**Soft UV Nanoimprint Lithography and Its Applications**

## Hongbo Lan

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/56186

## **1. Introduction**

Large-area nanopatterning technology has demonstrated high potential which can signifi‐ cantly enhance the performance of many devices and products, such as LEDs, solar cells, hard disk drives, laser diodes, display, etc [1]. For example, nano-patterned sapphire substrates (NPSS) and photonic crystals (PhC) have been considered as the most effective approaches to improve the light output efficiency (internal quantum efficiency and external quantum efficiency) of LEDs and beam shaping [2,3]. The solar cells with sub-micro anti-reflective coating exhibited higher photocurrent and higher power conversion efficiency compared to those without nanostructures [4]. Moreover, the ability to produce large-area micro- and nanostructures on non-planar surfaces is of importance for many applications such as optics, optoelectronics, nanophotonics, imaging technology, NEMS, and microfluidics [5]. However, creating large-area nanostructures on curved or non-planar surfaces are extremely difficult using existing patterning approaches. Furthermore, a variety of existing nanopatterning technologies such as electron beam lithography (ELB), optical lithography, interference lithography (IL), etc., cannot cope with all the practical demands of industrial applications with respect to high resolution, high throughput, low cost, large area, and patterning on nonflat and curved surface. Therefore, new high volume nanomanufacturing technology strongly needs to be exploited and developed so as to meet the tremendous requires of rapid growing markets.

Nanoimprint lithography (NIL) has now been considered as a promising nanopatterning method with low cost, high throughput and high resolution, especially for producing the largearea micro/nano scale patterns and complex 3-D structures and as well as high-aspect-ratio features. Due to these outstanding advantages, it was accepted by International Technology Roadmap for Semiconductors (ITRS) in 2009 for the 16 and 11 nm nodes, scheduled for industrial manufacturing in 2013. Toshiba has validated NIL for 22 nm and beyond. NIL has also been listed as one of 10 emerging technologies that will strongly impact the world by

© 2013 Lan; 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 Lan; 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.

MIT's Technology Review. The resolution potential has been demonstrated by the replication of 2.4-nm features. It is expected to play a critical role in the commercialization of nanostructure applications [6-8].

Compared to other NIL processes (thermal NIL or hot embossing, UV-NIL with rigid mold) and nanopatterning methods, soft UV-NIL using a flexible (or soft) mold has been proven to be a very promising approach for making large-area patterns up to wafer-level in the micro‐ meter and nanometer scale, fabricating 3-D micro/nano structures and high-aspect-ratio features, especially producing large-area patterns on the non-planar surfaces even curved substrates at low-cost and with high throughput. Since the soft mold (stamp, template) is adopted, the soft UV-NIL process has some unique advantages compared with the traditional UV-NIL with rigid mold. These strengths include: (1) Cost reduction. Cheap soft molds can be easily replicated from one expensive master, significantly reducing cost of the master template fabrication. (2) Conformal contact. Conformal contact between the undulated (or curved, waviness, bow, warp) substrate and the mold can be achieved over large areas without applying high external pressure. (3) Insensitive to particle contaminants. Particle contaminants are less problematic as the soft mold can locally deform around a particle avoiding damage to the mold or substrate which lead to improve the yield of the process and to extend the application fields. (4) Avoiding anti-adhesive layer due to the low surface energy of flexible mold materials. (5) Low imprinting and demolding force. (6) Utilizing gradually sequential micro-contact and "peel-off" separation method for thin film type molds. However, soft UV-NIL process has also some inherent drawbacks. (1) Deformation and distortion of soft molds. Due to the relatively low Young's modulus, the deformation of soft molds under pressure remains a major issue which limits the resolution, uniformity and reproducibility of imprinted patterns. High aspect ratios structures and dense patterns are not stable and tend to collapse. (2) Poor dimensional stability. Due to the poor solvent resistance as well as deformation of pressure and thermal expansion, the dimensional stability of imprinted patterns is determined and degraded. (3) Short mold lifetime. Since the hardness and resistance to solvent are poor, soft molds have relatively low mold lifetime. These limitations must be solved and overcame for extensive applications [8-13].

**Figure 1.** An infrastructure of soft UV-NIL process

**2.1. Principle and process flow**

**2. Principle and process of Soft UV-NIL**

The whole process flow of forming micro/nanostructures by soft UV-NIL is composed of four steps: the fabrication of a master template, the replication of a soft mold (or working mold) by this master template, the imprinting in the UV curable resist using the replicated soft mold, and the replicated patterns transfer form UV curable resist to the substrate or functional materials by etching or lift-off process. Together, these steps affect the quality of the final replica in terms of resolution, uniformity, fidelity, patterning area, and line edge roughness.

Soft UV Nanoimprint Lithography and Its Applications

http://dx.doi.org/10.5772/56186

171

A master template is firstly fabricated by EBL, IL or other patterning technologies (e.g. block copolymers, AAO, FIB, photolithography, etc.). Then, the surface of the master is treated forming an anti-adhesive layer. The liquid mold material is spin coated or casted the master template to duplicate a patterning layer. Subsequently, a backplane or a flexible layer is bonded to the patterned layer. After cured thermally or UV curing, the soft composite mold is peeled off form the master template. The soft mold obtained is a negative copy of the master template.

Currently, full wafer imprint up to 300mm, and 12.5nm resolution patterns have been achieved by using the soft UV-NIL. Soft UV-NIL has been considered as one of the most promising solution implementing mass production of micro/nanostructures over large areas at low cost for the applications in compound semiconductor optoelectronics and nanophotonic devices, especially for LED patterning [2, 14].

As an emerging cost-effective nanopatterning technique, soft UN-NIL involves two basic aspects: fundamental investigation and application research. The fundamental investigation comprises of theoretical basis and key enabling techniques including process, mold (material, fabrication of working stamp and master template), material (resist, functional material, etc.), tool. The application research mainly covers a variety of practical applications suitable for soft UV-NIL, such as LED patterning, optical components, nanophotonics, biological applications, etc. Author proposed an infrastructure of soft UV-NIL, as shown in Figure 1.

**Figure 1.** An infrastructure of soft UV-NIL process

## **2. Principle and process of Soft UV-NIL**

#### **2.1. Principle and process flow**

MIT's Technology Review. The resolution potential has been demonstrated by the replication of 2.4-nm features. It is expected to play a critical role in the commercialization of nanostructure

Compared to other NIL processes (thermal NIL or hot embossing, UV-NIL with rigid mold) and nanopatterning methods, soft UV-NIL using a flexible (or soft) mold has been proven to be a very promising approach for making large-area patterns up to wafer-level in the micro‐ meter and nanometer scale, fabricating 3-D micro/nano structures and high-aspect-ratio features, especially producing large-area patterns on the non-planar surfaces even curved substrates at low-cost and with high throughput. Since the soft mold (stamp, template) is adopted, the soft UV-NIL process has some unique advantages compared with the traditional UV-NIL with rigid mold. These strengths include: (1) Cost reduction. Cheap soft molds can be easily replicated from one expensive master, significantly reducing cost of the master template fabrication. (2) Conformal contact. Conformal contact between the undulated (or curved, waviness, bow, warp) substrate and the mold can be achieved over large areas without applying high external pressure. (3) Insensitive to particle contaminants. Particle contaminants are less problematic as the soft mold can locally deform around a particle avoiding damage to the mold or substrate which lead to improve the yield of the process and to extend the application fields. (4) Avoiding anti-adhesive layer due to the low surface energy of flexible mold materials. (5) Low imprinting and demolding force. (6) Utilizing gradually sequential micro-contact and "peel-off" separation method for thin film type molds. However, soft UV-NIL process has also some inherent drawbacks. (1) Deformation and distortion of soft molds. Due to the relatively low Young's modulus, the deformation of soft molds under pressure remains a major issue which limits the resolution, uniformity and reproducibility of imprinted patterns. High aspect ratios structures and dense patterns are not stable and tend to collapse. (2) Poor dimensional stability. Due to the poor solvent resistance as well as deformation of pressure and thermal expansion, the dimensional stability of imprinted patterns is determined and degraded. (3) Short mold lifetime. Since the hardness and resistance to solvent are poor, soft molds have relatively low mold lifetime. These limitations must be solved and overcame

Currently, full wafer imprint up to 300mm, and 12.5nm resolution patterns have been achieved by using the soft UV-NIL. Soft UV-NIL has been considered as one of the most promising solution implementing mass production of micro/nanostructures over large areas at low cost for the applications in compound semiconductor optoelectronics and nanophotonic devices,

As an emerging cost-effective nanopatterning technique, soft UN-NIL involves two basic aspects: fundamental investigation and application research. The fundamental investigation comprises of theoretical basis and key enabling techniques including process, mold (material, fabrication of working stamp and master template), material (resist, functional material, etc.), tool. The application research mainly covers a variety of practical applications suitable for soft UV-NIL, such as LED patterning, optical components, nanophotonics, biological applications,

etc. Author proposed an infrastructure of soft UV-NIL, as shown in Figure 1.

applications [6-8].

170 Updates in Advanced Lithography

for extensive applications [8-13].

especially for LED patterning [2, 14].

The whole process flow of forming micro/nanostructures by soft UV-NIL is composed of four steps: the fabrication of a master template, the replication of a soft mold (or working mold) by this master template, the imprinting in the UV curable resist using the replicated soft mold, and the replicated patterns transfer form UV curable resist to the substrate or functional materials by etching or lift-off process. Together, these steps affect the quality of the final replica in terms of resolution, uniformity, fidelity, patterning area, and line edge roughness.

A master template is firstly fabricated by EBL, IL or other patterning technologies (e.g. block copolymers, AAO, FIB, photolithography, etc.). Then, the surface of the master is treated forming an anti-adhesive layer. The liquid mold material is spin coated or casted the master template to duplicate a patterning layer. Subsequently, a backplane or a flexible layer is bonded to the patterned layer. After cured thermally or UV curing, the soft composite mold is peeled off form the master template. The soft mold obtained is a negative copy of the master template. The fabrication process of soft UV-NIL includes four steps (as shown in Figure.2): (a) Firstly, a UV-curable resist, which is liquid at room temperature, is spin-coated or dispensed on the substrate. (b) Subsequently, the soft mold is pressed into the resist on the substrate with a low pressure, and adjusts to the waviness or curvature of the substrate until completely conformal contact is achieved. Due to high flexibility, the soft mold can well adapt its shape to the waviness of the substrate obtaining good conformal contact between the non-flat substrate and the mold. (c) After filling all cavities or trenches of the mold, UV curing that solidifies the liquid resist due to cross-linking is carried out by a UV light source. (d) Finally, the soft mold is released, leaving the UV-curable resist patterned. Low viscosity UV-curable resist, commonly comprised of a low molecular weight polymer and photoinitiator, is significantly essential for easily filling the nanocavities of the mold [8-9, 15].

a substrate conformal contact between a working mold and a substrate, SCIL process relies on a sequential imprinting process. The approaching of the flexible mold starts from one side and spreads to the whole mold subsequently by releasing the vacuum in the grooves step by step and applying a small over pressure of 20 mBar on the mold (step 1 to 3). After conformal contact over the entire substrate is carried out, the imprint resist is cured by UV exposure or in case of using imprint sol-gel based resists diffusion of the sol-gel solvent into the PDMS mold (step 4). The automatic separation of the mold from the substrate is performed by switching on the vacuum in the grooves consequently, which is opposite to the imprint process (step 5 to 7). This results in a low force peeling action which removes the stamp from the patterned resist layer and avoids damage to mold or transferred patterns. SCIL has demonstrated sub-10nm resolution over 150mm-diameter substrates. The SCIL technology can well cope with non-ideal substrates and implement full wafer imprinting in a single step. For the SCIL process, the curing of sol-gel resist relies on the diffusion of solvents into the PDMS mold. Depending on the operation and preparation conditions, the curing time varies from 5 to 15 minutes. In order to improve throughput (reducing curing time) and repeatability of the process, a UV enhanced SCIL process using UV curable resist has been developing. Fader *et al*. recently introduced UV-SCIL with purely organic UV-curing materials showing curing times of 17s [20]. The excellent performance of SCIL in respect to substrate conformity and pattern fidelity over large areas makes this imprint technology a powerful tool, especially for applications like LED/VCSEL,

Soft UV Nanoimprint Lithography and Its Applications

http://dx.doi.org/10.5772/56186

173

Author developed a full wafer soft UV-NIL with a tri-layer composite mold. The composite mold includes a thin layer of fluoropolymer-based material as the patterning layer, a thick layer of *s*-PMDS as intermediate flexible or cushion layer, and a thin glass sheet as the support layer. Figure 4 illustrated the schematic diagram of the proposed full wafer soft UV-NIL process. The imprinting process is performed by a sequential and micro-contacting solution starting from the center to two sides of the mold. The separation process employs a continuous 'peel-off' demolding mode starting from two sides to the center of the mold. Compared to the SCIL, the distinct advantages of the process include: (1) The imprinting and demolding procedure take the mold center as axis of symmetry, are carried out at the same time on two sides with higher throughput and easier eliminating trapped air bubble. (2) An enhanced demolding approach is adopted. (3) Since the imprinting procedure is performed under a low vacuum pressure environment, it can better remove the trapped air bubbles and provide

There is a big difference for the imprinting and demolding mode between soft UV-NIL using flexible mold and NIL with rigid mold. Using gas-assisted imprinting method can easily achieve uniform distribution of applied pressure over the entire substrate, gradually and exactly load imprint force. The sequent contact mechanism prevents the flexible mold from trapping air bubbles and therefore ensures that the mold follows exactly the undulating topography over whole substrate surface. A combination of the sequent micro-contact and gas-assistedmprinting can ensure to achieve uniform pressure and good conformal contact, avoid the trapped air bubble defects, reduce the deformation of soft mold. The peel-off

optical elements or patterned media [21].

completely conformal contact [22].

**Figure 2.** Process flow of soft UV-NIL [15]

The removal of the residual layer is typically performed by using a reactive ion etching (RIE) process with an oxygen plasma. The molded resist can then serve as a mask for further processing steps or be used as a functional layer itself. The most common methods for further processing are using the structured resist as an etch mask for patterning the substrate or as a mask for a lift off process for structuring of functional materials like metals. Besides using the structured resist as a mask for further processing, the resist pattern can also be used directly as a functional layer [16, 17].

#### **2.2. Variations of soft UV-NIL**

Some variations of soft UV-NIL have been proposed and developed, e.g., SCIL (Substrate conformal imprint lithography) and UV-enhanced SCIL developed by Philips and SUSS, Soft Molecular Scale Nanoimprint Lithography (SMS-NIL) developed by EVG, and full wafer soft UV-NIL, etc. Verschuuren *et al.*[1, 14, 18-19] proposed substrate conformal imprint lithography (SCIL), which combines the advantages of a soft composite mold for large area patterning with the advantages of a rigid glass carrier for low pattern deformation and high resolution. SCIL is based on a combination of the sequential imprinting method and the sol-gel resist. Figure 3 presented the schematic illustration of the SCIL imprint and separation sequences. To achieve a substrate conformal contact between a working mold and a substrate, SCIL process relies on a sequential imprinting process. The approaching of the flexible mold starts from one side and spreads to the whole mold subsequently by releasing the vacuum in the grooves step by step and applying a small over pressure of 20 mBar on the mold (step 1 to 3). After conformal contact over the entire substrate is carried out, the imprint resist is cured by UV exposure or in case of using imprint sol-gel based resists diffusion of the sol-gel solvent into the PDMS mold (step 4). The automatic separation of the mold from the substrate is performed by switching on the vacuum in the grooves consequently, which is opposite to the imprint process (step 5 to 7). This results in a low force peeling action which removes the stamp from the patterned resist layer and avoids damage to mold or transferred patterns. SCIL has demonstrated sub-10nm resolution over 150mm-diameter substrates. The SCIL technology can well cope with non-ideal substrates and implement full wafer imprinting in a single step. For the SCIL process, the curing of sol-gel resist relies on the diffusion of solvents into the PDMS mold. Depending on the operation and preparation conditions, the curing time varies from 5 to 15 minutes. In order to improve throughput (reducing curing time) and repeatability of the process, a UV enhanced SCIL process using UV curable resist has been developing. Fader *et al*. recently introduced UV-SCIL with purely organic UV-curing materials showing curing times of 17s [20]. The excellent performance of SCIL in respect to substrate conformity and pattern fidelity over large areas makes this imprint technology a powerful tool, especially for applications like LED/VCSEL, optical elements or patterned media [21].

The fabrication process of soft UV-NIL includes four steps (as shown in Figure.2): (a) Firstly, a UV-curable resist, which is liquid at room temperature, is spin-coated or dispensed on the substrate. (b) Subsequently, the soft mold is pressed into the resist on the substrate with a low pressure, and adjusts to the waviness or curvature of the substrate until completely conformal contact is achieved. Due to high flexibility, the soft mold can well adapt its shape to the waviness of the substrate obtaining good conformal contact between the non-flat substrate and the mold. (c) After filling all cavities or trenches of the mold, UV curing that solidifies the liquid resist due to cross-linking is carried out by a UV light source. (d) Finally, the soft mold is released, leaving the UV-curable resist patterned. Low viscosity UV-curable resist, commonly comprised of a low molecular weight polymer and photoinitiator, is significantly essential for

(a) (b)

(c) (d)

The removal of the residual layer is typically performed by using a reactive ion etching (RIE) process with an oxygen plasma. The molded resist can then serve as a mask for further processing steps or be used as a functional layer itself. The most common methods for further processing are using the structured resist as an etch mask for patterning the substrate or as a mask for a lift off process for structuring of functional materials like metals. Besides using the structured resist as a mask for further processing, the resist pattern can also be used directly

Some variations of soft UV-NIL have been proposed and developed, e.g., SCIL (Substrate conformal imprint lithography) and UV-enhanced SCIL developed by Philips and SUSS, Soft Molecular Scale Nanoimprint Lithography (SMS-NIL) developed by EVG, and full wafer soft UV-NIL, etc. Verschuuren *et al.*[1, 14, 18-19] proposed substrate conformal imprint lithography (SCIL), which combines the advantages of a soft composite mold for large area patterning with the advantages of a rigid glass carrier for low pattern deformation and high resolution. SCIL is based on a combination of the sequential imprinting method and the sol-gel resist. Figure 3 presented the schematic illustration of the SCIL imprint and separation sequences. To achieve

easily filling the nanocavities of the mold [8-9, 15].

**Figure 2.** Process flow of soft UV-NIL [15]

172 Updates in Advanced Lithography

as a functional layer [16, 17].

**2.2. Variations of soft UV-NIL**

Author developed a full wafer soft UV-NIL with a tri-layer composite mold. The composite mold includes a thin layer of fluoropolymer-based material as the patterning layer, a thick layer of *s*-PMDS as intermediate flexible or cushion layer, and a thin glass sheet as the support layer. Figure 4 illustrated the schematic diagram of the proposed full wafer soft UV-NIL process. The imprinting process is performed by a sequential and micro-contacting solution starting from the center to two sides of the mold. The separation process employs a continuous 'peel-off' demolding mode starting from two sides to the center of the mold. Compared to the SCIL, the distinct advantages of the process include: (1) The imprinting and demolding procedure take the mold center as axis of symmetry, are carried out at the same time on two sides with higher throughput and easier eliminating trapped air bubble. (2) An enhanced demolding approach is adopted. (3) Since the imprinting procedure is performed under a low vacuum pressure environment, it can better remove the trapped air bubbles and provide completely conformal contact [22].

