique fabrication ability and its potential ability regarding to highthroughput (5 m.s-1 scan have been predicted for pure STED lithography), an exponential

To conclude about these STED-like section, it has to be mentioned that because of the relative novelty of STED-like optical lithographies (since 2009) and the fact that until now only 5 groups in the world have shown their skills to design such type of experiment, new insights are expected to appear rapidly in the near future. Interdisciplinary research has to be lead in order to propose a STED lithographic set-up with the dedicated optimized materials for few tens of nanometers in three dimensions. This will give birth of the first 3D arbitrary nanofabrication

An alternative method to improve the resolution is to add a quencher in the photopolymer‐ izable system. In presence of quencher, the photoinduced radical can be quenched which consequently prevents polymerization. By this way, it has been shown that the radical diffusion can be controlled resulting in the confinement of the polymerization region (Tanaka et al. 2005). However, in this reported work, the concentration of quenching molecules has to be much larger than those of radical produced in order to result in an effective deactivation. Therefore, Lu and coworkers designed a novel photoinitiator with a radical quenching moiety (Lu et al. 2011). In this case, an intramolecular radical deactivation can occur leading to a more efficiently control of radical diffusion than in the case of an intermolecular one. As a result, finer features can be formed. However, by these methods no sub-diffraction gaps between two lines have been demonstrated, and only small effects on the feature size have been observed.

increase of works on this becoming hot topic is expected in the near future.

depletion of DETC. Reproduced from references (Fischer et al. 2012 and Harke et al. 2012).

more spherical voxel (ratio of 2.1 in this example).

technique.

*4.1.3. Diffusion-assisted TPS*

Interestingly, STED lithography pump-probe experiment with the same photopolymerizable system, but with a depletion wavelength set at 642 nm has been performed by Harke and coworkers (Harke et al. 2012). In this experiment, pulse delay experiment has been realized, but no evidence of STED has been observed. Nevertheless, the unique component of depletion effect presents a timescale in the same range as typical triplet lifetime. The authors assume that the depleted excited state is not the singlet state S1 as in STED, but the triplet state T1. Besides, one possible and well-known pathway to deplete the triplet state proposed by the authors is the reverse intersystem crossing (ReISC).

Until now, only 3 pump-probe experiments have been performed by two distinct groups. Even if these studies lead to different observations and conclusions, it can certainly be explained by the use of different experimental conditions (depletion wavelength, pulse delay, excitation wavelength). Thus results illustrate the need to improve the knowledge in STED-like lithog‐ raphy process to define requirements list for efficient STED lithography photoinitiators.

The recent progresses of STED-like lithography have allowed new very promissing applica‐ tions in photonics. 3D polarization-independent carpet cloak for visible light have been fabricated for the first time which demonstrate the unique ability of TPS as 3D fabrication method (Fischer et al. 2011). In this case, 3D photonic crystal exhibits distance between two lines of 375 nm and 175 nm in axial and lateral directions, respectively. This has to be compared with the 510 and 210 nm values found in the frame of conventional TPS for axial and lateral directions, respectively. Whereas noticeable improvement has been shown for axial resolution,

**Figure 7.** A. Spectral sensitivity of the different processes for 10 mW depletion power. Due to pronounced single pho‐ ton absorption, depletion is not possible in grey area. B. schematic illustration of the different pathway involved in the depletion of DETC. Reproduced from references (Fischer et al. 2012 and Harke et al. 2012).

the gain in lateral resolution is less remarkable. This is explained by the use of a bottleneck beam shaping for the depletion beam. While combination of bottleneck and donut phase masks could be used to shape the beam and so to improve simultaneously lateral and axial resolution, it may be interested for specific applications to use only bottleneck beam since it can induce a more spherical voxel (ratio of 2.1 in this example).

Owing to its current and unique fabrication ability and its potential ability regarding to highthroughput (5 m.s-1 scan have been predicted for pure STED lithography), an exponential increase of works on this becoming hot topic is expected in the near future.

To conclude about these STED-like section, it has to be mentioned that because of the relative novelty of STED-like optical lithographies (since 2009) and the fact that until now only 5 groups in the world have shown their skills to design such type of experiment, new insights are expected to appear rapidly in the near future. Interdisciplinary research has to be lead in order to propose a STED lithographic set-up with the dedicated optimized materials for few tens of nanometers in three dimensions. This will give birth of the first 3D arbitrary nanofabrication technique.

