**3. Fabrication of the replicated soft stamp**

#### **3.1 Polymeric stamp materials**

8 Will-be-set-by-IN-TECH

We have recently proposed and developed a T-NIL based process to replicate and invert an EBL master mold (Chen et al. (2010)). The inversion by means of T-NIL allows for the fabrication of a silicon mold with inverted features, so that pillars in the original master mold become holes in the daughter mold (Figure 5). We have studied how each pattern transfer process can affect feature size, pattern shape and homogeneity. Sub-200nm nanostructures have been inverted with good reproducibility and homogeneity on field as large as 1cm2.

**T-NIL for master inversion**

**NXR1020 (180 nm) Au (20 nm)**

Fig. 5. (a) Schematic of master mold inversion based on T-NIL and SEM images of daughters

A schematic of this inversion process is presented in Figure 5. A thin gold layer (20nm) is first deposited on the silicon wafer. The thermal imprints are then performed on the NXR-1020 resist (Nanonex), spin coated at a thickness of 180 nm on Au/Si wafers. These resist films are annealed at 120◦C for 10 minutes to remove the solvents. The Si master mold is gently placed in contact with the resist substrate and sandwiched between the two membranes of the NXR-2500 Nanonex, which provides optimal uniformity over the whole imprinted field. Imprinting is carried out at 130◦C in two successive steps: 10 sec at 120 PSI (8 Bars) followed by 30 sec at 200 PSI (14 Bars). The imprinting tool is then cooled down below the estimated glass transition temperature of the resist, before carefully demolding the master. After removing the residual NXR1020 resist layer with O2 plasma at a rate of 30 nm/min, the thin Au mask layer is etched by ion beam etching (IBE). The resist layer is then simply removed with acetone. Finally, the patterns are transferred into the silicon wafer with a standard Si reactive ion etching process using the Au mask, obtaining hole arrays. A trichloromethylsilane (TMCS) anti-sticking treatment is then applied to the daughter Si mold. With this process, we demonstrated that inversion is feasibile at the whole wafer scale. Figure 5 shows the SEM images of a 2 inch silicon wafer inverted daughter mold. It was patterned on a surface of 1 cm2, with 200 nm-wide hole arrays. A good homogeneity was observed over the whole imprint field. These results clearly show the ability of the process to reach a resolution

**2.3 The whole wafer inversion based on T-NIL**

1. T- NIL on NXR1020 resist

2. RIE of residual layer and IBE of Au

**Si master mold**

3. Pattern transfer by RIE of Si

4. Removing of Au mask

molds

of 100 nm.

The first unquestionable advantage of using a polymeric replica of the original master mold in the nanoimprint process is the cost reduction. As previously stated, from a single and expensive master mold, it is possible replicate a large number of polymeric stamps to use in the nanoimprint process. This prevents damages to the original master mold if it is directly used in the nanoimprint process. Moreover, the long range flexibility of the elastomeric material used for the stamp ensures the contact between the stamp and the substrate on large surfaces at low pressures (tens of bars) and on curved or flexible substrates. Several kind of polymeric materials have been tested as candidate for stamp replication including polycarbonate resins (Posognano et al. (2004)), cross-linked novolak based epoxy resin (Pfeiffer et al. (2002)), fluoropolymer materials and tetrafluoroethylene(PTFE) (Kang et al. (2006)). Nevertheless, poly(dimethylsiloxanes) (PDMS) still offers numerous attractive properties as a stamp elastomer. First of all, PDMS ensures a conformal adhesion of the stamp with the substrate on large areas without applying any pressure. To stress this point, Soft UV NIL performed with PDMS based stamps was recently renamed as UV enhanced Substrate Conformal Imprint Lithography (UV-SCIL) by Philips Research and SUSS MicroTec (Ji et al. (2010)).