There is a big difference for the imprinting and demolding mode between soft UV-NIL using flexible mold and NIL with rigid mold. Using gas-assisted imprinting method can easily achieve uniform distribution of applied pressure over the entire substrate, gradually and exactly load imprint force. The sequent contact mechanism prevents the flexible mold from trapping air bubbles and therefore ensures that the mold follows exactly the undulating topography over whole substrate surface. A combination of the sequent micro-contact and gas-assistedmprinting can ensure to achieve uniform pressure and good conformal contact, avoid the trapped air bubble defects, reduce the deformation of soft mold. The peel-off

ously growing application needs, much more innovative processes or methods regarding soft

Soft UV Nanoimprint Lithography and Its Applications

http://dx.doi.org/10.5772/56186

175

The flexible mold is the most important elements for soft UV-NIL. The performance of flexible mold has a decisive effect on the soft UV-NIL in term of resolution, patterning area, through‐ put, uniformity of the imprinted patterns and the residual layer, and reproducibility of

Since the soft UV-NIL was introduced, various structural types or configurations have been developed and employed. The most common type of the soft mold is a bi-layer structure which is composed of a rigid glass backplane for mechanical stability (optional) and a patterned soft PDMS (the commercially available PDMS brand Sylgard 184, also known as soft-PDMS or *s*-PDMS from Dow Corning Inc.) imprint layer that adapts perfectly to the waviness or bow of a substrate [12, 24-26], illustrated in Figure 5(a). However, due to the low Young's modulus, high viscosity and swell problem of *s*-PDMS, hard-PDMS (*h*-PDMS) which has higher Young's modulus (a Young's modulus of around 9 MPa) and lower viscosity, has been developed as patterned layer material. Figure 5(b) illustrated such a composite mold, in which a thin *h*-PDMS layer with relief structure is supported by a thick layer of *s*-PDMS. The thin layer of *h*-PDMS is able to ensure a good replication of the nanostructures due to his higher Young's modulus, the thick commercial *s*-PDMS top layer maintains a global flexibility of the whole mold allowing perfect conformal contact even for a non-flat substrate at low imprint pressure [27-28]. Figure 5(c) showed a tri-layer composite flexible mold which includes a thin glass backplane, a soft PDMS flexible layer and a *h*-PDMS patterned layer. Although the *h*-PDMS stamp worked well for replication, there were still limitations in using *h*-PDMS as a patterning layer, such as cracking during the mold fabrication step and degraded conformal contact with the substrate compared with the PDMS material normally used. *X*-PDMS, which has a higher Young's modulus up to 80 MPa, the highest cross linking density and lower diffusion, has been developed by Philips and SUSS as a patterned layer for the tri-layer mold for the SCIL and UV enhanced SCIL applied. The composite mold includes a *X*-PDMS patterned layer, a low modulus PDMS intermediary layer, a thin glass sheet, as shown in Figure 5 (d). The inplane stiffness of the mold avoids pattern deformation over large areas, while out-of-plane flexibility allows conformal contact to underlying surface features [18, 29]. In Figure 5(e) offers the fifth structure of the flexible mold. The first layer is a 2-mm thick cushion layer cast from PDMS. The second layer is a thin flexible plate attached to the PDMS cushion using plasmaactivation, which results in an irreversible strong bond between the glass and the PDMS. The third layer is a thin layer of *h-*PDMS spin-coated onto the master. One of the advantages of this composite mold is the reduction of the lateral deformation since the patterned layer is

In order to improve Young's modulus of a patterned layer for enhancing the resolution of replicated pattern, PMMS is also used to be a structured layer material. A tri-layer mold

UV-NIL will emerge in the future.

**3. Various types of soft molds**

closely anchored to the rigid glass plate [12].

imprinted structures.

**Figure 3.** Schematic of substrate conformal imprint lithography (SCIL) [1]

**Figure 4.** Schematic of a full wafer soft UV-NIL process for non-ideal substrates

demolding approach can be utilized which results in a low separating force that avoids damage to the patterned resist as well as to the mold, and improving the mold lifetime [18, 23].

In order to satisfy the demands of a variety of practical applications, lots of new soft UV-NIL processes have been being proposed and developed. Here only presents some principal and typical variants of soft UV-NIL processes. As the progresses in the soft UV-NIL and continu‐ ously growing application needs, much more innovative processes or methods regarding soft UV-NIL will emerge in the future.

## **3. Various types of soft molds**

demolding approach can be utilized which results in a low separating force that avoids damage to the patterned resist as well as to the mold, and improving the mold lifetime [18, 23].

**Figure 3.** Schematic of substrate conformal imprint lithography (SCIL) [1]

174 Updates in Advanced Lithography

**Figure 4.** Schematic of a full wafer soft UV-NIL process for non-ideal substrates

In order to satisfy the demands of a variety of practical applications, lots of new soft UV-NIL processes have been being proposed and developed. Here only presents some principal and typical variants of soft UV-NIL processes. As the progresses in the soft UV-NIL and continu‐ The flexible mold is the most important elements for soft UV-NIL. The performance of flexible mold has a decisive effect on the soft UV-NIL in term of resolution, patterning area, through‐ put, uniformity of the imprinted patterns and the residual layer, and reproducibility of imprinted structures.

Since the soft UV-NIL was introduced, various structural types or configurations have been developed and employed. The most common type of the soft mold is a bi-layer structure which is composed of a rigid glass backplane for mechanical stability (optional) and a patterned soft PDMS (the commercially available PDMS brand Sylgard 184, also known as soft-PDMS or *s*-PDMS from Dow Corning Inc.) imprint layer that adapts perfectly to the waviness or bow of a substrate [12, 24-26], illustrated in Figure 5(a). However, due to the low Young's modulus, high viscosity and swell problem of *s*-PDMS, hard-PDMS (*h*-PDMS) which has higher Young's modulus (a Young's modulus of around 9 MPa) and lower viscosity, has been developed as patterned layer material. Figure 5(b) illustrated such a composite mold, in which a thin *h*-PDMS layer with relief structure is supported by a thick layer of *s*-PDMS. The thin layer of *h*-PDMS is able to ensure a good replication of the nanostructures due to his higher Young's modulus, the thick commercial *s*-PDMS top layer maintains a global flexibility of the whole mold allowing perfect conformal contact even for a non-flat substrate at low imprint pressure [27-28]. Figure 5(c) showed a tri-layer composite flexible mold which includes a thin glass backplane, a soft PDMS flexible layer and a *h*-PDMS patterned layer. Although the *h*-PDMS stamp worked well for replication, there were still limitations in using *h*-PDMS as a patterning layer, such as cracking during the mold fabrication step and degraded conformal contact with the substrate compared with the PDMS material normally used. *X*-PDMS, which has a higher Young's modulus up to 80 MPa, the highest cross linking density and lower diffusion, has been developed by Philips and SUSS as a patterned layer for the tri-layer mold for the SCIL and UV enhanced SCIL applied. The composite mold includes a *X*-PDMS patterned layer, a low modulus PDMS intermediary layer, a thin glass sheet, as shown in Figure 5 (d). The inplane stiffness of the mold avoids pattern deformation over large areas, while out-of-plane flexibility allows conformal contact to underlying surface features [18, 29]. In Figure 5(e) offers the fifth structure of the flexible mold. The first layer is a 2-mm thick cushion layer cast from PDMS. The second layer is a thin flexible plate attached to the PDMS cushion using plasmaactivation, which results in an irreversible strong bond between the glass and the PDMS. The third layer is a thin layer of *h-*PDMS spin-coated onto the master. One of the advantages of this composite mold is the reduction of the lateral deformation since the patterned layer is closely anchored to the rigid glass plate [12].

In order to improve Young's modulus of a patterned layer for enhancing the resolution of replicated pattern, PMMS is also used to be a structured layer material. A tri-layer mold configuration was proposed which consists of a rigid carrier, a PDMS buffer layer (Young Modulus *E* = 0.75 MPa) and a PMMA patterned layer (*E* = 5.2 GPa), as shown in Figure 5 (f). The introduction of a thin layer of hard material over the PDMS buffer ensures a good hardness for stamping all types of patterns and a reduced pattern deformation over a large area because of the flexibility of PDMS buffer layer. In order to overcome the cracks and fractures during the imprinting and detaching process for high modulus elastomers, a UV cured rigid polymer has been used as the patterning layer. The hybrid mold composed of a thin (100-200 nm) photocured feature layer and the thick (∼2 mm) elastic PDMS support. An interpenetrating polymer network was formed between the interface of the UV-cured rigid patterning layer and the flexible PDMS substrate to provide excellent adhesion of the two distinct materials [30].

viscosity, and thermal and chemical stability. The buffer layer is an intermediate cushion layer which has high flexibility to ensure the intimate conformal contact between the non-flat substrate and the mold. The support layer has mainly twofold: avoiding the lateral dimension of the mold and attaching or fixing the stamp to holder (or chuck). High Young's modulus materials with proper flexibility commonly need be adopted, such as a thin glass backplane or a PET sheet. Of course, there are more variations following the basic configuration. The composite mold should have global flexibility and local rigidity for achieving high resolution

A flexible thin-film (or membrane) can provide better conformal contact with the non-flat substrate (or large area surface) to be patterned without applying large pressure during the imprinting. A further benefit is that demolding can be achieved by peeling the mold from the substrate with an effectively smaller demolding area and low release force. This is much easier than the detaching of a rigid mold, where the hard mold needs to be separated from the substrate as a whole in a parallel way. Therefore, apart from the structure and material used of a flexible composite mold, the thickness of each layer of the mold is also be optimized.

**4. Materials used and fabrication methods for a variety of soft molds**

Apart from the structural style discussed in Section 3, the performances and capabilities of soft molds are largely dependent on the properties of the materials used. Nowadays, a wide variety of materials have been utilized to fabricate flexible molds [7, 8, 10]. This section primarily

Up to now, PDMS is undoubtedly the most widely used soft mold material. There are several reasons that PDMS emerged as a kind of standard material for the soft mold. It has a low Young Modulus and low surface energy that allows for conformal contact and easy release from both a master template and imprinted patterns. It is a durable material with fair chemical resistance and has good optical transparency down to a light wavelength of approximately 256 nm. It is a relatively tough material with a high elongation at break (> 150%) that allows for significant deformation before failure during patterning conditions. It can be easy handled and has high

**Patterned Layer**

**Buffer Layer**

Soft UV Nanoimprint Lithography and Its Applications

**Support Layer**

http://dx.doi.org/10.5772/56186

177

patterns over large areas.

**Figure 6.** Structural model of a common soft mold

discusses the various material used for flexible molds.

**4.1. PDMS-based materials**

*4.1.1. s-PDMS*

Due to the excellent properties of fluorinated materials for an flexible mold used in soft UV-NIL such as low surface energy, excellent mechanical property, high UV-transparency, low viscosity, high chemical durability, thermal stability, etc., fluorinated materials (e.g., PTFE, ETFE, Teflon, ) have been considered as the most promising candidate material of patterning layer. Figure 5 (g) and (h) demonstrates a composite mold including a thin layer of PFPE (*a*-PFPE) as the patterning layer and a thin, flexible, polyethylene terephthalate (PET) sheet as the flexible backing layer. The mold has the ability to fabricate high dense and sub-20 nm feature patterns without cracking or tear-out defects that typically occur with high modulus elastomers [31-33].

**Figure 5.** Various types of soft molds used by soft UV-NIL

Based on the presentations and analysis above, we can generalize a structural model of a common soft mold, as shown in Figure 6. It commonly comprised of three layers: a patterning layer, a buffer layer, and a support layer. The fundamental properties of the patterning layer involve the low surface energy, excellent mechanical property, high UV-transparency, low viscosity, and thermal and chemical stability. The buffer layer is an intermediate cushion layer which has high flexibility to ensure the intimate conformal contact between the non-flat substrate and the mold. The support layer has mainly twofold: avoiding the lateral dimension of the mold and attaching or fixing the stamp to holder (or chuck). High Young's modulus materials with proper flexibility commonly need be adopted, such as a thin glass backplane or a PET sheet. Of course, there are more variations following the basic configuration. The composite mold should have global flexibility and local rigidity for achieving high resolution patterns over large areas.

**Figure 6.** Structural model of a common soft mold

A flexible thin-film (or membrane) can provide better conformal contact with the non-flat substrate (or large area surface) to be patterned without applying large pressure during the imprinting. A further benefit is that demolding can be achieved by peeling the mold from the substrate with an effectively smaller demolding area and low release force. This is much easier than the detaching of a rigid mold, where the hard mold needs to be separated from the substrate as a whole in a parallel way. Therefore, apart from the structure and material used of a flexible composite mold, the thickness of each layer of the mold is also be optimized.

## **4. Materials used and fabrication methods for a variety of soft molds**

Apart from the structural style discussed in Section 3, the performances and capabilities of soft molds are largely dependent on the properties of the materials used. Nowadays, a wide variety of materials have been utilized to fabricate flexible molds [7, 8, 10]. This section primarily discusses the various material used for flexible molds.

### **4.1. PDMS-based materials**

## *4.1.1. s-PDMS*

configuration was proposed which consists of a rigid carrier, a PDMS buffer layer (Young Modulus *E* = 0.75 MPa) and a PMMA patterned layer (*E* = 5.2 GPa), as shown in Figure 5 (f). The introduction of a thin layer of hard material over the PDMS buffer ensures a good hardness for stamping all types of patterns and a reduced pattern deformation over a large area because of the flexibility of PDMS buffer layer. In order to overcome the cracks and fractures during the imprinting and detaching process for high modulus elastomers, a UV cured rigid polymer has been used as the patterning layer. The hybrid mold composed of a thin (100-200 nm) photocured feature layer and the thick (∼2 mm) elastic PDMS support. An interpenetrating polymer network was formed between the interface of the UV-cured rigid patterning layer and the flexible PDMS substrate to provide excellent adhesion of the two distinct materials [30]. Due to the excellent properties of fluorinated materials for an flexible mold used in soft UV-NIL such as low surface energy, excellent mechanical property, high UV-transparency, low viscosity, high chemical durability, thermal stability, etc., fluorinated materials (e.g., PTFE, ETFE, Teflon, ) have been considered as the most promising candidate material of patterning layer. Figure 5 (g) and (h) demonstrates a composite mold including a thin layer of PFPE (*a*-PFPE) as the patterning layer and a thin, flexible, polyethylene terephthalate (PET) sheet as the flexible backing layer. The mold has the ability to fabricate high dense and sub-20 nm feature patterns without cracking or tear-out defects that typically occur with high modulus

elastomers [31-33].

176 Updates in Advanced Lithography

**Thin Glass**

*s***-PDMS X PDMS**

**Figure 5.** Various types of soft molds used by soft UV-NIL

*h***-PDMS PMMA**

(c) (d)

(a) (b)

(e) (f)

(g) (h)

Based on the presentations and analysis above, we can generalize a structural model of a common soft mold, as shown in Figure 6. It commonly comprised of three layers: a patterning layer, a buffer layer, and a support layer. The fundamental properties of the patterning layer involve the low surface energy, excellent mechanical property, high UV-transparency, low

*s***-PDMS**

**PET**

**a-PFPE**

**PFPE**

Up to now, PDMS is undoubtedly the most widely used soft mold material. There are several reasons that PDMS emerged as a kind of standard material for the soft mold. It has a low Young Modulus and low surface energy that allows for conformal contact and easy release from both a master template and imprinted patterns. It is a durable material with fair chemical resistance and has good optical transparency down to a light wavelength of approximately 256 nm. It is a relatively tough material with a high elongation at break (> 150%) that allows for significant deformation before failure during patterning conditions. It can be easy handled and has high gas permeability. Most importantly, *s*-PDMS (Sylgard™184 from Dow Corning) is commer‐ cially available in kits that allow for inexpensive fabrication of flexible molds from polymer precursors. However, there are some inherent disadvantages to *s*-PDMS which severely limit its capabilities in soft UV-NIL. The low Young Modulus below to 2.0 MPa limits the replication of both the high-density and high resolution features and is also a detriment for forming highaspect ratio structures as fabricating such features will be apt to collapse, merge, or buckle. Moreover, *s*-PDMS tends to absorb organic solvents and monomers. This leads to fluctuations in the resist composition and swelling issue of the mold. This becomes a serious problem when trying to pattern certain biological materials or for the fabrication of functional nanostructures with controlled surfaces. Its poor solvent resistance has a serious effect on reproducibility due to degradation in the course of patterning repeatedly. Its high elasticity and thermal expansion can lead to deformation and distortions during the fabrication can result in loss of critical dimensions. The surface energy of *s*-PDMS is not low enough to duplicate profiles with high fidelity. In addition, the *s*-PDMS remains its high viscosity (for Sylgard 184, this is 3900 mPa s). As a consequence, 3D nanostructures and fine features of a master mold are difficult to be fully filled which results in a loss of feature height and some defects and uniformity of replicated patterns. Besides, the dimensional change (i.e., thermal shrinkage) due to thermal expansion after thermal curing makes it difficult to apply it for large-area mass production, especially multilevel pattern registration over a large area [10, 12, 13, 26, 34-37].

junction functionality. A nanoimprint resolution record of 2 nm has been achieved by using soft molds based on *h*-PDMS [37]. Viheriälä *et al*. described a formulation of *h*-PDMS [12].

Soft UV Nanoimprint Lithography and Its Applications

http://dx.doi.org/10.5772/56186

179

The procedure for producing a bi-layer *h*-PDMS/*s*-PDMS mold is illustrated in Figure 7. The *h*-PDMS prepolymer, prepared by referring to the formulation in [12], is spin coated onto a Si master template fabricate by EBL at 5000 rpm for 30 sec and then degassed in vacuum for 10 min (the thickness of the *h*-PDMS is about 5-8μm). A mixture of conventional *s*-PDMS (1:10) is then casted the spin coated *h*-PDMS layer curing at 60 °C for 24 hours, the bi-layer soft mold can then be peeled off from the master template and it is treated with trichloromethylsilane

In addition, Pei *et al*. presented a *h*-PDMS-based tri-layer composite mold and its fabrication process for nanostructured amorphous silicon photovoltaics [38]. Although the *h*-PDMS-based mold worked well for replication, there are still some limitations for its applications: (1) releasing it from the master caused cracking across the face of the mold; (2) external pressure is required to achieve conformal contact with a substrate, which created long-range, nonuni‐

**Figure 7.** Schematic of the fabrication process of a *h*-PDMS/*s*-PDMS bilayer composite mold and a pictures of a Silicon

form distortions over the large areas of contact [39, 40].

master template and of a replicated soft mold after peeling off [13]

(TMCS) [13].

Simple PDMS molds are typically replicated by first mixing two commercial PDMS compo‐ nents: 10:1 PDMS RTV 615 (part A) siloxane oligomer and RTV 615 (part B) cross-linking oligomers (General Electric). The mixture is then casted on a master template and degassed in a low pressure vacuum chamber. A curing time of 24 h and a curing temperature of 60 °C are usually recommended in order to reduce roughness and to avoid a build up of tension because of the thermal shrinkage. The replicated mold is left to cool to room temperature, carefully peeled off from the master template. In the subsequent fabrication step the unstructured backside of the PDMS layer was treated with oxygen plasma for 30 s in order to temporarily increase the surface energy and thereby ensure a stable bonding of the PDMS layer to the quartz or glass backplane. Sometimes, the mold is also treated with silane based anti-sticking treatment to further reduce the low PDMS surface energy. During the fabrication, curing times and temperatures, which have large effect on the elastic modulus and hardness of the repli‐ cated soft mold, must be exactly controlled. The vacuum degree in degassing phase is also a key process parameter. The previous results have indicated that a higher vacuum degree and a lower temperature are the favorable conditions to get better comprehensive physical properties of a PDMS-based soft mold [13, 26].