## *4.1.3. Diffusion-assisted TPS*

wavelength short pulse lasers for both excitation and deactivation. In addition, pulse delay between the two beams has to be controlled carefully. While this configuration seems more constraining than 2PII and RAPID, higher scan speed (around 5 m.s-1) is expected (Fischer et

STED lithography experiment has been attempted with isopropylthioxantone (ITX) as a photoinitiator. But further experiments such as pump-probe experiment have shown that the STED mechanism was not the main depletion pathway (Wolf et al. 2011) as claimed in previous work (Fischer et al. 2010). Based on the pump-probe experiment in ethanol (Wolf et al. 2011), a better suitable candidate appears to be the dye (7-diethylamino-3-thenoylcoumarin) (DETC)

However, because the S1 lifetime of a molecule usually depends on the solvent, a detailed and adapted pump probe study of DETC in the monomer has been realized (Fischer et al; 2012). While it has been shown that stimulated emission was not the only possible pathway, it was the first clear evidence of the possibility to perform true STED lithography. Fast and slow components of depletion were observed to exhibit opposite wavelength dependencies which indicate the existence of two distinct depletion mechanisms (Figure 7A). The fast component was ascribed to stimulated emission depletion, since its spectral dependence fits nicely the spectrum of the stimulated-emission (SE) cross-section. The slow component was not assigned in the frame of this study and further studies have to be accomplished to unravel this point. It has to be noted that at longer wavelength the relative strength of the fast component is weak

Interestingly, STED lithography pump-probe experiment with the same photopolymerizable system, but with a depletion wavelength set at 642 nm has been performed by Harke and coworkers (Harke et al. 2012). In this experiment, pulse delay experiment has been realized, but no evidence of STED has been observed. Nevertheless, the unique component of depletion effect presents a timescale in the same range as typical triplet lifetime. The authors assume that the depleted excited state is not the singlet state S1 as in STED, but the triplet state T1. Besides, one possible and well-known pathway to deplete the triplet state proposed by the authors is

Until now, only 3 pump-probe experiments have been performed by two distinct groups. Even if these studies lead to different observations and conclusions, it can certainly be explained by the use of different experimental conditions (depletion wavelength, pulse delay, excitation wavelength). Thus results illustrate the need to improve the knowledge in STED-like lithog‐ raphy process to define requirements list for efficient STED lithography photoinitiators.

The recent progresses of STED-like lithography have allowed new very promissing applica‐ tions in photonics. 3D polarization-independent carpet cloak for visible light have been fabricated for the first time which demonstrate the unique ability of TPS as 3D fabrication method (Fischer et al. 2011). In this case, 3D photonic crystal exhibits distance between two lines of 375 nm and 175 nm in axial and lateral directions, respectively. This has to be compared with the 510 and 210 nm values found in the frame of conventional TPS for axial and lateral directions, respectively. Whereas noticeable improvement has been shown for axial resolution,

since stimulated emission was clearly demonstrated.

regarding those of slow component.

the reverse intersystem crossing (ReISC).

al. 2012).

48 Updates in Advanced Lithography

An alternative method to improve the resolution is to add a quencher in the photopolymer‐ izable system. In presence of quencher, the photoinduced radical can be quenched which consequently prevents polymerization. By this way, it has been shown that the radical diffusion can be controlled resulting in the confinement of the polymerization region (Tanaka et al. 2005). However, in this reported work, the concentration of quenching molecules has to be much larger than those of radical produced in order to result in an effective deactivation. Therefore, Lu and coworkers designed a novel photoinitiator with a radical quenching moiety (Lu et al. 2011). In this case, an intramolecular radical deactivation can occur leading to a more efficiently control of radical diffusion than in the case of an intermolecular one. As a result, finer features can be formed. However, by these methods no sub-diffraction gaps between two lines have been demonstrated, and only small effects on the feature size have been observed.

More recently, Sakellari and coworkers proposed another route to control the extent of the polymerization region (Sakellari et al. 2012). From their point of view, since a nondiffusing quencher results only in an increase of the polymerization threshold, they proposed to add a mobile quencher. Contrary to other works (Tanaka et al. 2005, Lu et al. 2011) where the quencher plays its inhibitor role by interacting with the photoinitiator or the generated radical, the quencher used in this work is an amine-based monomer. It interacts with other monomer or become part of the polymer backbone without compromising the mechanical stability of the structure. Last but not least, the amine functions allow a future metallization or further chemical functionalization. By this method, fabrication of woodpile structures with 400 nm intralayer period has been achieved for the first time with a *single beam* (Figure 8). Without the amine-based monomer, the authors have already shown in previous works that the minimum intralayer period achievable for an equivalent crystal photonic was around 900 nm (Sun et al. 2010). Moreover, this 400 nm intralayer period obtain by diffusion method has to be compare with the best result obtained by STED-like lithography, i.e. 375 nm intralayer period for the same type of photonic crystal (Fischer et al. 2011). Interestingly, while a single beam is used in the frame of this diffusion assisted high resolution TPS (as named by their authors), comparable resolution are obtained in both cases. While this method is easier to implement in laboratory compare to STED-like lithography, it has to be mentioned that in order to get such impressive resolution, the scan speed is intrinsically low to allow diffusion of the quencher into the scanned area. Typical scan speed in this study is around 20 μm.s-1. In the last section, solutions to overcome this speed limitation will be addressed.