#### *4.1.2. h-PDMS*

An obvious way to overcome unwanted deformations is to use a mold with a higher Young's modulus. Therefore, there have been attempts to utilize materials of high Young's modulus for flexible molds. *h*-PDMS was developed at IBM as early as 2000 [36]. They tried to formulate a better imprint material by trying different combinations of vinyl and hydrosilane end-linked polymers and vinyl and hydrosilane copolymers, with varying mass between cross-links and junction functionality. A nanoimprint resolution record of 2 nm has been achieved by using soft molds based on *h*-PDMS [37]. Viheriälä *et al*. described a formulation of *h*-PDMS [12].

gas permeability. Most importantly, *s*-PDMS (Sylgard™184 from Dow Corning) is commer‐ cially available in kits that allow for inexpensive fabrication of flexible molds from polymer precursors. However, there are some inherent disadvantages to *s*-PDMS which severely limit its capabilities in soft UV-NIL. The low Young Modulus below to 2.0 MPa limits the replication of both the high-density and high resolution features and is also a detriment for forming highaspect ratio structures as fabricating such features will be apt to collapse, merge, or buckle. Moreover, *s*-PDMS tends to absorb organic solvents and monomers. This leads to fluctuations in the resist composition and swelling issue of the mold. This becomes a serious problem when trying to pattern certain biological materials or for the fabrication of functional nanostructures with controlled surfaces. Its poor solvent resistance has a serious effect on reproducibility due to degradation in the course of patterning repeatedly. Its high elasticity and thermal expansion can lead to deformation and distortions during the fabrication can result in loss of critical dimensions. The surface energy of *s*-PDMS is not low enough to duplicate profiles with high fidelity. In addition, the *s*-PDMS remains its high viscosity (for Sylgard 184, this is 3900 mPa s). As a consequence, 3D nanostructures and fine features of a master mold are difficult to be fully filled which results in a loss of feature height and some defects and uniformity of replicated patterns. Besides, the dimensional change (i.e., thermal shrinkage) due to thermal expansion after thermal curing makes it difficult to apply it for large-area mass production,

especially multilevel pattern registration over a large area [10, 12, 13, 26, 34-37].

properties of a PDMS-based soft mold [13, 26].

*4.1.2. h-PDMS*

178 Updates in Advanced Lithography

Simple PDMS molds are typically replicated by first mixing two commercial PDMS compo‐ nents: 10:1 PDMS RTV 615 (part A) siloxane oligomer and RTV 615 (part B) cross-linking oligomers (General Electric). The mixture is then casted on a master template and degassed in a low pressure vacuum chamber. A curing time of 24 h and a curing temperature of 60 °C are usually recommended in order to reduce roughness and to avoid a build up of tension because of the thermal shrinkage. The replicated mold is left to cool to room temperature, carefully peeled off from the master template. In the subsequent fabrication step the unstructured backside of the PDMS layer was treated with oxygen plasma for 30 s in order to temporarily increase the surface energy and thereby ensure a stable bonding of the PDMS layer to the quartz or glass backplane. Sometimes, the mold is also treated with silane based anti-sticking treatment to further reduce the low PDMS surface energy. During the fabrication, curing times and temperatures, which have large effect on the elastic modulus and hardness of the repli‐ cated soft mold, must be exactly controlled. The vacuum degree in degassing phase is also a key process parameter. The previous results have indicated that a higher vacuum degree and a lower temperature are the favorable conditions to get better comprehensive physical

An obvious way to overcome unwanted deformations is to use a mold with a higher Young's modulus. Therefore, there have been attempts to utilize materials of high Young's modulus for flexible molds. *h*-PDMS was developed at IBM as early as 2000 [36]. They tried to formulate a better imprint material by trying different combinations of vinyl and hydrosilane end-linked polymers and vinyl and hydrosilane copolymers, with varying mass between cross-links and

The procedure for producing a bi-layer *h*-PDMS/*s*-PDMS mold is illustrated in Figure 7. The *h*-PDMS prepolymer, prepared by referring to the formulation in [12], is spin coated onto a Si master template fabricate by EBL at 5000 rpm for 30 sec and then degassed in vacuum for 10 min (the thickness of the *h*-PDMS is about 5-8μm). A mixture of conventional *s*-PDMS (1:10) is then casted the spin coated *h*-PDMS layer curing at 60 °C for 24 hours, the bi-layer soft mold can then be peeled off from the master template and it is treated with trichloromethylsilane (TMCS) [13].

In addition, Pei *et al*. presented a *h*-PDMS-based tri-layer composite mold and its fabrication process for nanostructured amorphous silicon photovoltaics [38]. Although the *h*-PDMS-based mold worked well for replication, there are still some limitations for its applications: (1) releasing it from the master caused cracking across the face of the mold; (2) external pressure is required to achieve conformal contact with a substrate, which created long-range, nonuni‐ form distortions over the large areas of contact [39, 40].

**Figure 7.** Schematic of the fabrication process of a *h*-PDMS/*s*-PDMS bilayer composite mold and a pictures of a Silicon master template and of a replicated soft mold after peeling off [13]

## *4.1.3. X-PDMS*

Phlips and SUSS developed a new high modulus silicone rubber (*X*-PDMS) which is made from combination of vinyl-modified linear di-methyl-siloxanes and vinyl-modified quaterna‐ ry siloxanes. The latter component increases the intrinsic crosslink density in the rubber and thereby the Young's modulus. The mixture is cross linked with hydride modified linear siloxanes using a platinum catalyzed vinyl-hydride addition reaction. By changing the linear to quaternary siloxane ratio, they synthesized rubbers with Young's Modulus up to 80 MPa. The rubber material with the highest attained stiffness allows the faithful replication of dense sub-10 nm features while still providing conformal contact over a full wafer [18, 29].

*4.2.1. Perfluoropolyether (PFPE)*

*4.2.1.1. HPFPE*

*4.2.1.2. a-PFPE*

are formed via UV curing in several minutes [45-49].

area, and patterning a 50 nm linewidth and a 200 nm period [50].

Recently perfluoropolyether (PFPE) and its derivatives have gained popularity as flexible mold materials. PFPE is the main component of fluoropolymer that is composed of only carbon, fluorine and oxygen [44]. The pre-polymer mixture is a liquid at room temperature, and can be cross-linked under UV exposure or thermal annealing. The PFPE-based materials contain several distinctive properties such as high chemical resistance, extremely low surface energy, high gas permeability, high solvent resistance, high elastic recovery and good mechanical strength. In particular, the modulus of the elastomer can be easily tuned by changing the molecular weight between crosslinks of the precursor allowing for higher fidelity molds, and

Soft UV Nanoimprint Lithography and Its Applications

http://dx.doi.org/10.5772/56186

181

HPFPE is a copolymer resin of a perfluoropolyether (PFPE) and a hyperbranched polymer (HP). The HPFPE exhibited low viscosity, high Young's modulus, and lower surface energy, and enhanced stability. The low surface energy of the HPFPE resist resulting from the acrylic acid functional groups on the backbone of the fluorinated polyether precludes any adhesion of the polymer to the mold. By adjusting the weight percent of the multifunctional HP and diluter, 1, 6-hexanediol diacrylate (HDDA), it is possible to modify the viscosity of the obtained HPFPE copolymer resist and yield a HPFPE copolymer resist with a high Young's modulus. By optimizing the soft UV-NIL process with this flexible HPFPE mold, the imprinting results exhibited near zero residues at the bottom of the resist grooves, and no sticking over a large

Unlike certain PFPE formulations described earlier, which have a Young's modulus of around 4 MPa, the Young's modulus of acryloxy PFPE (denoted as *a*-PFPE) is 10.5 MPa. The low surface energy (∼18.5 mN m−1) as well as the chemical inertness of *a*-PFPE elimi‐ nates the necessity of treating the surfaces of the masters with fluorinated silanes to avoid sticking during casting and curing of the mold. Figure 8 illustrated the fabrication of a composite *a*-PFPE mold using backing layers of *s*-PDMS or polyethylene terephtha‐ late (PET). The photocurable fluorinated acrylate oligomer CN 4000 is mixed with 0.5 wt % of a photoinitiator (Darocurr 4265, Ciba Specialty Chemicals) and filtered through a 0.22 μm syringe filter. A thin layer (∼2 μm) of the *a*-PFPE resin is spin coated (4000 rpm, 30 s) on the master and cured in UV light (350–380 nm, 4 mW cm−2) under nitrogen purge for 2 *h*. Then a layer (∼4 mm) of Sylgard 184 PDMS (*s*-PDMS ) is poured onto the *a*-PFPE layer and cured at room temperature for ∼48 *h* or at 65◦C for 2 *h*. The *a*-PFPE/*s*-PDMS composite mold is peeled off from the master [51]. The high resolution capabilities of *a*-PFPE, together with other properties such as resistance to swelling, chemical inert‐ ness, and photocurability make it a promising alternative to PDMS for soft UV-NIL.

## *4.1.4. hν-PDMS*

Conventional PDMS requires a long, thermal cure under pressure which is very time con‐ suming and can take many hours. Photocurable PDMS (*hν*-PDMS) can overcome deformations associated with thermal curing of conventional PDMS. *hν-*PDMS also provides a tensile modulus higher than that of *s*-PDMS and an elongation at break that is much higher than that of *h*-PDMS, which makes it easier to handle than *h*-PDMS and to be less susceptible to mechanical deformation. Choi and Rogers developed a photocurable PDMS system [41]. *h*ν-PDMS has been used as a mold material to successfully replicate 300 nm width by 300 nm spacing by 600 nm height lines which could not be replicated in either *s*-PDMS(feature collapse failure) or *h*-PDMS (fracture failure).

## *4.1.5. Diluted PDMS*

The main drawback of conventional *s*-PDMS materials is the high viscosity, which is for Sylgard 184 (Dow Corning) 3900 mPa.s. It is extremely difficult for high viscous s-PDMS to fill the nanocavities of the master template for fabricating high resolution soft molds. Thus, the resolution of the soft mold is limited by an inappropriate material flow for pattern geometries within the sub-100 nm regime. To overcome this problem, using triethylamine, toluene and hexane as solvent to low the viscosity of *h*-PDMS has been reported and demonstrated the imprinting of 75 nm lines with a pitch of 150 nm. koo *et al*. have reported an improved mold fabrication process using Sylgard 184 diluted with toluene. Dots with a resolution of 50 nm are well replicated and an excellent imprint homogeneity across a 4 inch wafer with one imprint step only has been demonstrated [42].

## **4.2. Fluorinated polymer materials**

Fluorinated polymer offers an ideal material for soft molds. Compared to other materials used by soft molds, Fluorinated polymer materials have many outstanding advantages: extremely low surface energy, suitable Young's modulus (10Mpa~2GPa), high gas permeability, good mechanical strength, solvent resistance, chemical stability, visible transparency, and tunable modulus. These characteristics open up the possibility of fabricating high performance flexible molds for soft UV-NIL to pattern a wide variety of nanostructures and materials for real applications [43,44].

## *4.2.1. Perfluoropolyether (PFPE)*

Recently perfluoropolyether (PFPE) and its derivatives have gained popularity as flexible mold materials. PFPE is the main component of fluoropolymer that is composed of only carbon, fluorine and oxygen [44]. The pre-polymer mixture is a liquid at room temperature, and can be cross-linked under UV exposure or thermal annealing. The PFPE-based materials contain several distinctive properties such as high chemical resistance, extremely low surface energy, high gas permeability, high solvent resistance, high elastic recovery and good mechanical strength. In particular, the modulus of the elastomer can be easily tuned by changing the molecular weight between crosslinks of the precursor allowing for higher fidelity molds, and are formed via UV curing in several minutes [45-49].

#### *4.2.1.1. HPFPE*

*4.1.3. X-PDMS*

180 Updates in Advanced Lithography

*4.1.4. hν-PDMS*

*4.1.5. Diluted PDMS*

failure) or *h*-PDMS (fracture failure).

imprint step only has been demonstrated [42].

**4.2. Fluorinated polymer materials**

applications [43,44].

Phlips and SUSS developed a new high modulus silicone rubber (*X*-PDMS) which is made from combination of vinyl-modified linear di-methyl-siloxanes and vinyl-modified quaterna‐ ry siloxanes. The latter component increases the intrinsic crosslink density in the rubber and thereby the Young's modulus. The mixture is cross linked with hydride modified linear siloxanes using a platinum catalyzed vinyl-hydride addition reaction. By changing the linear to quaternary siloxane ratio, they synthesized rubbers with Young's Modulus up to 80 MPa. The rubber material with the highest attained stiffness allows the faithful replication of dense

Conventional PDMS requires a long, thermal cure under pressure which is very time con‐ suming and can take many hours. Photocurable PDMS (*hν*-PDMS) can overcome deformations associated with thermal curing of conventional PDMS. *hν-*PDMS also provides a tensile modulus higher than that of *s*-PDMS and an elongation at break that is much higher than that of *h*-PDMS, which makes it easier to handle than *h*-PDMS and to be less susceptible to mechanical deformation. Choi and Rogers developed a photocurable PDMS system [41]. *h*ν-PDMS has been used as a mold material to successfully replicate 300 nm width by 300 nm spacing by 600 nm height lines which could not be replicated in either *s*-PDMS(feature collapse

The main drawback of conventional *s*-PDMS materials is the high viscosity, which is for Sylgard 184 (Dow Corning) 3900 mPa.s. It is extremely difficult for high viscous s-PDMS to fill the nanocavities of the master template for fabricating high resolution soft molds. Thus, the resolution of the soft mold is limited by an inappropriate material flow for pattern geometries within the sub-100 nm regime. To overcome this problem, using triethylamine, toluene and hexane as solvent to low the viscosity of *h*-PDMS has been reported and demonstrated the imprinting of 75 nm lines with a pitch of 150 nm. koo *et al*. have reported an improved mold fabrication process using Sylgard 184 diluted with toluene. Dots with a resolution of 50 nm are well replicated and an excellent imprint homogeneity across a 4 inch wafer with one

Fluorinated polymer offers an ideal material for soft molds. Compared to other materials used by soft molds, Fluorinated polymer materials have many outstanding advantages: extremely low surface energy, suitable Young's modulus (10Mpa~2GPa), high gas permeability, good mechanical strength, solvent resistance, chemical stability, visible transparency, and tunable modulus. These characteristics open up the possibility of fabricating high performance flexible molds for soft UV-NIL to pattern a wide variety of nanostructures and materials for real

sub-10 nm features while still providing conformal contact over a full wafer [18, 29].

HPFPE is a copolymer resin of a perfluoropolyether (PFPE) and a hyperbranched polymer (HP). The HPFPE exhibited low viscosity, high Young's modulus, and lower surface energy, and enhanced stability. The low surface energy of the HPFPE resist resulting from the acrylic acid functional groups on the backbone of the fluorinated polyether precludes any adhesion of the polymer to the mold. By adjusting the weight percent of the multifunctional HP and diluter, 1, 6-hexanediol diacrylate (HDDA), it is possible to modify the viscosity of the obtained HPFPE copolymer resist and yield a HPFPE copolymer resist with a high Young's modulus. By optimizing the soft UV-NIL process with this flexible HPFPE mold, the imprinting results exhibited near zero residues at the bottom of the resist grooves, and no sticking over a large area, and patterning a 50 nm linewidth and a 200 nm period [50].

#### *4.2.1.2. a-PFPE*

Unlike certain PFPE formulations described earlier, which have a Young's modulus of around 4 MPa, the Young's modulus of acryloxy PFPE (denoted as *a*-PFPE) is 10.5 MPa. The low surface energy (∼18.5 mN m−1) as well as the chemical inertness of *a*-PFPE elimi‐ nates the necessity of treating the surfaces of the masters with fluorinated silanes to avoid sticking during casting and curing of the mold. Figure 8 illustrated the fabrication of a composite *a*-PFPE mold using backing layers of *s*-PDMS or polyethylene terephtha‐ late (PET). The photocurable fluorinated acrylate oligomer CN 4000 is mixed with 0.5 wt % of a photoinitiator (Darocurr 4265, Ciba Specialty Chemicals) and filtered through a 0.22 μm syringe filter. A thin layer (∼2 μm) of the *a*-PFPE resin is spin coated (4000 rpm, 30 s) on the master and cured in UV light (350–380 nm, 4 mW cm−2) under nitrogen purge for 2 *h*. Then a layer (∼4 mm) of Sylgard 184 PDMS (*s*-PDMS ) is poured onto the *a*-PFPE layer and cured at room temperature for ∼48 *h* or at 65◦C for 2 *h*. The *a*-PFPE/*s*-PDMS composite mold is peeled off from the master [51]. The high resolution capabilities of *a*-PFPE, together with other properties such as resistance to swelling, chemical inert‐ ness, and photocurability make it a promising alternative to PDMS for soft UV-NIL.

made possible because of the good conformal contact originating from the high flexibility and low-adhesion of the ETFE mold. ETFE-based molds also provide a high fidelity of reproduc‐ tion. ETFE is transparent in the visible and in the UV spectrum (91–95% transmittance in the 200–800 nm range for a 25μm sheet), it can be used to structure a photocurable polymer and to solidify it by irradiation with UV light at room temperature, providing a cheap alternative

Soft UV Nanoimprint Lithography and Its Applications

http://dx.doi.org/10.5772/56186

183

Combined with the advantage of their low adhesion and flexibility, ETFE molds can therefore be used to replace brittle and expensive inorganic materials, as well as deformable elastomers and other plastic molds which lack strength when pressure is applied. The strength and robustness of ETFE molds is essential for embossing large microstructures as well as small nanostructures with lateral dimensions approaching 10 nm. Some commercial ETFE products

Teflon AF 2400 form Dupont has been adopted as a typical rigiflex mold material. Teflon AF 2400 is a copolymer of 2,2-bistrifluoromethyl- 4,5-difluoro-1,3-dioxole and tetrafluoroethylene. The polymer has a tensile modulus of ca. 1.6 GPa (almost a thousand times harder than the elastomeric PDMS material), which is stiff enough for patterning small features without mold deformation. It can be used for UV-NIL in place of a quartz mold. This material has a low surface energy of ~16 mN m–1 which can be used as a mold material without any surface treatment. It is inert to all solvents except for perfluorinated solvents (3M, FC-77) such that there is no swelling problem. It is also inert to all chemicals. Therefore, the inert nature with a low surface energy makes it easy to demold after the imprinting process without any surface treatment and without deterioration in surface properties over many imprinting cycles. In addition, it is transparent to light in the region between deep UV and near infrared. It has high gas permeability. The property can eliminate the trapped air bubble during full wafer imprinting. Teflon® AF (amorphous fluoropolymer by Dupont) can have a life greater than 1000 impressions in a very clean environment. There are two ways of preparing the Teflon

A typical concern for high modulus materials is their tendency to crack and break due to their brittle nature. Choi *et al*. fabricated a fluorinated organic-inorganic hybrid mold with high modulus of 115 MPa by using a nonhydrolytic *sol-gel* process which can produce a crack-free mold without leaving any trace of solvent. Various nanometer scale patterns including sub-100 nm patterns have been obtained using the fluorinated hybrid mold [56]. A new commercially fluorinated mold material has been developed by AGC Chemicals Co. The fluoropolymer

region (>95% at 375 nm wavelength), high tensile modulus (1.2GPa), CTE (70–80 ppm/K), and low surface energy. *Tg* of this fluororesin is about 110°C. The thickness of the fluororesin could be controlled from 60 nm to 15 μm by changing concentration and spin coat conditions. Furthermore, it has high chemical durability [57-58]. Haatainen *et al*. demonstrated the

) against UV irradiation, good transparency at UV

including Fluon ETFE, Tefzel® ETFE resin, have been directly utilized [53].

mold: solvent-casting method and compression molding [24, 54-56].

*4.2.4. Others fluoropolymer materials*

material has good durability (>250 J/cm2

to quartz or other transparent inorganic mold materials.