should play a positive effect on the resolution since it allow to the suspended line to be maintained during the development step. In the other hand, an additional resolution enhance‐ ment has been achieved by using shorter laser pulse: the use of 8 fs instead of 50 fs allows improving feature size of the line from 150 nm to 90 nm for photoresin without cross linker.

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51

In addition to the above optical and chemical tricks, further improvement of resolution can be achieved by other minor technical development like high hybrid optics diffractive (Burmeiter

To conclude, recent technical developments of TPS open the doors to strong improvement of the resolution. Even if the diffraction limit has been beaten both in lateral and axial direction thanks to different methods such as the STED-like lithography or the diffusion-assisted high resolution TPS, effort research has to be focus on new photopolymerizable system to benefit completely of the intrinsic resolution achievable by the different techniques. For STED-like lithography, optimization of the photopolymerizable system should lead to feature size around 10 nm. Finally, when comparing the different technique, a particular attention has to be taken into account concerning the maximum scan speed for future use in industry. This will

**4.2. Recent advances in throughput: From prototyping towards production of semi-series**

Despite the possibilities to fabricate 3D objects with sub-100 nm features in a single step, TPS use is as far as we know limited to scientific community. Indeed, owing to the point-by-point writing process, TPS appears as an extremely slow technique for mass production in industry. Typical writing speed range in academic research goes from few μm.s-1 to few mm.s-1 which has to be compared with the few m.s-1 used in industry for different laser process (ablation, laser control, rapid prototyping by inspired-stereolithography methods,…). Until 2003, as for resolution, the research effort for increasing the throughput of TPS was mainly focused to the synthesis of high-efficiency photoinitiators. Nevertheless, in the past decade several research

As an attempt to solve this problem, Sun et al. demonstrated the impact of the laser beam trajectory over the manufacturing time by significantly increasing the fabrication efficiency of 90% when using CSM (contour scanning method, also called vector mode) mode rather than RSM (raster scanning method) mode. As shown in Figure 9, in the raster mode, all voxels which constitute the volume whose contains the microstructure are scanned. In case of CSM mode, only the voxels defining the surface of the microstructure are scanned. As a result, it took 3 hours or 13 minutes to manufacture the micro-bull using RSM and CSM mode respectively (Sun, 2003). Further information on the role played by trajectories can be found laser and

For applications where the objects have to be completely filled such as microlens, an additional UV exposure step is done. However cracks in the structure might occur to leading to a dramatic

groups have proposed various strategies to break down this technological bolt.

et al. 2012).

be the object of the next section.

*4.2.1. Influence of the scanning mode*

photonics review (Park et al., 2009).

decrease of desired performance.

**Figure 8.** A. Microstructures realized by intramolecular quenching method. Scale bars are 5 µm. B. SEM images of pho‐ tonic crystal fabricated by diffusion assisted high resolution TPS and diffraction pattern generated by the photonic de‐ vice. Reproduced from references (λexc: 780 nm; average power : 7.70 mW; writing speed: 66 µm.s-1, Lu et al 2011) and (λexc: 800 nm; average power : 20 mW; writing speed: 20 µm.s-1 Sakellari et al. 2012), respectively.

#### *4.1.4. Other methods*

Recently, 40 nm feature size has been obtained by combining chemical and optical approaches (Emons et al. 2012). The measurement of feature size has been performed by suspending bridge method. From a chemical point of view, the authors have demonstrated that the addition of a crosslinker (pentacrylate derivative) allow a resolution enhancement of a factor 2 (150 nm feature size versus 82.5 nm with a 50 fs pulse laser). As expected, the addition of crosslinker should play a positive effect on the resolution since it allow to the suspended line to be maintained during the development step. In the other hand, an additional resolution enhance‐ ment has been achieved by using shorter laser pulse: the use of 8 fs instead of 50 fs allows improving feature size of the line from 150 nm to 90 nm for photoresin without cross linker.