*4.2.3. Teflon*

**Figure 8.** Fabrication of a composite a-PFPE mold with either s-PDMS or PET films as backing supports [51]

#### *4.2.1.3. PFPE-DMA (Photochemically curable fluoropolymer)*

The a-,w-methacryloxy functionalized PFPE (PFPE-DMA) is a photochemically curable fluoropolymer. A commercially available hydroxy-terminated PFPE (Solvay Solexis, *Mn*=3800 gmol-1) with isocyanato ethyl methacrylate (Aldrich) is used to yield a methacryloxy-func‐ tionalized polymer which is a pourable liquid of low viscosity (0.36 Pas) at room temperature. A photoinitiator, 1-hydroxycyclohexyl phenyl ketone (1 wt%, Aldrich), is dissolved into the PFPE-DMA to make a photocurable liquid resin. The PFPE-DMA based mold involves the following features: easy fabrication, the ability to make conformal contact, remarkably low surface energy, resistance to swelling by small organic molecules, and enduring multiple printing procedures. It is able to replicate sub-100-nm sized features with no indications of limits to going to even smaller in size [52].

#### *4.2.2. Ethylene(Tetrafluoroethylene) (ETFE)*

The ETFE is a copolymer of ethylene and tetrafluoroethylene. It has some outstanding properties: an exceptional toughness and flexibility, a relatively high stiffness (elastic modulus ∼ 1.2 GPa), a high thermal stability (a high melting point in the range of 255–280 °C). ETFE has superior mechanical properties compared to Teflon AF in pressure assisted imprinting at high temperatures. The flexibility and low surface energy of ETFE provide a clean mold release without fracture or deformation of the embossed structures. Patterning over large areas is made possible because of the good conformal contact originating from the high flexibility and low-adhesion of the ETFE mold. ETFE-based molds also provide a high fidelity of reproduc‐ tion. ETFE is transparent in the visible and in the UV spectrum (91–95% transmittance in the 200–800 nm range for a 25μm sheet), it can be used to structure a photocurable polymer and to solidify it by irradiation with UV light at room temperature, providing a cheap alternative to quartz or other transparent inorganic mold materials.

Combined with the advantage of their low adhesion and flexibility, ETFE molds can therefore be used to replace brittle and expensive inorganic materials, as well as deformable elastomers and other plastic molds which lack strength when pressure is applied. The strength and robustness of ETFE molds is essential for embossing large microstructures as well as small nanostructures with lateral dimensions approaching 10 nm. Some commercial ETFE products including Fluon ETFE, Tefzel® ETFE resin, have been directly utilized [53].

## *4.2.3. Teflon*

**Figure 8.** Fabrication of a composite a-PFPE mold with either s-PDMS or PET films as backing supports [51]

The a-,w-methacryloxy functionalized PFPE (PFPE-DMA) is a photochemically curable fluoropolymer. A commercially available hydroxy-terminated PFPE (Solvay Solexis, *Mn*=3800 gmol-1) with isocyanato ethyl methacrylate (Aldrich) is used to yield a methacryloxy-func‐ tionalized polymer which is a pourable liquid of low viscosity (0.36 Pas) at room temperature. A photoinitiator, 1-hydroxycyclohexyl phenyl ketone (1 wt%, Aldrich), is dissolved into the PFPE-DMA to make a photocurable liquid resin. The PFPE-DMA based mold involves the following features: easy fabrication, the ability to make conformal contact, remarkably low surface energy, resistance to swelling by small organic molecules, and enduring multiple printing procedures. It is able to replicate sub-100-nm sized features with no indications of

The ETFE is a copolymer of ethylene and tetrafluoroethylene. It has some outstanding properties: an exceptional toughness and flexibility, a relatively high stiffness (elastic modulus ∼ 1.2 GPa), a high thermal stability (a high melting point in the range of 255–280 °C). ETFE has superior mechanical properties compared to Teflon AF in pressure assisted imprinting at high temperatures. The flexibility and low surface energy of ETFE provide a clean mold release without fracture or deformation of the embossed structures. Patterning over large areas is

*4.2.1.3. PFPE-DMA (Photochemically curable fluoropolymer)*

limits to going to even smaller in size [52].

182 Updates in Advanced Lithography

*4.2.2. Ethylene(Tetrafluoroethylene) (ETFE)*

Teflon AF 2400 form Dupont has been adopted as a typical rigiflex mold material. Teflon AF 2400 is a copolymer of 2,2-bistrifluoromethyl- 4,5-difluoro-1,3-dioxole and tetrafluoroethylene. The polymer has a tensile modulus of ca. 1.6 GPa (almost a thousand times harder than the elastomeric PDMS material), which is stiff enough for patterning small features without mold deformation. It can be used for UV-NIL in place of a quartz mold. This material has a low surface energy of ~16 mN m–1 which can be used as a mold material without any surface treatment. It is inert to all solvents except for perfluorinated solvents (3M, FC-77) such that there is no swelling problem. It is also inert to all chemicals. Therefore, the inert nature with a low surface energy makes it easy to demold after the imprinting process without any surface treatment and without deterioration in surface properties over many imprinting cycles. In addition, it is transparent to light in the region between deep UV and near infrared. It has high gas permeability. The property can eliminate the trapped air bubble during full wafer imprinting. Teflon® AF (amorphous fluoropolymer by Dupont) can have a life greater than 1000 impressions in a very clean environment. There are two ways of preparing the Teflon mold: solvent-casting method and compression molding [24, 54-56].

#### *4.2.4. Others fluoropolymer materials*

A typical concern for high modulus materials is their tendency to crack and break due to their brittle nature. Choi *et al*. fabricated a fluorinated organic-inorganic hybrid mold with high modulus of 115 MPa by using a nonhydrolytic *sol-gel* process which can produce a crack-free mold without leaving any trace of solvent. Various nanometer scale patterns including sub-100 nm patterns have been obtained using the fluorinated hybrid mold [56]. A new commercially fluorinated mold material has been developed by AGC Chemicals Co. The fluoropolymer material has good durability (>250 J/cm2 ) against UV irradiation, good transparency at UV region (>95% at 375 nm wavelength), high tensile modulus (1.2GPa), CTE (70–80 ppm/K), and low surface energy. *Tg* of this fluororesin is about 110°C. The thickness of the fluororesin could be controlled from 60 nm to 15 μm by changing concentration and spin coat conditions. Furthermore, it has high chemical durability [57-58]. Haatainen *et al*. demonstrated the fabrication of the F-template by combining the fluorinated mold material and thermal step and stamp nanoimprint lithography (SSIL) method, as well as presented imprinted results using the F-template. The patterns including gratings with 50 nm features and dot arrays of 350 nm diameter features have been achieved [59].

as high-aspect-ratio features. Physical properties of Ormostamp are modulus of elasticity of 0.650 GPa, hardness of 0.036 GPa, liquid viscosity of 0.75 Pa s. The values of Ormocomp modulus of elasticity and hardness are sufficient for patterning both micro- and nanostructures without any cracks and fractures (too brittle material) or deformations (too soft material). In addition, the relatively low viscosity of Ormostamp is important for efficient filling of the master template cavities and allows various deposition techniques, which make the material handling and further processing easier [12, 63-65]. The Ormostamp working mold possesses high transparency and thermal and UV stability which are essential for soft UV-NIL. Up to now, only quartz stamps exhibited all these features. Excellent pattern transfer fidelity has

Soft UV Nanoimprint Lithography and Its Applications

http://dx.doi.org/10.5772/56186

185

In order to meet various differently functional demands of the soft UV-NIL process, applied materials of soft molds should possess a number of desirable properties such as physical rigidity for high resolution, flexibility for intimate conformal contact, low surface energy for high quality demolding, high UV transparency for fast curing resist, low viscosity for easy and fast filling into the nano-cavities or features of a master at low pressure to achieve a high resolution mold, small curing-induced shrinkage for dimension accuracy and stability, and chemical inertness for mold durability, as well as thermal stability, easy material processing and high pattern transfer fidelity, etc. A proper balance between the mold rigidity required for patterning a very fine and dense structure and the flexibility needed for a conformal contact with the substrate is at the core of the successful applications. Table 1 summaries these mold

Various fluorinated polymer materials with ultra-low surface energy and suitable rigidity have received enough attention to become ideal materials of flexible molds for soft UV-NIL. The crack and too friable issues for some materials (such as Teflon) with high Young's modulus should be overcome. A composite mold combining a rigid patterning layer using fluoropoly‐ mer-based materials and a flexible support layer shows better performance with higher resolution, easy demolding and intimate confocal contact capability as well as longer mold lifetime. Furthermore, the discovery of newly suitable materials (mold and resist) and control

Soft UV-NIL has been employed to fabricate various micro/nanostructures and devices for nanoelectronics, optoelectronics, nanophotonics, optical components, glass, biological applications, etc. It has become a perfect match for some emerging application fields that are in great need of large area patterning of submicro and nano scale features at a low cost, such as patterned magnetic media, light emitting diodes, optical metamaterials and plasmonic devices for chemical and bio-sensing applications, etc. In particular, this technique has demonstrated great commercial prospects in several market segments, LEDs, laser diodes,

their properties will play a critical role for enhanced soft UV-NIL process.

demonstrated for 100 nm structures.

**5. Applications of soft UV-NIL**

**4.4. Summary and discussion**

materials.

## **4.3. UV curable materials**

## *4.3.1. Poly(Urethaneacrylate) (PUA)*

The PUA based materials exhibit higher rigidity and better impact strength compared with PDMS without significant compromises in flexibility enabling micro-and nanopatterning when the PUA mold thickness is below 50 mm. It is almost impermeable to gases, inert to chemicals and solvents such that there is no swelling problem, and it is transparent to light in the UV and visible regions. However, the rigidity of PUA material is not enough high. It is necessary to further improve the Young's modulus of PUA for fulfilling higher resolution pattern replication. The mechanical properties of PUA based materials largely depend on their cross-linking density, UV-exposure time, UV light wavelength/intensity, as well as the PUA precursor composition. According to the optimizing results of mechanical properties (hard‐ ness and effective modulus) form Kim, the PUA replica mold demonstrated very high mechanical properties of hardness (0.15 GPa) and elastic modulus (2.7 GPa) due to the increased cross-linking density of the PUA precursor at an optimized UV-exposure time of 600 s. The PUA replica mold demonstrated potential for the fabrication of multi-scale line-andspace patterns with sizes of 350 nm or less with good uniformity and reproducibility over large areas [48, 60-61].

#### *4.3.2. Modulus-tunable UV-curable materials*

A UV-curable mold material which consists of a functionalized prepolymer with acrylate group for cross-linking, a monomeric modulator, a photoinitiator, and a radiation-curable releasing agent for surface activity, has been developed. The mechanical properties of the mold can be tailored by the chain length of an acrylate modulator in the cross-linking reaction. This tunability can be utilized to obtain a proper balance that is needed for a given patterning technique between the rigidity requirement (tensile modulus of 320 MPa) of a mold for patterning a fine structure and the flexibility requirement (tensile modulus of 19.8 MPa) for a conformal contact [62].

### *4.3.3. UV-curable inorganic–organic hybrid polymer — Ormostamp*

Both the transparency and the thermal stability are principal material properties for flexible molds used in soft UV-NIL. A facing challenge in the UV-NIL process is the high transparency of the mold material in the UV-range, which is the characteristic wavelength range for the majority of photo initiators used in photoresist materials. The novel mold material system based on a – Ormostamp – offers high UV-transparency even after thermal exposure at 270 °C. In this case 90% transparency remains at 350 nm. The elasticity and hardness of the mold material are also critical factors for the transfer of nanostructures and dense patterns as well as high-aspect-ratio features. Physical properties of Ormostamp are modulus of elasticity of 0.650 GPa, hardness of 0.036 GPa, liquid viscosity of 0.75 Pa s. The values of Ormocomp modulus of elasticity and hardness are sufficient for patterning both micro- and nanostructures without any cracks and fractures (too brittle material) or deformations (too soft material). In addition, the relatively low viscosity of Ormostamp is important for efficient filling of the master template cavities and allows various deposition techniques, which make the material handling and further processing easier [12, 63-65]. The Ormostamp working mold possesses high transparency and thermal and UV stability which are essential for soft UV-NIL. Up to now, only quartz stamps exhibited all these features. Excellent pattern transfer fidelity has demonstrated for 100 nm structures.

## **4.4. Summary and discussion**

fabrication of the F-template by combining the fluorinated mold material and thermal step and stamp nanoimprint lithography (SSIL) method, as well as presented imprinted results using the F-template. The patterns including gratings with 50 nm features and dot arrays of 350 nm

The PUA based materials exhibit higher rigidity and better impact strength compared with PDMS without significant compromises in flexibility enabling micro-and nanopatterning when the PUA mold thickness is below 50 mm. It is almost impermeable to gases, inert to chemicals and solvents such that there is no swelling problem, and it is transparent to light in the UV and visible regions. However, the rigidity of PUA material is not enough high. It is necessary to further improve the Young's modulus of PUA for fulfilling higher resolution pattern replication. The mechanical properties of PUA based materials largely depend on their cross-linking density, UV-exposure time, UV light wavelength/intensity, as well as the PUA precursor composition. According to the optimizing results of mechanical properties (hard‐ ness and effective modulus) form Kim, the PUA replica mold demonstrated very high mechanical properties of hardness (0.15 GPa) and elastic modulus (2.7 GPa) due to the increased cross-linking density of the PUA precursor at an optimized UV-exposure time of 600 s. The PUA replica mold demonstrated potential for the fabrication of multi-scale line-andspace patterns with sizes of 350 nm or less with good uniformity and reproducibility over large

A UV-curable mold material which consists of a functionalized prepolymer with acrylate group for cross-linking, a monomeric modulator, a photoinitiator, and a radiation-curable releasing agent for surface activity, has been developed. The mechanical properties of the mold can be tailored by the chain length of an acrylate modulator in the cross-linking reaction. This tunability can be utilized to obtain a proper balance that is needed for a given patterning technique between the rigidity requirement (tensile modulus of 320 MPa) of a mold for patterning a fine structure and the flexibility requirement (tensile modulus of 19.8 MPa) for a

Both the transparency and the thermal stability are principal material properties for flexible molds used in soft UV-NIL. A facing challenge in the UV-NIL process is the high transparency of the mold material in the UV-range, which is the characteristic wavelength range for the majority of photo initiators used in photoresist materials. The novel mold material system based on a – Ormostamp – offers high UV-transparency even after thermal exposure at 270 °C. In this case 90% transparency remains at 350 nm. The elasticity and hardness of the mold material are also critical factors for the transfer of nanostructures and dense patterns as well

diameter features have been achieved [59].

*4.3.2. Modulus-tunable UV-curable materials*

*4.3.3. UV-curable inorganic–organic hybrid polymer — Ormostamp*

**4.3. UV curable materials**

184 Updates in Advanced Lithography

areas [48, 60-61].

conformal contact [62].

*4.3.1. Poly(Urethaneacrylate) (PUA)*

In order to meet various differently functional demands of the soft UV-NIL process, applied materials of soft molds should possess a number of desirable properties such as physical rigidity for high resolution, flexibility for intimate conformal contact, low surface energy for high quality demolding, high UV transparency for fast curing resist, low viscosity for easy and fast filling into the nano-cavities or features of a master at low pressure to achieve a high resolution mold, small curing-induced shrinkage for dimension accuracy and stability, and chemical inertness for mold durability, as well as thermal stability, easy material processing and high pattern transfer fidelity, etc. A proper balance between the mold rigidity required for patterning a very fine and dense structure and the flexibility needed for a conformal contact with the substrate is at the core of the successful applications. Table 1 summaries these mold materials.

Various fluorinated polymer materials with ultra-low surface energy and suitable rigidity have received enough attention to become ideal materials of flexible molds for soft UV-NIL. The crack and too friable issues for some materials (such as Teflon) with high Young's modulus should be overcome. A composite mold combining a rigid patterning layer using fluoropoly‐ mer-based materials and a flexible support layer shows better performance with higher resolution, easy demolding and intimate confocal contact capability as well as longer mold lifetime. Furthermore, the discovery of newly suitable materials (mold and resist) and control their properties will play a critical role for enhanced soft UV-NIL process.

## **5. Applications of soft UV-NIL**

Soft UV-NIL has been employed to fabricate various micro/nanostructures and devices for nanoelectronics, optoelectronics, nanophotonics, optical components, glass, biological applications, etc. It has become a perfect match for some emerging application fields that are in great need of large area patterning of submicro and nano scale features at a low cost, such as patterned magnetic media, light emitting diodes, optical metamaterials and plasmonic devices for chemical and bio-sensing applications, etc. In particular, this technique has demonstrated great commercial prospects in several market segments, LEDs, laser diodes,


another method of generating periodic patterns over large areas at low cost. Although the patterns made by IL are highly uniform and have superior long-range order, these patterns are usually in very simple geometric forms of grating lines and 2-D dots, and their dimensions are difficult to reduce to sub-100 nm due to light diffraction. Furthermore, it is unsuitable for high volume production processes because the optical configuration has to be modified to realize different patterns. In addition, this approach requires a strict control of the environment to maintain stable fringe patterns. Soft UV-NIL with flexible mold has the capability of nanopatterning on non-flat surface over large areas and is less-sensitive to the production atmosphere. Compared to ICs industry, the LED application is much more relaxed than IC's for overlay and defect density. Therefore, soft UV-NIL has been considered as one of the most suitable solution for LED patterning. Due to its cost-effectiveness combined with superior processing performance, soft UV-NIL will play a crucial role in moving the LED industry into a new realm of nanopattered LEDs with ultra-high efficiency [2, 14, 17, 65]. Figure 9 showed some cases related to LED patterning using soft UV-NIL. In addition, some commercial companies such as SUUS, Obducat, EVG, Toshiba, Aurotek, Luminus, etc. have been devel‐ oping the process and equipment of soft UV-NIL for high volume producing PhC LEDs and

Soft UV Nanoimprint Lithography and Its Applications

http://dx.doi.org/10.5772/56186

187

**Figure 9.** LED patterning using soft UV-NIL [2, 14, 66, 67] (a): NPSS-based LEDs; (b): PhC LEDs)

structures have been realized on a large area (1 mm2

Nanophotonics is a rapidly growing field with great commercial potential. It is a wide field covering many interesting applications branching from cutting edge science including plasmonics, metamaterials, cavity quantum electrodynamics in high-Q cavities all the way to applied sciences like silicon nanophotonics for on chip optical interconnections and single frequency semiconductor light sources. Most of the practical device demonstrations in these fields utilize nanopatterned surfaces. Applications require patterning of nanoscopic gratings, photonic crystals, waveguides and metal structures. Viheriälä *et al*. demonstrated soft UV-NIL nanophotonics applications including distributed feedback laser diodes, plasmonic nano‐ structures, and patterned facets of optical fibres. Soft UV-NIL will play a critical important role in the commercialization of many nanophotonic applications since it offers excellent cost effectiveness and requires relatively low capital investment. High-density plasmonic nano‐

obtained dimensions of the nanodisks are 65 nm in diameter, 180 nm in periodicity and 25 nm

) using the soft UV-NIL technique. The

NPSS.

**5.2. Nanophotonic devices**

**Table 1.** Summary of various soft mold materials

solar cells, optical elements, patterned media, flat panel displays, micro-lens, and functional polymer devices [7-12, 15-17].

#### **5.1. LED patterning**

Light efficiency enhancement and manufacturing cost reduction have always been regarding as the two most crucial issues in LED industry, particularly for the large-scale realization of solid state lighting. Compared to other technologies improving the LED performance, two emerging techniques, photonic crystals (PhC) and nanopatterned sapphire substrate (NPSS), have shown higher potential in output efficiency enhancement and beam shaping. NPSS and Photonic Crystal based LEDs have been considered as the most promising solutions for high brightness LEDs. The typical characteristics of LED epitaxial wafers and sapphire substrates are with large variation in wafer topography (varying TTD), high bow and warp, surface roughness with surface protrusions with micron size, and particle contaminations, etc. And these materials tend to be fragile or brittle. Due to the non-planar and rough nature of the LED epitaxial wafer and substrate, existing nanopatterning technologies cannot well meet the requirements of producing these nanostructures in both technology and cost level which mainly originated form the new challenging issues form LED patterning. Due to a very small depth of focus, optical lithography techniques have insufficiently fidelity for LED patterning. As warpage increases with larger wafer sizes, the ability of the photolithography tool to compensate for substrate warpage becomes even more critical. Interference lithography is another method of generating periodic patterns over large areas at low cost. Although the patterns made by IL are highly uniform and have superior long-range order, these patterns are usually in very simple geometric forms of grating lines and 2-D dots, and their dimensions are difficult to reduce to sub-100 nm due to light diffraction. Furthermore, it is unsuitable for high volume production processes because the optical configuration has to be modified to realize different patterns. In addition, this approach requires a strict control of the environment to maintain stable fringe patterns. Soft UV-NIL with flexible mold has the capability of nanopatterning on non-flat surface over large areas and is less-sensitive to the production atmosphere. Compared to ICs industry, the LED application is much more relaxed than IC's for overlay and defect density. Therefore, soft UV-NIL has been considered as one of the most suitable solution for LED patterning. Due to its cost-effectiveness combined with superior processing performance, soft UV-NIL will play a crucial role in moving the LED industry into a new realm of nanopattered LEDs with ultra-high efficiency [2, 14, 17, 65]. Figure 9 showed some cases related to LED patterning using soft UV-NIL. In addition, some commercial companies such as SUUS, Obducat, EVG, Toshiba, Aurotek, Luminus, etc. have been devel‐ oping the process and equipment of soft UV-NIL for high volume producing PhC LEDs and NPSS.

**Figure 9.** LED patterning using soft UV-NIL [2, 14, 66, 67] (a): NPSS-based LEDs; (b): PhC LEDs)

#### **5.2. Nanophotonic devices**

solar cells, optical elements, patterned media, flat panel displays, micro-lens, and functional

Tunable 19.8-320

**Modulus (MPa)**

**Surface Energy (mN/m)**

PFPE 4 12 UV-Light HPFPE 4-5.4 17-22 300-900 UV-Light a-PFPE 10.5 ∼18.5 60 cps at 25 °C UV light PFPE-DMA 4 16.3 360 UV light

*s*-PDMS < 2 21-24 ~3900 Thermal *h*-PDMS 8-12 ~20 Tunable Thermal *X*-PDMS ~80 Thermal *hv*-PDMS ~3-4 ~20 UV-Light

ETFE ~1.2GPa 15.6 Thermal Teflon AF 2400 1.6 GPa ~16 Thermal

PUA 2.7 GPa 23 UV light

Ormostamp 650 750 UV-curable

**Viscosity (mPa.s) Curing**

**mode**

UV light

Light efficiency enhancement and manufacturing cost reduction have always been regarding as the two most crucial issues in LED industry, particularly for the large-scale realization of solid state lighting. Compared to other technologies improving the LED performance, two emerging techniques, photonic crystals (PhC) and nanopatterned sapphire substrate (NPSS), have shown higher potential in output efficiency enhancement and beam shaping. NPSS and Photonic Crystal based LEDs have been considered as the most promising solutions for high brightness LEDs. The typical characteristics of LED epitaxial wafers and sapphire substrates are with large variation in wafer topography (varying TTD), high bow and warp, surface roughness with surface protrusions with micron size, and particle contaminations, etc. And these materials tend to be fragile or brittle. Due to the non-planar and rough nature of the LED epitaxial wafer and substrate, existing nanopatterning technologies cannot well meet the requirements of producing these nanostructures in both technology and cost level which mainly originated form the new challenging issues form LED patterning. Due to a very small depth of focus, optical lithography techniques have insufficiently fidelity for LED patterning. As warpage increases with larger wafer sizes, the ability of the photolithography tool to compensate for substrate warpage becomes even more critical. Interference lithography is

polymer devices [7-12, 15-17].

materials

**Table 1.** Summary of various soft mold materials

PFPEbased materials

**Item Sub-class Young's**

Modulus-tunable UV-curable

**5.1. LED patterning**

PDMS-based materials

186 Updates in Advanced Lithography

Fluorinated polymer materials

UV curable materials

> Nanophotonics is a rapidly growing field with great commercial potential. It is a wide field covering many interesting applications branching from cutting edge science including plasmonics, metamaterials, cavity quantum electrodynamics in high-Q cavities all the way to applied sciences like silicon nanophotonics for on chip optical interconnections and single frequency semiconductor light sources. Most of the practical device demonstrations in these fields utilize nanopatterned surfaces. Applications require patterning of nanoscopic gratings, photonic crystals, waveguides and metal structures. Viheriälä *et al*. demonstrated soft UV-NIL nanophotonics applications including distributed feedback laser diodes, plasmonic nano‐ structures, and patterned facets of optical fibres. Soft UV-NIL will play a critical important role in the commercialization of many nanophotonic applications since it offers excellent cost effectiveness and requires relatively low capital investment. High-density plasmonic nano‐ structures have been realized on a large area (1 mm2 ) using the soft UV-NIL technique. The obtained dimensions of the nanodisks are 65 nm in diameter, 180 nm in periodicity and 25 nm

in height with the soft *h*-PDMS/*s*-PDMS mold. Cattoni *et al.*, have successfully realized a Localized Surface Plasmon Resonance (LSPR) biosensor based on λ<sup>3</sup> /1000 plasmonic nano‐ cavities fabricated by Soft UV-NIL on large surfaces (0.5-1 cm2 ). These structures present nearly perfect omnidirectional absorption in the infra-red regime independently of the incident angle and light polarization and outstanding biochemical sensing performances with high refractive index sensitivity and figure of merit 10 times higher than conventional LSPR based biosensor [12, 13, 18, 68-71]. Figure 10 presented some nanophotonic devices made using soft UV-NIL.

Soft UV-NIL has also been used to pattern GaAs (1 0 0) substrate into periodic nucleation sites for the growth of InAs site-controlled quantum dots. The incorporation of soft UV-NIL and MOCVD may be a promising method of forming large-area, site-controlled, highly uniform and ordered arrays of quantum dots with low-cost and high throughput to satisfy the require‐

Soft UV Nanoimprint Lithography and Its Applications

http://dx.doi.org/10.5772/56186

189

**Figure 11.** QDs-based devices fabricated using soft UV-NIL [73-76] (a: Site-controlled InAs QDs on UV-NIL patterned surface; b: Nanoimprinted QD arrays for sensing and detecting biomolecules; c: Quantum dot optoelectronics fabrica‐

Soft UV-NIL by using a flexible mold has been proven to be a cost-effective high volume nanopatterning method for large-area structure replication up to wafer-level (300mm) in the micrometer and nanometer scale, fabricating complex 3-D micro/nano structures, especially making large-area patterns on the non-planar surfaces even curved substrates at low-cost and with high throughput. In particular, it provides an ideal solution and a powerful tool for mass producing micro/nanostructures over large areas at low cost for the applications in compound semiconductor optoelectronics and nanophotonic devices, especially for LED patterning. That opens the way for many applications not previously conceptualized or economically feasible. Soft UV-NIL has been regarded as the closest process for the industrial application of NIL. In particular, the applications in LED patterning and wafer level micro optics have demonstrated significantly commercial prospect. Soft UV-NIL and its variations (e.g., Roll-type nanoimprint using soft molds) will become more and more important for these applications in large area patterning, fabricating 3-D micor/nanostructures and forming patterns on the non-planar or curved surface. There is a plenty of room to enhance the resolution, patterning area, mold

This work was financially supported by National Science Foundation of China - Major Research Plan "Fundamental Study on Nanomanufacturing" (Grant No. 91023023) and

Program for New Century Excellent Talents in University (Grant No.NCET-11-1029).

ments of mass production for QDs and QD arrays [73-76].

(a) (b) (c)

lifetime, yield for the promising patterning method.

**Acknowledgements**

tion using soft UV-NIL)

**6. Conclusions**

**Figure 10.** Nanophotonic devices made using soft UV-NIL [12, 13, 72] (a: DFB-laser diodes; b: Silicon microring resona‐ tors; c: Metal nanostructures; d: Optical fibre with the imprinted blazed grating)

## **5.3. QDs-based devices**

Quantum dot (QD) arrays have now been attracting tremendous attention due to the potential applications in various high performance devices (e.g., QD lasers, 3rd generation solar cells, single photon emitters, QD memories, etc.), the fundamental investigation of quantum computing and quantum communication, and in the exploration or observation of novel physical phenomena. Currently, the major challenging issues in commercialized application of QD arrays include fabrication of large-area, defect-free, highly uniform and ordering QDs, accurate positioning for individual QD nucleation site, and reproducibility in size and spatial distribution, which all crucially determines optoelectronic performance and consistency for these QDs-based functional devices and the investigation of fundamental physical properties for QDs. In order to accurately control the size, position, density and composition of epitaxially grown self-assembled quantum dots, a variety of strategies including buried stressor disloca‐ tion networks, multiple-layer heteroepitaxy structures to control the stress distribution, quantum dots growth on patterned substrates using various nanofabrication techniques have been proposed in the past decade. Among these, the QDs growth on patterned substrates has been considered as the most straightforward approach to control the size, density and position of QDs so as to achieve highly uniform and ordering QD arrays. Furthermore, growing QD arrays on the patterned substrates has the ability to control the absolute lateral position of quantum dots on a long-range scale. Compared to other nanopatterning approaches, soft UV-NIL technique has high potential to create large-area, low defects patterned substrates with low cost and high throughput. Tommila *et al.* reported on the development of UV-NIL process for patterning GaAs substrates, which are used as templates in seeded S–K growth of QDs. Soft UV-NIL has also been used to pattern GaAs (1 0 0) substrate into periodic nucleation sites for the growth of InAs site-controlled quantum dots. The incorporation of soft UV-NIL and MOCVD may be a promising method of forming large-area, site-controlled, highly uniform and ordered arrays of quantum dots with low-cost and high throughput to satisfy the require‐ ments of mass production for QDs and QD arrays [73-76].

**Figure 11.** QDs-based devices fabricated using soft UV-NIL [73-76] (a: Site-controlled InAs QDs on UV-NIL patterned surface; b: Nanoimprinted QD arrays for sensing and detecting biomolecules; c: Quantum dot optoelectronics fabrica‐ tion using soft UV-NIL)

## **6. Conclusions**

in height with the soft *h*-PDMS/*s*-PDMS mold. Cattoni *et al.*, have successfully realized a

cavities fabricated by Soft UV-NIL on large surfaces (0.5-1 cm2 ). These structures present nearly perfect omnidirectional absorption in the infra-red regime independently of the incident angle and light polarization and outstanding biochemical sensing performances with high refractive index sensitivity and figure of merit 10 times higher than conventional LSPR based biosensor [12, 13, 18, 68-71]. Figure 10 presented some nanophotonic devices made using soft

**Figure 10.** Nanophotonic devices made using soft UV-NIL [12, 13, 72] (a: DFB-laser diodes; b: Silicon microring resona‐

Quantum dot (QD) arrays have now been attracting tremendous attention due to the potential applications in various high performance devices (e.g., QD lasers, 3rd generation solar cells, single photon emitters, QD memories, etc.), the fundamental investigation of quantum computing and quantum communication, and in the exploration or observation of novel physical phenomena. Currently, the major challenging issues in commercialized application of QD arrays include fabrication of large-area, defect-free, highly uniform and ordering QDs, accurate positioning for individual QD nucleation site, and reproducibility in size and spatial distribution, which all crucially determines optoelectronic performance and consistency for these QDs-based functional devices and the investigation of fundamental physical properties for QDs. In order to accurately control the size, position, density and composition of epitaxially grown self-assembled quantum dots, a variety of strategies including buried stressor disloca‐ tion networks, multiple-layer heteroepitaxy structures to control the stress distribution, quantum dots growth on patterned substrates using various nanofabrication techniques have been proposed in the past decade. Among these, the QDs growth on patterned substrates has been considered as the most straightforward approach to control the size, density and position of QDs so as to achieve highly uniform and ordering QD arrays. Furthermore, growing QD arrays on the patterned substrates has the ability to control the absolute lateral position of quantum dots on a long-range scale. Compared to other nanopatterning approaches, soft UV-NIL technique has high potential to create large-area, low defects patterned substrates with low cost and high throughput. Tommila *et al.* reported on the development of UV-NIL process for patterning GaAs substrates, which are used as templates in seeded S–K growth of QDs.

/1000 plasmonic nano‐

Localized Surface Plasmon Resonance (LSPR) biosensor based on λ<sup>3</sup>

(a) (b) (c) (d)

tors; c: Metal nanostructures; d: Optical fibre with the imprinted blazed grating)

UV-NIL.

188 Updates in Advanced Lithography

**5.3. QDs-based devices**

Soft UV-NIL by using a flexible mold has been proven to be a cost-effective high volume nanopatterning method for large-area structure replication up to wafer-level (300mm) in the micrometer and nanometer scale, fabricating complex 3-D micro/nano structures, especially making large-area patterns on the non-planar surfaces even curved substrates at low-cost and with high throughput. In particular, it provides an ideal solution and a powerful tool for mass producing micro/nanostructures over large areas at low cost for the applications in compound semiconductor optoelectronics and nanophotonic devices, especially for LED patterning. That opens the way for many applications not previously conceptualized or economically feasible.

Soft UV-NIL has been regarded as the closest process for the industrial application of NIL. In particular, the applications in LED patterning and wafer level micro optics have demonstrated significantly commercial prospect. Soft UV-NIL and its variations (e.g., Roll-type nanoimprint using soft molds) will become more and more important for these applications in large area patterning, fabricating 3-D micor/nanostructures and forming patterns on the non-planar or curved surface. There is a plenty of room to enhance the resolution, patterning area, mold lifetime, yield for the promising patterning method.

## **Acknowledgements**

This work was financially supported by National Science Foundation of China - Major Research Plan "Fundamental Study on Nanomanufacturing" (Grant No. 91023023) and Program for New Century Excellent Talents in University (Grant No.NCET-11-1029).

## **Author details**

Hongbo Lan1,2\*

Address all correspondence to: hblan99@gmail.com

1 Nanomanufacturing and Nano-Optoelectronics Lab, Qingdao Technological University, Qingdao, China

[12] Viheriälä, J, Niemi, T, & Kontio, J. Nanoimprint Lithography- Next Generation Nano‐ patterning Methods for Nanophotonics Fabrication. In: Kim K. (ed.) Recent Optical

Soft UV Nanoimprint Lithography and Its Applications

http://dx.doi.org/10.5772/56186

191

[13] Cattoni, A, Chen, J, & Decanini, D. Soft UV Nanoimprint Lithography: A Versatile Tool for Nanostructuration at the 20nm Scale. In: Cui B. (ed.) Recent Advances in Nanofabrication Techniques and Applications Rijeka: InTech; (2011). , 139-156.

[14] Ji, R, Hornung, M, & Verschuuren, M. UV Enhanced Substrate Conformal Imprint Lithography (UV-SCIL) Technique for Photonic Crystals Patterning in LED Manufac‐

[15] Glinsner, T, Plachetka, U, & Matthias, T. Soft UV-based Nanoimprint Lithography

[16] Hiroshima, H. Nanoimprint with Thin and Uniform Residual Layer for Various Pat‐

[17] Lan, H, & Ding, Y. Nanoimprint lithography. In: Wang M. (ed.) Lithography Rijeka:

[18] Verschuuren, M. A. Substrate Conformal Imprint Lithography for Nanophotonics.

[19] Hornung, M, Ji, R, & Verschuuren, M. Inch Full Field Wafer Size Nanoimprint Lith‐ ography for Photonic Crystals Patterning. (2010). th IEEE Conference on Nanotech‐

[20] Schmitt, H, Duempelmann, P, & Fader, R. el at. Life Time Evaluation of PDMS Stamps for UV-enhanced Substrate Conformal Imprint Lithography. Microelectronic

[22] Lan, H. A full wafer-scale UV Nanoimprint Lithography Process and Tool for Pat‐ terning Photonic Crystal for HB-LEDs. BIT's 1st Annual world congress of Nano-

[23] Hiroshima, H. Release Force Reduction in UV Nanoimprint by Mold Orientation Control and by Gas Environment. J. Vac. Sci. Technol. B. (2009). , 27(6), 2862-2865.

[24] Rogers, J A, & Lee, H H. Unconventional Nanopatterning Techniques and Applica‐

[25] Plachetka, U, Bender, M, & Fuchs, A. Comparison of Multilayer Stamp Concepts in

[26] Bender, M, Plachetka, U, & Ran, J. High Resolution Lithography with PDMS Molds.

for Large Area Imprinting Applications. Proc. of SPIE (2007).

tern Densities, Microelectronic Engineering (2008).

and Photonic Technologies (2010). , 275-298.

turing. Microelectronic Engineering (2010).

InTech; (2010). , 457-494.

nology. , 2010, 339-342.

Engineering. (2012). S., 275-278.

S&T-2011. Oct. Dalian; (2011).

[21] SUSShttp://www.suss.com/index.php,(2012).

tions. John Wiley & Sons, Inc.; (2008).

UV-NIL. Microelectronic Engineering (2006). , 83-944.

J. Vac. Sci. Technol. B. (2004). , 22(6), 3229-3232.

PhD thesis. Utrecht University; (2011).

2 Qingdao Bona Optoelectronics Equipment Co., Ltd., Qingdao National High-Tech Indus‐ trial Development Zone, China

## **References**


**Author details**

190 Updates in Advanced Lithography

Address all correspondence to: hblan99@gmail.com

1 Nanomanufacturing and Nano-Optoelectronics Lab, Qingdao Technological University,

2 Qingdao Bona Optoelectronics Equipment Co., Ltd., Qingdao National High-Tech Indus‐

[1] Verschuuren, M A, & Sprang, H A. Large-area Nanopatterns: Improving LEDs, La‐

[2] Lee, Y C, & Tu, S H. Improving the Light-emitting Efficiency of GaN LEDs Using Nanoimprint lithography. In: Cui B. (ed.) Recent Advances in Nanofabrication Tech‐

[3] Baek, J H, Kim, S M, & Lee, I H. Control of Characteristic Performance by Patterned

[4] Yang, Y, Mielczarek, K, & Aryal, M. Nanoimprinted Polymer Solar Cell. ACS NANO

[5] Farshchian, B, Amirsadeghi, A, & Hurst, S M. Soft UV-nanoimprint Lithography on

[6] Chou, Y, Krauss, P, & Renstrom, P. Imprint Lithography with 25-nanometer Resolu‐

[7] Schift, H. Nanoimprint lithography: An Old Story in Modern Times? A Review, J.

[8] Lan, H, Ding, Y, & Liu, H. Nanoimprint Lithography Principles, Processes and Mate‐

[9] Guo, J. Recent Progress in Nanoimprint Technology and its Applications. Journal of

[10] Guo, J. Nanoimprint Lithography: Methods and Material Requirements. Advanced

[11] Bender, M, Fuchs, A, & Plachetka, U. Status and Prospects of UV-Nanoimprint Tech‐

sers, and Photovoltaics. SPIE. http://spie.org/x87355.xml.(2012).

niques and Applications. Rijeka: InTech; (2011). , 173-195.

Structure in Light-emitting Diodes. Proc. of SPIE (2011). B.

Non-planar Surfaces. Microelectronic Engineering (2011). , 88-3287.

Hongbo Lan1,2\*

Qingdao, China

**References**

trial Development Zone, China

(2012). , 6(4), 2877-2892.

tion. Science (1996). , 272(5258), 85-87.

rials. Nova Science Pub Inc.; (2011).

Materials (2007). , 19(4), 495-513.

Vac. Sci. Technol. B. (2008). , 26(2), 458-480.

Physics D: Applied Physics (2004). RR141., 123.

nology. Microelectronic Engineering (2006). , 83-827.


[27] . Bilayer Transparent Molds for High Resolution Soft UV Nanoimprint Lithography. http://jnte08.trans-gdr.lpn.cnrs.fr/FILES/p12.pdf.