In addition to the above optical and chemical tricks, further improvement of resolution can be achieved by other minor technical development like high hybrid optics diffractive (Burmeiter et al. 2012).

To conclude, recent technical developments of TPS open the doors to strong improvement of the resolution. Even if the diffraction limit has been beaten both in lateral and axial direction thanks to different methods such as the STED-like lithography or the diffusion-assisted high resolution TPS, effort research has to be focus on new photopolymerizable system to benefit completely of the intrinsic resolution achievable by the different techniques. For STED-like lithography, optimization of the photopolymerizable system should lead to feature size around 10 nm. Finally, when comparing the different technique, a particular attention has to be taken into account concerning the maximum scan speed for future use in industry. This will be the object of the next section.

## **4.2. Recent advances in throughput: From prototyping towards production of semi-series**

Despite the possibilities to fabricate 3D objects with sub-100 nm features in a single step, TPS use is as far as we know limited to scientific community. Indeed, owing to the point-by-point writing process, TPS appears as an extremely slow technique for mass production in industry. Typical writing speed range in academic research goes from few μm.s-1 to few mm.s-1 which has to be compared with the few m.s-1 used in industry for different laser process (ablation, laser control, rapid prototyping by inspired-stereolithography methods,…). Until 2003, as for resolution, the research effort for increasing the throughput of TPS was mainly focused to the synthesis of high-efficiency photoinitiators. Nevertheless, in the past decade several research groups have proposed various strategies to break down this technological bolt.

## *4.2.1. Influence of the scanning mode*

More recently, Sakellari and coworkers proposed another route to control the extent of the polymerization region (Sakellari et al. 2012). From their point of view, since a nondiffusing quencher results only in an increase of the polymerization threshold, they proposed to add a mobile quencher. Contrary to other works (Tanaka et al. 2005, Lu et al. 2011) where the quencher plays its inhibitor role by interacting with the photoinitiator or the generated radical, the quencher used in this work is an amine-based monomer. It interacts with other monomer or become part of the polymer backbone without compromising the mechanical stability of the structure. Last but not least, the amine functions allow a future metallization or further chemical functionalization. By this method, fabrication of woodpile structures with 400 nm intralayer period has been achieved for the first time with a *single beam* (Figure 8). Without the amine-based monomer, the authors have already shown in previous works that the minimum intralayer period achievable for an equivalent crystal photonic was around 900 nm (Sun et al. 2010). Moreover, this 400 nm intralayer period obtain by diffusion method has to be compare with the best result obtained by STED-like lithography, i.e. 375 nm intralayer period for the same type of photonic crystal (Fischer et al. 2011). Interestingly, while a single beam is used in the frame of this diffusion assisted high resolution TPS (as named by their authors), comparable resolution are obtained in both cases. While this method is easier to implement in laboratory compare to STED-like lithography, it has to be mentioned that in order to get such impressive resolution, the scan speed is intrinsically low to allow diffusion of the quencher into the scanned area. Typical scan speed in this study is around 20 μm.s-1. In the last section,

**Figure 8.** A. Microstructures realized by intramolecular quenching method. Scale bars are 5 µm. B. SEM images of pho‐ tonic crystal fabricated by diffusion assisted high resolution TPS and diffraction pattern generated by the photonic de‐ vice. Reproduced from references (λexc: 780 nm; average power : 7.70 mW; writing speed: 66 µm.s-1, Lu et al 2011) and

Recently, 40 nm feature size has been obtained by combining chemical and optical approaches (Emons et al. 2012). The measurement of feature size has been performed by suspending bridge method. From a chemical point of view, the authors have demonstrated that the addition of a crosslinker (pentacrylate derivative) allow a resolution enhancement of a factor 2 (150 nm feature size versus 82.5 nm with a 50 fs pulse laser). As expected, the addition of crosslinker

(λexc: 800 nm; average power : 20 mW; writing speed: 20 µm.s-1 Sakellari et al. 2012), respectively.

*4.1.4. Other methods*

50 Updates in Advanced Lithography

solutions to overcome this speed limitation will be addressed.

As an attempt to solve this problem, Sun et al. demonstrated the impact of the laser beam trajectory over the manufacturing time by significantly increasing the fabrication efficiency of 90% when using CSM (contour scanning method, also called vector mode) mode rather than RSM (raster scanning method) mode. As shown in Figure 9, in the raster mode, all voxels which constitute the volume whose contains the microstructure are scanned. In case of CSM mode, only the voxels defining the surface of the microstructure are scanned. As a result, it took 3 hours or 13 minutes to manufacture the micro-bull using RSM and CSM mode respectively (Sun, 2003). Further information on the role played by trajectories can be found laser and photonics review (Park et al., 2009).