[43] Boday, D. J. The State of Fluoropolymers. In Smith D (ed.) Advances in Fluorine-

Soft UV Nanoimprint Lithography and Its Applications

http://dx.doi.org/10.5772/56186

193

[44] Con, C, Zhang, J, & Jahed, Z. Thermal Nanoimprint Lithography Using Fluoropoly‐

[45] Williams, S, Retterer, S, & Lopez, R. High-resolution PFPE-based Molding Techni‐ ques for Nanofabrication of High-pattern Density, Sub-20 nm Features: a Fundamen‐

[46] Gilles, S, Diez, M, & Offenhausser, A. Deformation of Nanostructures on Polymer Molds During Soft UV Nanoimprint Lithography. Nanotechnology (2010).

[47] Mühlberger, M, Bergmair, I, & Klukowska, A. UV-NIL with Working Stamps Made

[48] Kim, J K, Cho, H S, & Jung, H S. Effect of Surface Tension and Coefficient of Thermal Expansion in 30 nm Scale Nanoimprinting with Two Flexible Polymer Molds. Nano‐

[49] Jayakumar, P, Ho, Y T, & Soo, H. Adhesion Force Measurement Between the Stamp and the Resin in Ultraviolet Nanoimprint Lithography-an Investigative Approach.

[50] Zhu, Z, Li, Q, & Zhang, L. UV-based Nanoimprinting Lithography with a Fluorinat‐

[51] Truong, T T, Lin, R, & Jeon, S. Soft Lithography Using Acryloxy Perfluoropolyether

[52] Rolland, J P, Hagberg, E C, & Denison, G M. High-resolution Soft Lithography: Ena‐

[53] Barbero, D R, Saifullah, M, & Hoffmann, M. P., *et al*. High-resolution Nanoimprinting with a Robust and Reusable Polymer Mold. Advanced Functional Materials (2007). ,

[54] Kim, M J, Park, J E, & Song, S. Simple "Solutal" Method for Preparing Teflon Nano‐

[55] Khang, D Y, & Lee, H H. Sub-100 nm Patterning with an Amorphous Fluoropolymer

[56] Khang, D Y, Kang, H, & Kim, T. Low Pressure Nanoimprint Lithography. Nano Lett.

[57] Choi, D G, Jeong, J J, & Sim, Y S. Fluorinated Organic-inorganic Hybrid Mold as a New Stamp for Nanoimprint and Soft Lithography. Langmuir (2005). , 21-9390. [58] Kawaguchi, Y, Nonaka, F, & Sanada, Y. Fluorinated Materials for UV Nanoimprint

bling Materials for Nanotechnologies. Angew. Chem. Int. Ed. (2004).

structures and Molds. J. Vac. Sci. Technol. B. (2007). , 25-1412.

Lithography. Microelectronic Engineering (2007). , 84-973.

Containing Polymers. ACS; (2012). , 1-7.

technology (2012).

17(14), 2419-2425.

(2004). , 4-633.

Mold. Langmuir (2004). , 20-2445.

Nanotechnology (2009).

mer Mold. Microelectronic Engineering. (2012). In press.

tal Materials Approach. Nano Lett. (2010). , 10-1421.

From Ormostamp. Microelectronic Engineering (2009).

ed Flexible Stamp. J. Vac. Sci. Technol. B. (2011).

Composite Stamps. Langmuir (2007). , 23(5), 2898-2905.


[43] Boday, D. J. The State of Fluoropolymers. In Smith D (ed.) Advances in Fluorine-Containing Polymers. ACS; (2012). , 1-7.

[27] . Bilayer Transparent Molds for High Resolution Soft UV Nanoimprint Lithography.

[28] Barbillon, G. Plasmonic Nanostructures Prepared by Soft UV Nanoimprint Lithogra‐ phy and Their Application in Biological Sensing. Micromachines (2012). , 3-21.

[29] Verschuuren, M A, & Brakel, R. van de Laar H W J J., *et al*. VCSEL, LED, Thin-film PV production by Substrate Conformal Imprint Lithography. International Confer‐

[30] Roy, E, Kanamori, Y, & Belotti, M. Enhanced UV Imprint Ability with a Tri-layer

[31] Williams, S S, Retterer, S, & Lopez, R. High-resolution PFPE-based Molding Techni‐ ques for Nanofabrication of High-pattern Density, Sub-20 nm Features: a Fundamen‐

[32] Gilles, S, Meier, M, & Prömpers, M. UV Nanoimprint Lithography with Rigid Poly‐

[33] Choi, D G, Jeong, J H, & Sim, Y. Fluorinated Organic-inorganic Hybrid Mold as a New Stamp for Nanoimprint and Soft Lithography. Langmuir (2005). , 21-9390. [34] Choi, W M, & Park, O O. Soft-imprint Technique for Submicron-scale Patterns Using

[35] Koo, N, Plachetka, U, & Otto, M. The Fabrication of a Flexible Mold for High Resolu‐

[36] Schmid, H, & Michel, B. Siloxane Polymers for High-resolution, High-accuracy Soft

[37] Hua, F, Sun, Y, & Gaur, A. Polymer Imprint Lithography with Molecular-scale Reso‐

[38] Pei, L, Balls, A, & Tippets, C. Polymer Molded Templates for Nanostructured Amor‐

[39] Li, Z, Gu, Y, & Wang, L. Hybrid Nanoimprint-soft Lithography With Sub-15 nm Res‐

[40] Odom, T W. Christopher Love J., Wolfe D.B., *et al*. Improved Pattern Transfer in Soft

[41] Choi, K M, & Rogers, J A. A Photocurable Poly(dimethylsiloxane) Chemistry De‐ signed for Soft Lithographic Molding and Printing in the Nanometer regime. J. AM.

[42] Koo, N, Bender, M, & Plachetka, U. Improved Mold Fabrication for the Definition of High Quality Nanopatterns by Soft UV-Nanoimprint Lithography Using Diluted

tion Soft Ultraviolet Nanoimprint Lithography. Nanotechnology (2008).

http://jnte08.trans-gdr.lpn.cnrs.fr/FILES/p12.pdf.

192 Updates in Advanced Lithography

ence on Micro and Nano Engineering Berlin; (2011).

tal Materials Approach. Nano Lett. (2010). , 10-1421.

a PDMS Mold. Microelectronic Engineering (2004).

Lithography. Macromolecules (2000). , 33-3042.

lution. Nano Letters (2004). , 4(12), 2467-2471.

olution. Nano Lett. (2009). , 9(6), 2306-2310.

CHEM. SOC. (2003). , 125-4060.

phous Silicon Photovoltaics. J. Vac. Sci. Technol. A. (2011).

Lithography Using Composite Stamps. Langmuir (2002). , 18-5314.

PDMS Material. Microelectronic Engineering (2007). , 84-904.

mer Molds. Microelectronic Engineering (2009). , 86-661.

Stamp Configuration. Microelectronic Engineering (2005).


[59] Haatainen, T, Mäkelä, T, & Ahopelto, J. Imprinted Polymer Stamps for UV-NIL. Mi‐ croelectronic Engineering (2009). , 86-2293.

[75] Oh, Y, Lee, K, & Ko, S. Quantum Dot Arrays Patterned by Direct Nanoimprint. The

Soft UV Nanoimprint Lithography and Its Applications

http://dx.doi.org/10.5772/56186

195

[76] Tommila, J, Tukiainen, A, & Viheriälä, J. Nanoimprint Lithography Patterned GaAs Templates for Site-controlled InAs Quantum Dots. Journal of Crystal Growth

10th International Conference on NNT, Shilla Jeju, Korea, (2011).

(2011). , 232(1), 183-186.


[75] Oh, Y, Lee, K, & Ko, S. Quantum Dot Arrays Patterned by Direct Nanoimprint. The 10th International Conference on NNT, Shilla Jeju, Korea, (2011).

[59] Haatainen, T, Mäkelä, T, & Ahopelto, J. Imprinted Polymer Stamps for UV-NIL. Mi‐

[60] Kim, J Y, Park, H S, & Kim, Z S. Fabrication of Low-cost Submicron Patterned Poly‐ meric Replica Mold With High Elastic Modulus Over a Large Area. Soft Matter.

[61] Suh, D, Choi, S J, & Lee, H H. Rigiflex Lithography for Nanostructure Transfer. Ad‐

[62] Yoo, P J, Choi, S J, & Kim, J H. Unconventional Patterning with a Modulus-tunable Mold: From Imprinting to Microcontact Printing. Chem. Mater. (2004). , 16-5000.

[63] Klukowska, A, Kolander, A, & Bergmair, I. Novel Transparent Hybrid Polymer Working Stamp for UV-imprinting. Microelectronic Engineering (2009). , 86-697.

[64] Mühlberger, M, Bergmair, I, & Klukowska, A. UV-NIL with Working Stamps Made

[65] Byeon, K J, Hong, E, & Park, H. Full Wafer Scale Nanoimprint Lithography for GaN-

[67] Uhrmann, T. Patterned Sapphire Substrates (PSS): Making LEDs Brighter and Cheap‐

[68] Jukka, V, Milla-riina, V, & Juha, H. Soft Stamp Ultraviolet-nanoimprint Lithography

[69] Shi, J, Chen, J, Decanini, D, et al. Fabrication of Metallic Nanocavities by Soft UV

[70] Barbillon, G. Soft UV Nanoimprint Lithography: A Tool to Design Plasmonic Nano‐ biosensors. In : Kostovski E (ed.) Advances in Unconventional Lithography. Rijeka:

[71] Weng, Y J, Weng, Y. C, & Yang, S Y. Fabrication of Optical Waveguide Devices Using Gas-assisted UV micro/nanoimprinting with Soft Mold. Polymers for Advanced

[72] Spillane, S, Xu, Q, & Fattal, D. Fabrication of Nanophotonic Structures for Informa‐

[73] Lan, H, & Ding, Y. Ordering, Positioning and Uniformity of Quantum Dot Arrays.

[74] Meneou, K. Pathways for Quantum Dot Optoelectronics Fabrication Using Soft Nanoimprint Lithography. Dissertation, University of Illinois at Urbana-Champaign;

croelectronic Engineering (2009). , 86-2293.

vanced Materials (2005). , 17(12), 1554-1560.

[66] Hung, R. Luminus Devices, Inc. (2012).

From Ormostamp. Microelectronic Engineering (2009).

based Light-emitting Diodes. Thin Solid Films (2011). , 519-2241.

for Fabrication of Laser Diodes. Proceedings of the SPIE (2009). O.

Nanoimprint Lithography. Microelectronic Engineering (2009).

(2012). , 8-1184.

194 Updates in Advanced Lithography

er. (2012).

(2010).

InTech; (2011). , 3-14.

Technologies (2007). , 18(11), 876-882.

tion Processing. Proc. of SPIE (2008).

Nano today (2012). , 7(2), 94-123.

[76] Tommila, J, Tukiainen, A, & Viheriälä, J. Nanoimprint Lithography Patterned GaAs Templates for Site-controlled InAs Quantum Dots. Journal of Crystal Growth (2011). , 232(1), 183-186.

**Chapter 8**

**Provisional chapter**

**Sub-30 nm Plasmonic Nanostructures by Soft UV**

**Sub-30 nm Plasmonic Nanostructures by Soft UV**

The capability to fabricate nanostructures of high density and high resolution over large areas is important point for fundamental and applied research as subwavelength optical nanostructures, optoelectronics and biosensors [1–5]. Various lithographic techniques such as focused ion beam lithography [6] and electron beam lithography [7, 8] are mainly used to pattern sub-100 nm structures on large surfaces. However, these two techniques are slow to obtain these surfaces and their equipments are expensive. Moreover, charge effect on insulating surface can alter the regularity of the pattern shape. Thus, these techniques will not be suitable for a mass production. In addition, alternative methods emerged in the past decades, and are not very expensive and fast to realize high density nanostructures. Moreover, these methods offer a better compatibility for biology and chemical applications [9, 10]. One of these recent techniques is the soft UV Nanoimprint Lithography, which is very promising for the periodic nanostructures fabrication with a high density and high resolution on large surfaces for a reasonable cost [11, 12]. However, a limiting factor of UV-NIL is the resolution of the fabricated molds [13, 14]. In soft UV-NIL, cast molding processes are used for flexible mold realization. An advantage of the soft UV-NIL technique is the obtaining of a great patterns homogeneity on a large zone. This chapter proposes to present in details the soft UV-NIL and its use for the fabrication of sub-30 nm plasmonic structure on large area.

**2. Principle** & **fabrication process of soft UV nanoimprint lithography**

Soft UV-NIL has two fundamental steps and is illustrated in figure 1. Firstly, a UV transparent mold with nanostructures on its surface is pressed into a UV sensitive resist. The UV sensitive resist, which is liquid at room temperature, is typically spin coated on

> ©2012 Barbillon, licensee InTech. This is an open access chapter 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 Barbillon; 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 Barbillon, 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.

**Nanoimprint Lithography**

**Nanoimprint Lithography**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Grégory Barbillon

Grégory Barbillon

10.5772/56119

**1. Introduction**

**2.1. Principle of UV-NIL**

http://dx.doi.org/10.5772/56119

**Provisional chapter**

## **Sub-30 nm Plasmonic Nanostructures by Soft UV Nanoimprint Lithography Sub-30 nm Plasmonic Nanostructures by Soft UV Nanoimprint Lithography**

Grégory Barbillon Grégory Barbillon

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

http://dx.doi.org/10.5772/56119 10.5772/56119

## **1. Introduction**

The capability to fabricate nanostructures of high density and high resolution over large areas is important point for fundamental and applied research as subwavelength optical nanostructures, optoelectronics and biosensors [1–5]. Various lithographic techniques such as focused ion beam lithography [6] and electron beam lithography [7, 8] are mainly used to pattern sub-100 nm structures on large surfaces. However, these two techniques are slow to obtain these surfaces and their equipments are expensive. Moreover, charge effect on insulating surface can alter the regularity of the pattern shape. Thus, these techniques will not be suitable for a mass production. In addition, alternative methods emerged in the past decades, and are not very expensive and fast to realize high density nanostructures. Moreover, these methods offer a better compatibility for biology and chemical applications [9, 10]. One of these recent techniques is the soft UV Nanoimprint Lithography, which is very promising for the periodic nanostructures fabrication with a high density and high resolution on large surfaces for a reasonable cost [11, 12]. However, a limiting factor of UV-NIL is the resolution of the fabricated molds [13, 14]. In soft UV-NIL, cast molding processes are used for flexible mold realization. An advantage of the soft UV-NIL technique is the obtaining of a great patterns homogeneity on a large zone. This chapter proposes to present in details the soft UV-NIL and its use for the fabrication of sub-30 nm plasmonic structure on large area.

## **2. Principle** & **fabrication process of soft UV nanoimprint lithography**

## **2.1. Principle of UV-NIL**

Soft UV-NIL has two fundamental steps and is illustrated in figure 1. Firstly, a UV transparent mold with nanostructures on its surface is pressed into a UV sensitive resist. The UV sensitive resist, which is liquid at room temperature, is typically spin coated on

©2012 Barbillon, licensee InTech. This is an open access chapter distributed under the terms of the Creative

the substrate. The UV transparent stamp is deposited on the substrate with a low pressure between 0 and 1 bar [15], at room temperature. Next, the soft stamp is released. The first step duplicates the nanopattern of the transparent mold in the UV sensitive resist. The second step is the removal of residual layer of UV sensitive resist. This step is realized by anisotropic etching such as Reactive Ion Etching (RIE) in order to obtain the desired patterns into the UV resist. During the nanoimprint step, the resist is cured by a UV source (for example, a simple UV lamp or other available systems).

10.5772/56119

199

http://dx.doi.org/10.5772/56119

an accelerating voltage of 100 kV. Then, the sample is developed in a methylisobutylketone (MIBK)/isopropanol (IPA) solution at room temperature for 35 s followed by a rinsing of 10 s in IPA and thoroughly dried with N2 gas. Next, the patterns designed in PMMA are transferred into the silicon substrate via a suitable RIE process. The RIE conditions are: 10 sccm for O2, 45 sccm for SF6 with P = 30 W, a pressure of 50 mTorr and an autopolarization voltage of 85V [15, 17]. Then, the PMMA mask is removed with a lift-off process in trichloroethylene at 80 ◦C. Next, the Si master mold surface is treated with HF and H2O2 in order to obtain a SiO2 thin surface layer, then modified with an anti-sticking layer (TMCS: TriMethylChloroSilane) for decreasing the surface energy (Si+TMCS = 28.9 mN/m) in order to easy remove the PDMS molds [17, 19, 20]. In figure 2, SEM images of

Sub-30 nm Plasmonic Nanostructures by Soft UV Nanoimprint Lithography

27 nm

**Figure 2.** SEM images of Si master mold designed by EBL: nanoholes of diameter ∼ 27 nm and ∼ 78 nm of periodicity. (a)

For the master mold, the dimensions of obtained nanoholes are ∼ 27 nm of diameter and ∼ 78 nm of periodicity on a zone of 1 cm2. The programmed dimensions for the electron beam lithography are 25 nm for the diameter and 75 nm for the pitch. Consequently, the dimensions obtained experimentally are in good agreement with those programmed by taking into account the errors of measurements, which could be observed on the

78 nm

nanostructures obtained on a Si substrate are presented.

(a)

450 nm

(b)

zone of some *µ*m<sup>2</sup> and (b) zoom of the previous zone.

nanostructure dimensions with SEM.

150 nm

The main advantages of soft UV-NIL are the transparent flexible stamp and a low viscosity UV-curable resist. The replication of the flexible stamps is typically obtained by molding and curing a polymer from a 3D template. The UV transparent flexible stamp fabrication is mainly realized with poly(dimethylsiloxane) PDMS [16–18]. PDMS offers a good chemical stability and a high optical transparency. Moreover, the deformation risk of the soft stamp is minimized with the use of low viscosity UV-curable resists, which permits 3D patterning at low pressure without any heating cycles.

**Figure 1.** Scheme of UV nanoimprint lithography process: (1) Imprint with the stamp (+ curing with UV light), (2) Stamp withdrawal and (3) Removal of the residual layer with RIE process.

## **2.2. Master mold fabrication**

The master mold used to fabricate flexible stamp is made using the electron beam lithography (EBL). EBL technique allows an excellent accuracy, a very high resolution, and an capability to pattern a large variety of geometries. In the example presented here for the Si master mold fabrication, a PMMA layer of 100 nm (PolyMethylMethAcrylate A2 resist: 950 PMMA A2, MicroChem Corp.) is deposited by spin-coating on Si substrate and baked at 170 ◦<sup>C</sup> during 30 min. An EBL system (Leica EBPG5000+) is used to expose PMMA A2, employing an accelerating voltage of 100 kV. Then, the sample is developed in a methylisobutylketone (MIBK)/isopropanol (IPA) solution at room temperature for 35 s followed by a rinsing of 10 s in IPA and thoroughly dried with N2 gas. Next, the patterns designed in PMMA are transferred into the silicon substrate via a suitable RIE process. The RIE conditions are: 10 sccm for O2, 45 sccm for SF6 with P = 30 W, a pressure of 50 mTorr and an autopolarization voltage of 85V [15, 17]. Then, the PMMA mask is removed with a lift-off process in trichloroethylene at 80 ◦C. Next, the Si master mold surface is treated with HF and H2O2 in order to obtain a SiO2 thin surface layer, then modified with an anti-sticking layer (TMCS: TriMethylChloroSilane) for decreasing the surface energy (Si+TMCS = 28.9 mN/m) in order to easy remove the PDMS molds [17, 19, 20]. In figure 2, SEM images of nanostructures obtained on a Si substrate are presented.

2 Lithography

simple UV lamp or other available systems).

low pressure without any heating cycles.

withdrawal and (3) Removal of the residual layer with RIE process.

**2.2. Master mold fabrication**

the substrate. The UV transparent stamp is deposited on the substrate with a low pressure between 0 and 1 bar [15], at room temperature. Next, the soft stamp is released. The first step duplicates the nanopattern of the transparent mold in the UV sensitive resist. The second step is the removal of residual layer of UV sensitive resist. This step is realized by anisotropic etching such as Reactive Ion Etching (RIE) in order to obtain the desired patterns into the UV resist. During the nanoimprint step, the resist is cured by a UV source (for example, a

The main advantages of soft UV-NIL are the transparent flexible stamp and a low viscosity UV-curable resist. The replication of the flexible stamps is typically obtained by molding and curing a polymer from a 3D template. The UV transparent flexible stamp fabrication is mainly realized with poly(dimethylsiloxane) PDMS [16–18]. PDMS offers a good chemical stability and a high optical transparency. Moreover, the deformation risk of the soft stamp is minimized with the use of low viscosity UV-curable resists, which permits 3D patterning at

Flexible stamp

1. Press Stamp

resist Si substrate

2. Release Stamp

3. RIE process

**Figure 1.** Scheme of UV nanoimprint lithography process: (1) Imprint with the stamp (+ curing with UV light), (2) Stamp

The master mold used to fabricate flexible stamp is made using the electron beam lithography (EBL). EBL technique allows an excellent accuracy, a very high resolution, and an capability to pattern a large variety of geometries. In the example presented here for the Si master mold fabrication, a PMMA layer of 100 nm (PolyMethylMethAcrylate A2 resist: 950 PMMA A2, MicroChem Corp.) is deposited by spin-coating on Si substrate and baked at 170 ◦<sup>C</sup> during 30 min. An EBL system (Leica EBPG5000+) is used to expose PMMA A2, employing

**Figure 2.** SEM images of Si master mold designed by EBL: nanoholes of diameter ∼ 27 nm and ∼ 78 nm of periodicity. (a) zone of some *µ*m<sup>2</sup> and (b) zoom of the previous zone.

For the master mold, the dimensions of obtained nanoholes are ∼ 27 nm of diameter and ∼ 78 nm of periodicity on a zone of 1 cm2. The programmed dimensions for the electron beam lithography are 25 nm for the diameter and 75 nm for the pitch. Consequently, the dimensions obtained experimentally are in good agreement with those programmed by taking into account the errors of measurements, which could be observed on the nanostructure dimensions with SEM.

## **2.3. Flexible UV-transparent stamp fabrication**

#### *2.3.1. Polymeric materials for stamp fabrication*

An advantage of polymeric materials for the replication of original master mold in the nanoimprint process is the low cost of fabrication compared to EBL. With a single and expensive master mold, a large number of polymeric stamps can be replicate and use in the nanoimprint process. Moreover, the excellent flexibility of the elastomeric material offers a good contact between the stamp and the substrate on large areas at low pressures (tens of bars) and on non-planar substrates. Various polymeric materials have been used for stamp fabrication as cross-linked novolak based epoxy resin [21], polycarbonate resins [22], fluoropolymer materials and tetrafluoroethylene (PTFE) [23]. In addition, poly(dimethylsiloxanes) (PDMS) have very interesting properties as a stamp elastomer. The first of these properties is a conformal adhesion of the stamp with the substrate on large areas without any external pressure.

10.5772/56119

201

http://dx.doi.org/10.5772/56119

*2.3.3. Bilayer hard-PDMS/PDMS stamp fabrication process*

cross-linking.

anti-sticking layer.

**2.4. Soft UV-NIL process**

pressure increases due to the low Young′

To increase the resolution and fidelity of structures, the mechanical properties of the soft stamp need to be improved. Odom *et al.* [28] developed a bilayer stamp of hard-PDMS and standard PDMS, which presents, as advantages, a rigid layer to obtain a high resolution pattern transfer and an elastic support for obtaining a conformal contact even at a low imprint pressure. The hard-PDMS has an attractive property: a lower viscosity of its prepolymer in comparison to standard PDMS. The hard-PDMS prepolymer viscosity is obtained by decreasing of the chain length during its preparation. Thus, the accuracy of replication is improved especially for high-density and small patterns. Another groups [23, 29] also studied the viscosity reduction of the prepolymer for an good replication of the master mold. In this case, the PDMS prepolymer viscosity was decreased with the introduction of a solvent in the mixture. This solvent used with an excessive amount of modulator allows to delay the

Sub-30 nm Plasmonic Nanostructures by Soft UV Nanoimprint Lithography

The figure 3 represents the fabrication process of the bilayer hard-PDMS/PDMS stamp. The hard-PDMS is a specific thermocured siloxane polymer based on copolymers Vinylmethylsiloxane-Dimethylsiloxane (VDT301) and Methyl-hydrosilane-Dimethylsiloxane (HMS-301) from ABCR, respectively, 34 g and 11 g [27]. In addition, before degassing the mixture with a mixing machine we add 50 *µ*L of platinum catalyst, and 0.5% w/w modulator tetramethyl-tetravinyl cyclotetrasiloxane from FLUKA to the mixture [30]. Then, the hard-PDMS is spin coated on the silicon master mold and the used thickness is mainly 5-8 *µ*m and supported by a standard PDMS layer (∼1.5 mm) (see figure 3). The standard PDMS layer keeps a good flexibility and adaptation on the spin coated wafer during imprint transfer [31]. Then, the bilayer stamp is placed on a glass carrier. The standard PDMS (RTV 615) with its curing agent are mixed before deposit on the thin hard layer PDMS (H-PDMS). Finally, the sample is cured at 60 ◦C during 24 hours and treated with a TriMethylChloroSilane (TMCS)

For the chosen example of nanostructures, the bilayer stamp is very suitable. Indeed, the obtained nanodots dimensions are ∼ 28 nm of diameter, the periodicity of ∼ 78 nm, and the height of ∼ 60 nm. The figure 4 represents an AFM image of the hard-PDMS/PDMS stamp.

In order to realize the best pattern replication, the control of possible deformations of the replicated pattern is a very important point. Indeed, the PDMS stamp flexibility allows to obtain a conformal adhesion with the substrate at low pressure, during the imprint process. Nevertheless, structures deformations with a high aspect ratio can occur when the applied

cannot be avoided, their control and reduction are keypoints and depend on the wished application. A study of the resolution was made by some groups. KarlSuss GmbH has studied the replication of nanoholes (340 nm of diameter) in AMONIL resist. They obtained a diameter of 340 nm ± 5%, and a period uniformity of 2 nm over a 6 inch area [32]. During the imprint step, the resist flow depends strongly on the applied pressure and determines the accuracy on the dimensions of the imprinted nanostructures. In order to reduce local distortions, the pressure of the imprint step must be minimized to obtain a good depth of

s modulus. In the case where these deformations

*2.4.1. Optimization of pressure and decreasing of possible deformations for imprint step*

Indeed, PDMS has other attractive properties: (1) its flexibility, which allows a good accuracy of relief shapes replication in the fabrication of the patterning elements, (2) its low Young′ s modulus (750 KPa) [24] and its low surface energy which allows conformal contact with surface without applied pressure and non-destructive release from designed structures [25], (3) its good optical transparency to a UV light source [26], and (4) its commercial availability in bulk quantities at low cost. The standard PDMS has some advantages, however a number of properties inherent to PDMS limits its performances in the soft UV-NIL. First, the Young′ s modulus of standard PDMS is low and can limit the fabrication of high density patterns at a sub-100 nm scale due to the collapse of structures. Second, the surface energy (∼ 20 mN/m) of PDMS is not low enough and that does not make it possible the duplication of profiles with a high fidelity. Moreover, the high elasticity and thermal expansion can lead to deformations and distortions during the fabrication process. In general, long range deformations can be avoided by using a thin glass backplane which preserves a global flexibility. In addition, short range deformations can be avoided only by increasing the elastic modulus of PDMS (see paragraph 2.3.2).

#### *2.3.2. Standard PDMS stamp fabrication process*

Standard PDMS stamps are mainly realized with a mixture of two commercial PDMS components: (10:1) PDMS RTV 615 siloxane oligomer and RTV 615 cross-linking oligomers (General Electric). This mixture is deposited then casted on the Si master mold and degassed in a dessicator. Standard PDMS is cured at 60 ◦C for 24 h in order to reduce roughness and to avoid a build up of tension due to thermal shrinkage. If longer curing times and higher temperatures are used, then the elastic modulus and hardness of the polymer are increased (up to x2). However, a higher roughness and deformations can be observed. The stamps are cooled to room temperature, thoroughly peeled off from the master mold and treated with the TriMethylChloroSilane (TMCS) anti-sticking layer in order to reduce the low PDMS surface energy. These stamps are not suitable for the replication of sub-100 nm structures (or with a high aspect ratio) due to the low elastic modulus of PDMS. To solve this problem of low Young′ s modulus, a modified PDMS called hard-PDMS was already developed.

## *2.3.3. Bilayer hard-PDMS/PDMS stamp fabrication process*

4 Lithography

**2.3. Flexible UV-transparent stamp fabrication**

An advantage of polymeric materials for the replication of original master mold in the nanoimprint process is the low cost of fabrication compared to EBL. With a single and expensive master mold, a large number of polymeric stamps can be replicate and use in the nanoimprint process. Moreover, the excellent flexibility of the elastomeric material offers a good contact between the stamp and the substrate on large areas at low pressures (tens of bars) and on non-planar substrates. Various polymeric materials have been used for stamp fabrication as cross-linked novolak based epoxy resin [21], polycarbonate resins [22], fluoropolymer materials and tetrafluoroethylene (PTFE) [23]. In addition, poly(dimethylsiloxanes) (PDMS) have very interesting properties as a stamp elastomer. The first of these properties is a conformal adhesion of the stamp with the substrate on large

Indeed, PDMS has other attractive properties: (1) its flexibility, which allows a good accuracy of relief shapes replication in the fabrication of the patterning elements, (2) its low Young′

modulus (750 KPa) [24] and its low surface energy which allows conformal contact with surface without applied pressure and non-destructive release from designed structures [25], (3) its good optical transparency to a UV light source [26], and (4) its commercial availability in bulk quantities at low cost. The standard PDMS has some advantages, however a number of properties inherent to PDMS limits its performances in the soft UV-NIL. First, the Young′

modulus of standard PDMS is low and can limit the fabrication of high density patterns at a sub-100 nm scale due to the collapse of structures. Second, the surface energy (∼ 20 mN/m) of PDMS is not low enough and that does not make it possible the duplication of profiles with a high fidelity. Moreover, the high elasticity and thermal expansion can lead to deformations and distortions during the fabrication process. In general, long range deformations can be avoided by using a thin glass backplane which preserves a global flexibility. In addition, short range deformations can be avoided only by increasing the elastic modulus of PDMS

Standard PDMS stamps are mainly realized with a mixture of two commercial PDMS components: (10:1) PDMS RTV 615 siloxane oligomer and RTV 615 cross-linking oligomers (General Electric). This mixture is deposited then casted on the Si master mold and degassed in a dessicator. Standard PDMS is cured at 60 ◦C for 24 h in order to reduce roughness and to avoid a build up of tension due to thermal shrinkage. If longer curing times and higher temperatures are used, then the elastic modulus and hardness of the polymer are increased (up to x2). However, a higher roughness and deformations can be observed. The stamps are cooled to room temperature, thoroughly peeled off from the master mold and treated with the TriMethylChloroSilane (TMCS) anti-sticking layer in order to reduce the low PDMS surface energy. These stamps are not suitable for the replication of sub-100 nm structures (or with a high aspect ratio) due to the low elastic modulus of PDMS. To solve this problem of

s modulus, a modified PDMS called hard-PDMS was already developed.

s

s

*2.3.1. Polymeric materials for stamp fabrication*

areas without any external pressure.

(see paragraph 2.3.2).

low Young′

*2.3.2. Standard PDMS stamp fabrication process*

To increase the resolution and fidelity of structures, the mechanical properties of the soft stamp need to be improved. Odom *et al.* [28] developed a bilayer stamp of hard-PDMS and standard PDMS, which presents, as advantages, a rigid layer to obtain a high resolution pattern transfer and an elastic support for obtaining a conformal contact even at a low imprint pressure. The hard-PDMS has an attractive property: a lower viscosity of its prepolymer in comparison to standard PDMS. The hard-PDMS prepolymer viscosity is obtained by decreasing of the chain length during its preparation. Thus, the accuracy of replication is improved especially for high-density and small patterns. Another groups [23, 29] also studied the viscosity reduction of the prepolymer for an good replication of the master mold. In this case, the PDMS prepolymer viscosity was decreased with the introduction of a solvent in the mixture. This solvent used with an excessive amount of modulator allows to delay the cross-linking.

The figure 3 represents the fabrication process of the bilayer hard-PDMS/PDMS stamp. The hard-PDMS is a specific thermocured siloxane polymer based on copolymers Vinylmethylsiloxane-Dimethylsiloxane (VDT301) and Methyl-hydrosilane-Dimethylsiloxane (HMS-301) from ABCR, respectively, 34 g and 11 g [27]. In addition, before degassing the mixture with a mixing machine we add 50 *µ*L of platinum catalyst, and 0.5% w/w modulator tetramethyl-tetravinyl cyclotetrasiloxane from FLUKA to the mixture [30]. Then, the hard-PDMS is spin coated on the silicon master mold and the used thickness is mainly 5-8 *µ*m and supported by a standard PDMS layer (∼1.5 mm) (see figure 3). The standard PDMS layer keeps a good flexibility and adaptation on the spin coated wafer during imprint transfer [31]. Then, the bilayer stamp is placed on a glass carrier. The standard PDMS (RTV 615) with its curing agent are mixed before deposit on the thin hard layer PDMS (H-PDMS). Finally, the sample is cured at 60 ◦C during 24 hours and treated with a TriMethylChloroSilane (TMCS) anti-sticking layer.

For the chosen example of nanostructures, the bilayer stamp is very suitable. Indeed, the obtained nanodots dimensions are ∼ 28 nm of diameter, the periodicity of ∼ 78 nm, and the height of ∼ 60 nm. The figure 4 represents an AFM image of the hard-PDMS/PDMS stamp.

## **2.4. Soft UV-NIL process**

## *2.4.1. Optimization of pressure and decreasing of possible deformations for imprint step*

In order to realize the best pattern replication, the control of possible deformations of the replicated pattern is a very important point. Indeed, the PDMS stamp flexibility allows to obtain a conformal adhesion with the substrate at low pressure, during the imprint process. Nevertheless, structures deformations with a high aspect ratio can occur when the applied pressure increases due to the low Young′ s modulus. In the case where these deformations cannot be avoided, their control and reduction are keypoints and depend on the wished application. A study of the resolution was made by some groups. KarlSuss GmbH has studied the replication of nanoholes (340 nm of diameter) in AMONIL resist. They obtained a diameter of 340 nm ± 5%, and a period uniformity of 2 nm over a 6 inch area [32]. During the imprint step, the resist flow depends strongly on the applied pressure and determines the accuracy on the dimensions of the imprinted nanostructures. In order to reduce local distortions, the pressure of the imprint step must be minimized to obtain a good depth of

10.5772/56119

203

http://dx.doi.org/10.5772/56119

X: 750 n<sup>m</sup>

Sub-30 nm Plasmonic Nanostructures by Soft UV Nanoimprint Lithography

**Figure 4.** AFM image of the dots in the bilayer hard-PDMS/PDMS stamp (periodicity: ∼ 78 nm, diameter (FWHM): ∼ 28 nm,

stamp pattern in order to decrease the residual thickness of resist layer. One of these groups conducted a study on the AMONIL resist and they obtained a structure depth of 170 nm and a residual layer of 36 nm [32]. Thus, a very small thickness of the residual AMONIL layer

Various UV-sensitive resists as the NXR 2010 and the AMONIL are available. These two resists exhibit good performance for resolution and etching resistance. The AMONIL resist was used for its low cost compared to the NXR 2010 resist, and its excellent time of conservation. AMONIL resist is a mixture of organic and inorganic compounds having a surface energy of 39.5 mN/m. AMONIL MMS10 from AMO GmbH is used and spin coated on the top of a Ge/PMMA A2 bilayer (10 nm/100 nm thick, respectively), which allows the AMONIL lift-off after curing (see figure 5(a)). The Ge layer is used to improve the selectivity of the former one over the PMMA layer [20]. An AMONIL thickness of 70 nm is chosen in order to minimize the residual thickness of AMONIL. Then, the imprint process is performed in AMONIL with UV exposure at 365 nm wavelength for 10 min and a pressure of 0.7 bar. All these parameters were optimized for the fabrication of nanostructures, which use the bilayer hard-PDMS/PDMS stamp obtained from Si master mold. The figure 5(b) represents the imprint in AMONIL. The dimensions obtained for nanoholes imprinted in AMONIL are ∼ 28 nm of diameter and ∼ 78 nm of periodicity and these values are in good agreement

Firstly, the residual AMONIL thickness in the ground of the nanoholes must be removed by a suitable RIE process. For the removal of the residual layer, the etch gases used for RIE

**2.5. Soft UV-NIL for sub-30 nm plasmonic nanostructures fabrication**

and height: ∼ 60 nm).

was observed and allowed a good replication.

with the dimensions of nanoholes of Si master mold.

*2.5.2. Plasmonic nanodisks fabrication*

*2.5.1. Soft UV-NIL in AMONIL*

**Figure 3.** Principle scheme of the fabrication process of the hard-PDMS/PDMS stamp.

the resist in the stamp nanostructures. Cattoni *et al.* [33] demonstrated that the pressure could be reduced to 0.7 bar (thus that Shi et al. [19] ) and combined with a UV exposure of 10 min (*λ* = 365 nm, dose of 2 J/cm2), a high quality of nanostructures shape was obtained.

#### *2.4.2. Optimization of the thickness of the resist residual layer*

Another keypoint of the NIL process is the removal of the residual layer of the resist. In Thermal-NIL and standard UV-NIL, a rigid mold and a high pressure are used, and a thin residual layer of resist is mainly left between the mold protrusions and the substrate. It acts as a soft cushion layer that prevents direct impact of these fragile nanostructures and the substrate. The residual layer is typically withdrawn by RIE. The RIE step can strongly affect the initial shape and size of nanostructures. In addition, in the soft UV-NIL, which uses flexible stamps, the residual layer can be reduced by adapting the original resist thickness to the height (or depth following the desired pattern) of the stamp pattern. Several groups demonstrated the adaptation of the initial resist thickness to the height or depth of the

**Figure 4.** AFM image of the dots in the bilayer hard-PDMS/PDMS stamp (periodicity: ∼ 78 nm, diameter (FWHM): ∼ 28 nm, and height: ∼ 60 nm).

stamp pattern in order to decrease the residual thickness of resist layer. One of these groups conducted a study on the AMONIL resist and they obtained a structure depth of 170 nm and a residual layer of 36 nm [32]. Thus, a very small thickness of the residual AMONIL layer was observed and allowed a good replication.

## **2.5. Soft UV-NIL for sub-30 nm plasmonic nanostructures fabrication**

## *2.5.1. Soft UV-NIL in AMONIL*

6 Lithography

1. Master mold designed by EBL in PMMA + Transfer of paerns in Si by RIE (O2 + SF6) + Li‐off of PMMA

Silicon

2. Hard‐PDMS spin coated (thickness 5‐8 µm)

H‐PDMS

Silicon

3. PDMS Casng (≈ 1.5 mm) and curing at 60°C during 24h

Silicon

4. Bilayer Hard‐PDMS/PDMS Stamp Release

H‐PDMS PDMS

Silicon

the resist in the stamp nanostructures. Cattoni *et al.* [33] demonstrated that the pressure could be reduced to 0.7 bar (thus that Shi et al. [19] ) and combined with a UV exposure of 10 min (*λ* = 365 nm, dose of 2 J/cm2), a high quality of nanostructures shape was obtained.