For applications where the objects have to be completely filled such as microlens, an additional UV exposure step is done. However cracks in the structure might occur to leading to a dramatic decrease of desired performance.

*4.2.3. Multi-focal TPS*

Another solution for boosting fabrication speed while avoiding geometric limitations associ‐ ated with molding is the use of multi-focal strategy. This technical innovation has been first demonstrated by Kawata and coworkers in 2005 (Kato, 2005). By combining TPS with a microlens array, more than one hundred identical and individual 3D objects have been written simultaneously resulting in a two-order increase in the fabrication yield compared to singlebeam TPS (Figure 11). In 2006, Kawata et al. succeed to write in parallel more than 700 hundred identical structures (Formanek, 2006) illustrating the high potential for large scale production.

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**Figure 11.** a) and b) examples of two- and three-dimensional fabrication by mean of microlens array.

Recently, Ritschdorff et al. proposed a more general approach of multi-focal TPS to allow parallel but independently writing of different objects (Ritschdorff et al. 2012). Indeed, until this work, advanced TPS based on multi-focal strategy was dedicated to the creation of identical replicas or to the fabrication of a single structure with many identical sub-units. In the frame of this recent work, a proof-of-concept has been illustrated with the construction of biocompatible networks by using two independent sub-beams. In order to control each beam separately, the main beam is directed through a dynamic mask (typically a spatial light modulator). To extend this appealing strategy to numerous parallel and independently subbeam, high-power lasers are required. Additionally to the increase of the fabrication's speed, this work opens the doors to numerous and more flexible applications. For example, one could imagine making unlimited modifications into microfluidic systems, but more interestingly, generation of pattern with different exposure time will result to display gradients in chemical functionality, mechanical functionality or porosity which play a key role in tissue engineering.

However the promising potential of multi-focal TPS for speed up fabrication is quite obvious, two points have to be taken in consideration to use it as a tool in laboratory or industries. The first point concerns the use of an expensive amplified femtosecond laser in order to provide enough energy after each lens or dynamic mask. In addition, the laser beam intensity distri‐ bution has also to be perfectly controlled to deliver the same amount of energy for each lens and to fabricate uniform structures. It has to be mentioned that though this point may be a brake for scientific community, this is clearly not the case for its use in industry. A more serious issue with parallel fabrication is the precise control of alignment of the hundred forming laser beams with respect to the plan of the substrate. A tilt of less than 1° of the substrate will result

**Figure 9.** Two fabrication strategies based on scanning mode: a) Raster mode, b) Vector mode (or contour mode), c) and d) SEM images of a microbull structures using RSM and CSM respectively. Reproduced from reference Sun 2003.

#### *4.2.2. Replication*

To address the low-throughput of the method, LaFratta et al. have proposed to use TPS in tandem with soft lithography technique known as microtransfer molding (Xia et al. 1998). By this way, high-fidelity molds of structures with extremely high aspect ratios and large overhangs have been realized (LaFratta et al. 2004, Figure 10). Besides, in the frame of this work, more than ten replicas have been made from a single master without any significant deterioration of the resulting structures. Even if this technique have been applied to more complex structures such as arches or coils (LaFratta et al. 2006), a range of geometries or objects such as closed traps or micropumps are still impossible or currently too challenging to replicate using microtransfer molding. Finally, from our knowledge, no improvement or additional example of this technique has been recently reported in the literature, underlying the difficul‐ ties to separate the molder to the master without damage. Recent advances in soft lithography might facilitate the delivering step.

**Figure 10.** a) and b) SEM images of master and replica array of towers respectively, c) and d) SEM images of master and replica of coil respectively. Scale bars are 10 µm. Reproduced from reference LaFratta 2004 and 2006.

## *4.2.3. Multi-focal TPS*

*4.2.2. Replication*

might facilitate the delivering step.

(a) (b)

52 Updates in Advanced Lithography

)

(a)

(b)

)

To address the low-throughput of the method, LaFratta et al. have proposed to use TPS in tandem with soft lithography technique known as microtransfer molding (Xia et al. 1998). By this way, high-fidelity molds of structures with extremely high aspect ratios and large overhangs have been realized (LaFratta et al. 2004, Figure 10). Besides, in the frame of this work, more than ten replicas have been made from a single master without any significant deterioration of the resulting structures. Even if this technique have been applied to more complex structures such as arches or coils (LaFratta et al. 2006), a range of geometries or objects such as closed traps or micropumps are still impossible or currently too challenging to replicate using microtransfer molding. Finally, from our knowledge, no improvement or additional example of this technique has been recently reported in the literature, underlying the difficul‐ ties to separate the molder to the master without damage. Recent advances in soft lithography

**Figure 9.** Two fabrication strategies based on scanning mode: a) Raster mode, b) Vector mode (or contour mode), c) and d) SEM images of a microbull structures using RSM and CSM respectively. Reproduced from reference Sun 2003.