Another keypoint of the NIL process is the removal of the residual layer of the resist. In Thermal-NIL and standard UV-NIL, a rigid mold and a high pressure are used, and a thin residual layer of resist is mainly left between the mold protrusions and the substrate. It acts as a soft cushion layer that prevents direct impact of these fragile nanostructures and the substrate. The residual layer is typically withdrawn by RIE. The RIE step can strongly affect the initial shape and size of nanostructures. In addition, in the soft UV-NIL, which uses flexible stamps, the residual layer can be reduced by adapting the original resist thickness to the height (or depth following the desired pattern) of the stamp pattern. Several groups demonstrated the adaptation of the initial resist thickness to the height or depth of the

**Figure 3.** Principle scheme of the fabrication process of the hard-PDMS/PDMS stamp.

*2.4.2. Optimization of the thickness of the resist residual layer*

H‐PDMS PDMS

> Various UV-sensitive resists as the NXR 2010 and the AMONIL are available. These two resists exhibit good performance for resolution and etching resistance. The AMONIL resist was used for its low cost compared to the NXR 2010 resist, and its excellent time of conservation. AMONIL resist is a mixture of organic and inorganic compounds having a surface energy of 39.5 mN/m. AMONIL MMS10 from AMO GmbH is used and spin coated on the top of a Ge/PMMA A2 bilayer (10 nm/100 nm thick, respectively), which allows the AMONIL lift-off after curing (see figure 5(a)). The Ge layer is used to improve the selectivity of the former one over the PMMA layer [20]. An AMONIL thickness of 70 nm is chosen in order to minimize the residual thickness of AMONIL. Then, the imprint process is performed in AMONIL with UV exposure at 365 nm wavelength for 10 min and a pressure of 0.7 bar. All these parameters were optimized for the fabrication of nanostructures, which use the bilayer hard-PDMS/PDMS stamp obtained from Si master mold. The figure 5(b) represents the imprint in AMONIL. The dimensions obtained for nanoholes imprinted in AMONIL are ∼ 28 nm of diameter and ∼ 78 nm of periodicity and these values are in good agreement with the dimensions of nanoholes of Si master mold.

#### *2.5.2. Plasmonic nanodisks fabrication*

Firstly, the residual AMONIL thickness in the ground of the nanoholes must be removed by a suitable RIE process. For the removal of the residual layer, the etch gases used for RIE

## 1. Imprint in AMONIL resist with a UV lamp (λ = 365 nm) during 10 min and P = 0.7 bar

10.5772/56119

205

http://dx.doi.org/10.5772/56119

(a) (b)

**Figure 6.** SEM images of plasmonic nanodisks: (a) a zone of some *µ*m<sup>2</sup> (scale bar = 150 nm), (b) diameter ∼ 28 nm, and

of soft UV-NIL to replicate sub-30 nm nanostructures with high homogeneity at the whole pattern surface is demonstrated. To conclude, we present an example of a fabrication by soft UV-NIL on large area (1 cm2) of plasmonic nanostructures with potential applications in biosensors and photonics. Finally, we believe that the soft UV-NIL technique will play quickly an important role as a powerful and versatile tool for the nanostructures fabrication.

<sup>⋆</sup> Address all correspondence to: gregory.barbillon@laposte.net

Electronics & Optoelectronics 12 91-97.

Chem. B 103: 9846-9853.

1 Laboratory of Photonics and Nanostructures CNRS UPR 20, Marcoussis, France

gel gratings for optical applications. J. Vac. Sci. Technol. B 21: 660-663.

2 Laboratory of Lasers Physics CNRS UMR 7538, University Paris 13, Villetaneuse, France

[1] Li M, Tan H, Wang J, Chou S.Y (2003) Large area direct nanoimprinting of SiO2-TiO2

[2] Chegel V, Lucas B, Guo J, Lopatynskyi A, Lopatynska O, Poperenko L (2009) Detection of biomolecules using optoelectronic biosensor based on localized surface plasmon resonance. Nanoimprint lithography approach. Semiconductor Physics, Quantum

[3] Jensen T.R, Duval M.L, Kelly K.L, Lazarides A.A, Schatz G.C, Van Duyne R.P (1999) Nanosphere Lithography : Effect of the External Dielectric Medium on the Surface Plasmon Resonance Spectrum of a Periodic Array of Silver Nanoparticles. J. Phys.

periodicity ∼ 78 nm.

**Author details**

**References**

Grégory Barbillon1,2,<sup>⋆</sup>

30 nm

Sub-30 nm Plasmonic Nanostructures by Soft UV Nanoimprint Lithography

**Figure 5.** (a) Scheme of the trilayer soft UV-NIL process, and (b) SEM image of imprint in AMONIL with bilayer hard-PDMS/PDMS stamp.

are O2 and CHF3. Ge is removed by RIE using SF6 [34]. For the removal of the PMMA A2, the gas used for RIE is O2. A good selectivity between PMMA and AMONIL is obtained [15, 17]. The next step is to evaporate a gold thin layer (30 nm) in order to realize the plasmonic nanodisks. Previously, an adhesion layer (Cr) for gold is evaporated (3-5 nm). Then, a lift-off in acetone is used to remove the PMMA underlayer (+ AMONIL/Ge) in order to obtain the sub-30 nm plasmonic nanodisks. The figure 6 presents the results obtained with the bilayer hard-PDMS/PDMS stamp in AMONIL. We observe that the dimensions of plasmonic nanodisks are in good agreement with the dimensions obtained with the imprint in AMONIL. Then, the plasmonic nanostructures can be used as Localized Surface Plasmon Resonance biosensors [35].

## **3. Conclusion**

In this chapter, we have demonstrated the fabrication of sub-30 nm plasmonic nanostructures with the soft UV-NIL technique. The soft UV-NIL is composed by three separate steps: the fabrication of the master mold, the replication of the flexible hard-PDMS/PDMS stamp from the Si master mold, and the imprinting process by using the bilayer hard-PDMS/PDMS stamp. All these steps are very important in order to obtain a very good quality of the final result, in terms of resolution and line edge roughness of the nanostructures. A master mold fabrication process based on EBL is presented in details. Then, the replication of the soft polymeric stamp, based on a composite hard-PDMS/PDMS bilayer, is presented. The ability

**Figure 6.** SEM images of plasmonic nanodisks: (a) a zone of some *µ*m<sup>2</sup> (scale bar = 150 nm), (b) diameter ∼ 28 nm, and periodicity ∼ 78 nm.

of soft UV-NIL to replicate sub-30 nm nanostructures with high homogeneity at the whole pattern surface is demonstrated. To conclude, we present an example of a fabrication by soft UV-NIL on large area (1 cm2) of plasmonic nanostructures with potential applications in biosensors and photonics. Finally, we believe that the soft UV-NIL technique will play quickly an important role as a powerful and versatile tool for the nanostructures fabrication.

## **Author details**

8 Lithography

stamp.

Resonance biosensors [35].

**3. Conclusion**

1. Imprint in AMONIL resist with a UV lamp (λ = 365 nm) during 10 min and P = 0.7 bar

> H‐PDMS PDMS

> > Silicon

2. RIE of residual trilayer

Silicon

3. Au Evaporaon & Li‐off of PMMA

Silicon

AMONIL (70 nm) Ge (10 nm)

‐ Ge : SF6 ‐ PMMA: O2

(b)

Gases for RIE of the trilayer: ‐ AMONIL: O2 and CHF3

> PMMA (100 nm) Au (30 nm)

30 nm

(a)

**Figure 5.** (a) Scheme of the trilayer soft UV-NIL process, and (b) SEM image of imprint in AMONIL with bilayer hard-PDMS/PDMS

are O2 and CHF3. Ge is removed by RIE using SF6 [34]. For the removal of the PMMA A2, the gas used for RIE is O2. A good selectivity between PMMA and AMONIL is obtained [15, 17]. The next step is to evaporate a gold thin layer (30 nm) in order to realize the plasmonic nanodisks. Previously, an adhesion layer (Cr) for gold is evaporated (3-5 nm). Then, a lift-off in acetone is used to remove the PMMA underlayer (+ AMONIL/Ge) in order to obtain the sub-30 nm plasmonic nanodisks. The figure 6 presents the results obtained with the bilayer hard-PDMS/PDMS stamp in AMONIL. We observe that the dimensions of plasmonic nanodisks are in good agreement with the dimensions obtained with the imprint in AMONIL. Then, the plasmonic nanostructures can be used as Localized Surface Plasmon

In this chapter, we have demonstrated the fabrication of sub-30 nm plasmonic nanostructures with the soft UV-NIL technique. The soft UV-NIL is composed by three separate steps: the fabrication of the master mold, the replication of the flexible hard-PDMS/PDMS stamp from the Si master mold, and the imprinting process by using the bilayer hard-PDMS/PDMS stamp. All these steps are very important in order to obtain a very good quality of the final result, in terms of resolution and line edge roughness of the nanostructures. A master mold fabrication process based on EBL is presented in details. Then, the replication of the soft polymeric stamp, based on a composite hard-PDMS/PDMS bilayer, is presented. The ability Grégory Barbillon1,2,<sup>⋆</sup>


## **References**


[4] Barbillon G, Bijeon J.L, Lérondel G, Plain J, Royer P (2008) Detection of chemical molecules with integrated plasmonic glass nanotips. Surface Science 602: L119-L122.

10.5772/56119

207

http://dx.doi.org/10.5772/56119

[17] Barbillon G, Hamouda F, Held S, Gogol P, Bartenlian B (2010) Gold nanoparticles by soft UV nanoimprint lithography coupled to a lift-off process for plasmonic sensing of

[18] Hamouda F, Sahaf H, Held S, Barbillon G, Gogol P, Moyen E, Aassime A, Moreau J, Canva M, Lourtioz J.M, Hanb*u*¨cken M, Bartenlian B (2011) Large area nanopatterning by combined anodic aluminum oxide and soft UV-NIL technologies for applications in

[19] Shi J, Chen J, Decanini D, Chen Y, Haghiri-Gosnet A.M (2009) Fabrication of metallic nanocavities by soft UV nanoimprint lithography. Microelectron. Eng. 86: 596-599.

[20] Chen J, Shi J, Decanini D, Cambril E, Chen Y, Haghiri-Gosnet A.M (2009) Gold nanohole arrays for biochemical sensing fabricated by soft UV nanoimprint

[21] Pfeiffer K, Fink M, Ahrens G, Gruetzner G, Reuther F, Seekamp J, Zankovych S, Torres C.S, Maximov I, Beck M, Graczyk M, Montelius L, Schulz H, Scheer H.C, Steingrueber F (2002) Polymer stamps for nanoimprinting. Microelectron. Eng. 61-62: 393-398.

nanoimprint lithography by replica of high glass transition temperature polymers. J.

[23] Kang H, Lee J, Park J, Lee H.H (2006) An improved method of preparing composite

[24] Bender M, Plachteka U, Ran J, Fuchs A, Vratzov B, Kurz H, Glinsner T, Lindner F (2004) High resolution lithography with PDMS molds. J. Vac. Sci. Technol. B 22: 3229-3232.

[25] Hsia K.J, Huang Y, Menard E, Park J.U, Zhou W, Rogers J, Fulton J.M (2005) Collapse of stamps for soft lithography due to interfacial adhesion. Appl. Phys. Lett. 86: 154106.

[26] Schmid H, Biebuyck H, Michel B, Martin O.J.M (1998) Light-coupling masks for

[27] Choi D.G, Yu H.K, Yang S.M (2004) 2D nano/micro hybrid patterning using soft/block

[28] Odom T.W, Love J.C, Wolfe D.B, Paul K.E, Whitesides G.M (2002) Improved pattern transfer in soft lithography using composite stamps. Langmuir 18: 5314-5320.

[29] Koo N, Bender M, Plachetka U, Fuchs A, Wahlbrink, T, Bolten J, Kurz H (2007) Improved mold fabrication for the definition of high quality nanopatterns by soft UV-nanoimprint lithography using diluted PDMS material. Microelectron. Eng. 84:

[30] Schmid H, Michel B (2000) Siloxane Polymers for High-Resolution, High-Accuracy Soft

lensless, sub-wavelength optical lithography. Appl. Phys. Lett. 72: 2379.

poly(dimethylsiloxane) moulds. Nanotechnology 17: 197-200.

copolymer lithography. Mater. Sci. Eng. C 24: 213-216.

Lithography. Macromolecules 33: 3042-3049.

Amone S, Gigli G, Cingolani R (2004) Rigid organic molds for

Sub-30 nm Plasmonic Nanostructures by Soft UV Nanoimprint Lithography

antibodies. Microelectron. Eng. 87: 1001-1004.

biology. Microelectron. Eng. 88: 2444-2446.

lithography. Microelectron. Eng. 86: 632-635.

Vac. Sci. Technol. B 22: 1759-1763.

[22] Posognano D, D′

904-908.


[17] Barbillon G, Hamouda F, Held S, Gogol P, Bartenlian B (2010) Gold nanoparticles by soft UV nanoimprint lithography coupled to a lift-off process for plasmonic sensing of antibodies. Microelectron. Eng. 87: 1001-1004.

10 Lithography

[4] Barbillon G, Bijeon J.L, Lérondel G, Plain J, Royer P (2008) Detection of chemical molecules with integrated plasmonic glass nanotips. Surface Science 602: L119-L122.

[5] Faure A.C, Barbillon G, Ou M.G, Ledoux G, Tillement O, Roux S, Fabregue D, Descamps A, Bijeon J.L, Marquette C.A, Billotey C, Jamois C, Benyatou T, Perriat P (2008) Core/Shell nanoparticles for multiple biological detection with enhanced

[6] Jiang X, Ji L, Chang A, Leung K.N (2003) Resolution improvement for a maskless microion beam reduction lithography system. J. Vac. Sci. Technol. B 21: 2724-2727.

[7] Vieu C, Carcenac F, Pépin A, Chen Y, Mejias M, Lebib A, Manin-Ferlazzo L, Couraud L, Launois H (2000) Electron beam lithography: resolution limits and applications.

[8] Barbillon G, Bijeon J.L, Plain J, Royer P (2009) Sensitive detection of biological species through localized surface plasmon resonance on gold nanodisks. Thin Solid Films 517:

[9] Gates B.D, Xu Q, Stewart M, Ryan D, Wilson C.G, Whitesides G.M (2005) New Approaches to Nanofabrication: Molding, Printing, and Other Techniques. Chemical

[10] Krauss P.R, Chou S.Y (1997) Nano-compact disks with 400 Gbit/in2 storage density fabricated using nanoimprint lithography and real with proximal probe. Appl. Phys.

[11] Chou S.Y, Krauss P.R, Renstorm P.J (1996) Imprint lithography with 25-Nanometer

[12] Austin M.D, Ge H, Wu W, Li M, Yu Z, Wasserman D, Lyon S.A, Chou S.Y (2004) Fabrication of 5 nm linewidth and 14 nm pitch features by nanoimprint lithography.

[13] Jung G.Y, Johnston-Halperin E, Wu W, Yu Z, Wang S.Y, Tong W.M, Li Z, Green J.E, Sheriff B.A, Boukai A, Bunimovich Y, Heath J.R, Stanley Williams R (2006) Circuit Fabrication at 17 nm Half-Pitch by Nanoimprint Lithography. Nano Lett. 6: 351-354.

[14] Austin M.D, Zhang W, Ge H, Wasserman D, Lyon S.A, Chou S.Y (2005) 6 nm half-pitch lines and 0.04 *µ*m<sup>2</sup> static random access memory patterns by nanoimprint lithography.

[15] Hamouda F, Barbillon G, Held S, Agnus G, Gogol P, Maroutian T, Scheuring S, Bartenlian B (2009) Nanoholes by soft UV nanoimprint lithography applied to study

[16] Hamouda F, Barbillon G, Gaucher F, Bartenlian B (2010) Sub-200 nm gap electrodes by soft UV nanoimprint lithography using polydimethylsiloxane mold without external

of membrane proteins. Microelectron. Eng. 86: 583-585.

pressure. J. Vac. Sci. Technol. B 28: 82-85.

sensitivity and kinetics. Nanotechnology 19: 485103.

Applied Surface Science 164: 111-117.

2997-3000.

Reviews 105: 1171-1196.

resolution. Science 272: 85-87.

Appl. Phys. Lett. 84: 5299-5301.

Nanotechnology 16: 1058.

Lett. 71: 3174-3176.


[31] Plachteka U, Bender M, Fuchs A, Vratzov B, Glinsner T, Lindner F, Kurz H (2005), Wafer scale patterning by soft UV-Nanoimprint Lithography. Microelectron. Eng. 73/74: 167-171.

**Section 4**

**Fabrication of 3D Nano-Structure**


**Fabrication of 3D Nano-Structure**

12 Lithography

167-171.

208 Updates in Advanced Lithography

Microelectron. Eng. 87: 963-967.

Eng. 87: 1015-1018.

Rijeka: InTech. pp. 3-14.

[31] Plachteka U, Bender M, Fuchs A, Vratzov B, Glinsner T, Lindner F, Kurz H (2005), Wafer scale patterning by soft UV-Nanoimprint Lithography. Microelectron. Eng. 73/74:

[32] Ji R, Hornung M, Verschuuren M.A, van de Laar R, van Eekelen J, Plachetka U, Moeller M, Moormann C (2010) UV enhanced substrate conformal imprint lithography (UV-SCIL) technique for photonic crystals patterning in LED manufacturing.

[33] Cattoni A, Cambril E, Decanini D, Faini G, Haghiri-Gosnet A.M (2010) Soft UV-NIL at 20 nm scale using flexible bi-layer stamp casted on HSQ master mold. Microelectron.

[34] Chen J, Shi J, Cattoni A, Decanini D, Liu Z, Chen Y, Haghiri-Gosnet A.M (2010) A versatile pattern inversion process based on thermal and soft UV nanoimprint

[35] Barbillon G (2011) Soft UV Nanoimprint Lithography: A Tool to Design Plasmonic Nanobiosensors. In: Kostovski G, editor. Advances in Unconventional Lithography.

lithography techniques. Microelectron. Eng. 87: 899-903.

**Chapter 9**

**The Fabrication of High Aspect Ratio**

**Nanostructures on Quartz Substrate**

Khairudin Mohamed and Maan M. Alkaisi

Additional information is available at the end of the chapter

Recent developments in nano-scale devices have imposed many complex patterns with high aspect ratio nanostructures in its design. High aspect-ratio nanostructures have many appli‐ cations such as X-ray diffractive optical elements [1,2], nano-electro-mechanical-system (NEMS), fuel cell electrodes [3] and nanoimprint molds [4]. However, fabricating the high aspect-ratio nanostructures is still a challenging problem. For silicon technology, Bosch process [5] which is based on alternating multiple steps of etching and sidewall passivation is normally employed to achieve a high aspect ratio nanostructure. Other alternative is the cryogenic process [6] which is cooling the silicon substrates to cryogenic temperature using liquid nitrogen in order to achieve vertical sidewall profiles during etching. These processes however

In addition, although a large number of articles on high aspect-ratio silicon structures have been reported, limited information is available for quartz etching process in achieving high aspect-ratio nanostructures. Fabricating high aspect-ratio nanostructures on glass or quartz would open several new possibilities in the MEMS/NEMS field and especially in BioMEMS. In this chapter, we explained the pattern transfer process required on quartz substrate using CHF3/Ar reactive ion etching (RIE) technique. The fabrication of feature sizes below 100 nm on quartz substrates will be demonstrated by understanding first the etching mechanism of

Quartz is an insulating and hard substrate material in which patterning on top of its surface using electron beam lithography (EBL) is very challenging. Surface charging is the major issue

> © 2013 Mohamed and Alkaisi; 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 Mohamed and Alkaisi; 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.

http://dx.doi.org/10.5772/56315

**1. Introduction**

are not suitable for quartz.

**2. The fabrication method**

quartz and then finding an optimised RIE process.

## **Chapter 9**