(c)

(c)

(d)

)

(d)

)

**Figure 10.** a) and b) SEM images of master and replica array of towers respectively, c) and d) SEM images of master

and replica of coil respectively. Scale bars are 10 µm. Reproduced from reference LaFratta 2004 and 2006.

Another solution for boosting fabrication speed while avoiding geometric limitations associ‐ ated with molding is the use of multi-focal strategy. This technical innovation has been first demonstrated by Kawata and coworkers in 2005 (Kato, 2005). By combining TPS with a microlens array, more than one hundred identical and individual 3D objects have been written simultaneously resulting in a two-order increase in the fabrication yield compared to singlebeam TPS (Figure 11). In 2006, Kawata et al. succeed to write in parallel more than 700 hundred identical structures (Formanek, 2006) illustrating the high potential for large scale production.

**Figure 11.** a) and b) examples of two- and three-dimensional fabrication by mean of microlens array.

Recently, Ritschdorff et al. proposed a more general approach of multi-focal TPS to allow parallel but independently writing of different objects (Ritschdorff et al. 2012). Indeed, until this work, advanced TPS based on multi-focal strategy was dedicated to the creation of identical replicas or to the fabrication of a single structure with many identical sub-units. In the frame of this recent work, a proof-of-concept has been illustrated with the construction of biocompatible networks by using two independent sub-beams. In order to control each beam separately, the main beam is directed through a dynamic mask (typically a spatial light modulator). To extend this appealing strategy to numerous parallel and independently subbeam, high-power lasers are required. Additionally to the increase of the fabrication's speed, this work opens the doors to numerous and more flexible applications. For example, one could imagine making unlimited modifications into microfluidic systems, but more interestingly, generation of pattern with different exposure time will result to display gradients in chemical functionality, mechanical functionality or porosity which play a key role in tissue engineering.

However the promising potential of multi-focal TPS for speed up fabrication is quite obvious, two points have to be taken in consideration to use it as a tool in laboratory or industries. The first point concerns the use of an expensive amplified femtosecond laser in order to provide enough energy after each lens or dynamic mask. In addition, the laser beam intensity distri‐ bution has also to be perfectly controlled to deliver the same amount of energy for each lens and to fabricate uniform structures. It has to be mentioned that though this point may be a brake for scientific community, this is clearly not the case for its use in industry. A more serious issue with parallel fabrication is the precise control of alignment of the hundred forming laser beams with respect to the plan of the substrate. A tilt of less than 1° of the substrate will result in the fabrication of inhomogeneous structures which is unacceptable from a metrology point of view in industries.

In particularly, a better understanding will give a list of criteria for novel photoinitiators

Concerning the throughput of the technique, speed of 5 m.s-1 has been recently announced by an European consortium (march 2012). For comparison, this speed is quite close to those used in conventional process in microelectronics industry such as control or ablation process (10 to 50 m.s-1). Such promising advances should allow overcome limit for mass production and

To conclude, this technology opens up new perspectives in a wide range of applications such as rapid prototyping of micro- and nanofluidics, small-scale production of microoptics components, or 3D frameworks for cell biology. Finally, owing to its currently fast expansion and to the versatile science involved in all the chain, TPS appears as a fantastic and so appealing

Agence Nationale pour la Recherche (ANR - Projects 2-PAGmicrofab ANR-BLAN-0815-03, NANOQUENCHING and NIR-OPTICS), CNRS and Région Alsace are gratefully acknowl‐

, Fabrice Stehlin1

, Patrice Baldeck2

2 Laboratoire de Spectrométrie Physique, Université Joseph-Fourier, Saint Martin d'Hères,

[1] Amato, L, Gu, Y, Bellini, N, Eaton, S. M, Cerullo, G, & Osellame, R. (2012). Integrated three-dimensional filter separates nanoscale from microscale elements in a microflui‐

[2] Becker, E. W, Ehrfeld, W, & Muenchmeyer, D. (1984). Accuracy of X-ray lithography using synchrotron radiation for the fabrication of technical separation nozzle ele‐

, Jean-Pierre Malval1

and Olivier Soppera1

,

Recent Advances in Two-Photon Stereolithography

http://dx.doi.org/10.5772/56165

55

consequently should reinforce the highly potential of TPS for industry.

devoted to this promising technique.

field of research for the next decades.

**Acknowledgements**

edged for financial supports.

Arnaud Spangenberg1\*, Nelly Hobeika1

, Prem Prabhakaran2

dic chip. *Lab on a Chip*, , 12, 1135-1142.

\*Address all correspondence to: arnaud.spangenberg@uha.fr

1 Institut de Science des Matériaux de Mulhouse, Mulhouse, France

**Author details**

Fernand Wieder1

France

**References**

## *4.2.4. Currently speed of TPS process*

In the literature, usual process speeds of several 100 μm.s-1 are reported with sub-100 nm resolution. More rarely, mm.s-1 can be reached while keeping a submicrometric resolution. Until recently, the fastest demonstration of microstructures with micrometric resolution has been realized by Fourkas's group by using a very sensitive photoinitiator (Kumi et al. 2010). In the frame of this work, speed of 1 cm.s-1 was reported.

Since march 2012, a 300-micrometer long model of a Formula 1 race car has been fabricated by TPS in only four minutes while keeping micrometric resolution. Thus spectacular result means that a process speed of 5 m.s-1 is involved, which is the same order of other laser process used in industry. A video of the construction can be found on the website of the Vienna University (Vienna, 2012). Unfortunately, certainly due to economic interests, little information can be freely accessed. According to their website, the increase in speed results from efforts from a chemical and mechanical point of view.

## **5. Conclusion and perspectives**

After the pioneers works on TPS (Maruo, 1997), the research efforts were mainly focused on the synthesis of high efficient photoinitiators and materials in order to respectively speed up the writing process and to improve the mechanical, optical or chemical properties of the resulting 3D objects. Thanks to both the versatility of photopolymerizable systems and to the possibility to incorporate additional materials into the structures, TPS has attracted consider‐ able attention over the past decade leading to enough mature technology. Indeed, despite the novelty of TPS, this is now daily used for broad range of applications such as creation of 3D components for microfluidic systems, tissue scaffolding, optical components, and so on. Besides, only 10 years after the first instance of 3D microstructures created by TPS, several companies have developed commercial 3D microfabrication set-ups which have supported the widespread of the technique in various research fields.

Recently, the rapid technical development of TPS provided much better structural resolution and high-throughput. The combination of all these improvements in a single commercial setup will certainly boost the use of TPS in industry. From resolution point of view, the Abbe diffraction limit in optical lithography has been overcome by using and/or adapting a concept called STED originally from optical microscopy. The latter is already commercialized since 2005 by several well-known optical microscopy companies and is well expanded in life sciences. Interestingly, even if both in optical microscopy and lithography the diffraction limit has been beaten thanks to the STED principle, record for lateral spatial resolution in optical lithography (175 nm) is still far away from thus in optical microscopy (5.6 nm). In order to obtain comparable resolution, further investigations are required to enhance the comprehen‐ sion of the photophysical and photochemical mechanism underlying the STED lithography. In particularly, a better understanding will give a list of criteria for novel photoinitiators devoted to this promising technique.

Concerning the throughput of the technique, speed of 5 m.s-1 has been recently announced by an European consortium (march 2012). For comparison, this speed is quite close to those used in conventional process in microelectronics industry such as control or ablation process (10 to 50 m.s-1). Such promising advances should allow overcome limit for mass production and consequently should reinforce the highly potential of TPS for industry.

To conclude, this technology opens up new perspectives in a wide range of applications such as rapid prototyping of micro- and nanofluidics, small-scale production of microoptics components, or 3D frameworks for cell biology. Finally, owing to its currently fast expansion and to the versatile science involved in all the chain, TPS appears as a fantastic and so appealing field of research for the next decades.

## **Acknowledgements**

in the fabrication of inhomogeneous structures which is unacceptable from a metrology point

In the literature, usual process speeds of several 100 μm.s-1 are reported with sub-100 nm resolution. More rarely, mm.s-1 can be reached while keeping a submicrometric resolution. Until recently, the fastest demonstration of microstructures with micrometric resolution has been realized by Fourkas's group by using a very sensitive photoinitiator (Kumi et al. 2010).

Since march 2012, a 300-micrometer long model of a Formula 1 race car has been fabricated by TPS in only four minutes while keeping micrometric resolution. Thus spectacular result means that a process speed of 5 m.s-1 is involved, which is the same order of other laser process used in industry. A video of the construction can be found on the website of the Vienna University (Vienna, 2012). Unfortunately, certainly due to economic interests, little information can be freely accessed. According to their website, the increase in speed results from efforts from a

After the pioneers works on TPS (Maruo, 1997), the research efforts were mainly focused on the synthesis of high efficient photoinitiators and materials in order to respectively speed up the writing process and to improve the mechanical, optical or chemical properties of the resulting 3D objects. Thanks to both the versatility of photopolymerizable systems and to the possibility to incorporate additional materials into the structures, TPS has attracted consider‐ able attention over the past decade leading to enough mature technology. Indeed, despite the novelty of TPS, this is now daily used for broad range of applications such as creation of 3D components for microfluidic systems, tissue scaffolding, optical components, and so on. Besides, only 10 years after the first instance of 3D microstructures created by TPS, several companies have developed commercial 3D microfabrication set-ups which have supported

Recently, the rapid technical development of TPS provided much better structural resolution and high-throughput. The combination of all these improvements in a single commercial setup will certainly boost the use of TPS in industry. From resolution point of view, the Abbe diffraction limit in optical lithography has been overcome by using and/or adapting a concept called STED originally from optical microscopy. The latter is already commercialized since 2005 by several well-known optical microscopy companies and is well expanded in life sciences. Interestingly, even if both in optical microscopy and lithography the diffraction limit has been beaten thanks to the STED principle, record for lateral spatial resolution in optical lithography (175 nm) is still far away from thus in optical microscopy (5.6 nm). In order to obtain comparable resolution, further investigations are required to enhance the comprehen‐ sion of the photophysical and photochemical mechanism underlying the STED lithography.

of view in industries.

54 Updates in Advanced Lithography

*4.2.4. Currently speed of TPS process*

chemical and mechanical point of view.

**5. Conclusion and perspectives**

In the frame of this work, speed of 1 cm.s-1 was reported.

the widespread of the technique in various research fields.

Agence Nationale pour la Recherche (ANR - Projects 2-PAGmicrofab ANR-BLAN-0815-03, NANOQUENCHING and NIR-OPTICS), CNRS and Région Alsace are gratefully acknowl‐ edged for financial supports.

## **Author details**

Arnaud Spangenberg1\*, Nelly Hobeika1 , Fabrice Stehlin1 , Jean-Pierre Malval1 , Fernand Wieder1 , Prem Prabhakaran2 , Patrice Baldeck2 and Olivier Soppera1

\*Address all correspondence to: arnaud.spangenberg@uha.fr

1 Institut de Science des Matériaux de Mulhouse, Mulhouse, France

2 Laboratoire de Spectrométrie Physique, Université Joseph-Fourier, Saint Martin d'Hères, France

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**Chapter 3**

**Femtosecond Laser Lithography in Organic and Non-**

The lithography is a well established technology for fabrication of microelectronic components, integrated optics, microfluidic devices and Micro-Electro-Mechanical-Systems (MEMS) [1,2]. Using energy sources like ultra-violet (UV) photons or X-ray, various patterns are transferred from photo lithographic masks to photoresist materials. Developed for MEMS fabrications, the photoresists are light-sensitive materials. During exposure, chemical reactions are initiated in the irradiated volume, changing the chemical properties of the material. An imprinted pattern can be obtained when the exposed or unexposed material is removed by chemical

The size of the structures created through lithographic methods depends on the radiation wavelength. The optical diffraction limit represents the limiting factor for obtaining the minimum feature size. As a result, in the UV lithography the exposure wavelength was

When smaller structures were required for the metal–oxide–semiconductor (MOS) circuits improvement, the UV lamps were replaced with excimer lasers. Deep UV lithography (DUV), based on 248 nm (KrF) and 194 nm (ArF) wavelengths [3,4], is used in semiconductor industry to produce transistors with 90 nm gate lengths. A further decrease of the radiation source to 157 nm wavelength (molecular fluorine) [5] was limited by the low transmission of fused silica

High quality crystalline calcium fluoride with low birefringence was grown for mask substrate and refractive lenses fabrication. Some technical difficulties, related to the mask protection,

> © 2013 Jipa et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

© 2013 Jipa et al.; licensee InTech. This is a paper 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.

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

reduced initially from g-line (436 nm) to h-line (405 nm) and then to i-line (365 nm).

**Organic Materials**

http://dx.doi.org/10.5772/56579

material in this spectral range.

**1. Introduction**

solvents.

Florin Jipa, Marian Zamfirescu, Alin Velea,

Additional information is available at the end of the chapter

Mihai Popescu and Razvan Dabu