Additionally, PDMS offers numerous other attractive properties: (1) its flexible backbone enables accurate replication of relief shapes in the fabrication of the patterning elements, (2) its low Young's modulus (∼750 KPa) and its low surface energy enable conformal contact with the substrate without applied pressure and nondestructive release from patterned structures, (3) its good optical transparency down to a light wavelength of approximately 256 nm, (4) its commercial availability in bulk quantities at low cost. While PDMS offers some advantages, there are a number of properties inherent to PDMS which severely limit its capabilities in soft UV-NIL. First, its low Young's modulus limits the fabrication of features with high aspect ratios due to collapse, merging and buckling of the relief structures. Second, its surface energy (<sup>≈</sup> 22-25 mNm−1) isn't low enough to replicate profiles with high accuracy. Third, its high elasticity and thermal expansion can lead to deformation and distortions during the fabrication. Finally, it shrinks by ∼1% after curing and can be readily swelled by some organic solvents. In general, long range deformations can be avoided by the introduction of a thin glass backplane which preserves a global flexibility. On the other hand, short range deformations can be avoided only by increasing the elastic modulus of PDMS, as we will discuss in deepth in the next paragraph. Therefore, compared to hot-embossing lithography (HEL) and UV-NIL which use rigid molds, it is particularly important to control the mold deformation as much as possible for the soft UV-NIL.

Fig. 6. Left panel: SEM images of (a) a 100 nm line of the master pattern, (b) patterned polymer with the composite stamp prepared by the method proposed by Schmid and Odom and (c) by the improved method with toluene as solvent for hard-PDMS (the scale bar is 500 nm). Reprinted with permission from (Kang et al. (2006)). Copyright 2006 IOPP Publication. Right panel: schematic of the influence of the material viscosity on the penetration depth into the master cavities for nanostructures. Exemplary SEM pictures of an 50 nm dots array imprinted with diluted PDMS molds prepared with toluene concentration of: A: 10 wt%, B: 20 wt%, C: 40 wt% and D: 60 wt% whom correspond to a pattern height increases from 10 nm (A) up to 70 nm (D). Reprinted with permission from (Koo et al. (2007)). Copyright 2007

<sup>149</sup> Soft UV Nanoimprint Lithography:

A Versatile Tool for Nanostructuration at the 20nm Scale

In order to achieve an accurate pattern replication, it is important to control the eventual broadening and deformations of the replicated structures. During T-NIL and UV-NIL processes, a parallel and uniform surface contact between the hard mold and the substrate is obtained by applying high pressure, and the replicated structures preserve a high accuracy when compared to the original ones in the master mold. During the Soft UV NIL process, the flexibility of the PDMS stamp ensures a conformal adhesion with the substrate at low pressure. On the other hand, the low elastic module of PDMS can produce deformations of high aspect ratio structures when too much pressure is applied. If it is not possible to completely avoid these deformations, it is important to control and reduce them under appropriate values, depending on the specific application. Because soft UV-NIL is a very recent variant of NIL,

Elsevier.

**4. Soft imprinting process**

**4.1 Minimization of pressure and dimension control**

### **3.2 Basic PDMS stamp fabrication process**

Simple PDMS stamps are typically replicated by first mixing two commercial PDMS components: 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 the nanostructured master molds and degassed in a dessicator. 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 due to thermal shrinkage. Longer curing times and higher temperatures allow up to twice the elastic modulus and hardness of the polymer, but can also lead to higher roughness and deformations. The stamps are left to cool to room temperature, carefully peeled off from the master mold and treated with silane based antisticking treatment to further reduce the low PDMS surface energy. These stamps are not suitable for the replication structures at the sub-100 nm scale or with a high aspect ratio because of the low elastic modulous of PDMS. To address this issue, a modified PDMS with a higher elastic modulus was already proposed 10 years ago.

#### **3.3 Improved hard-PDMS/PDMS bilayer stamp fabrication process**

In 2000, Schmid et al. (Schmid & Michel (2000)) used a modified PDMS with higher elastic modulus (hard-PDMS) to extend the range in which conventional soft lithography could be applied. The hardening of the PDMS was accomplished by decreasing the chain length of the prepolymer. Increasing the elastic modulus of PDMS allows for the replication of smaller features in the 100 nm range, but it results in poor flexibility and an increase in the brittleness of the stamp. To overcome this problem, Odom et al. (Odom et al. (2002)) proposed a composite stamp of hard-PDMS and standard PDMS which combined the advantages of a rigid layer to achieve a high resolution pattern transfer and an elastic support which enabled conformal contact even at a low imprint pressure. Another important property of hard-PDMS is the lower viscosity of its prepolymer in comparison to PDMS. PDMS prepolymer cannot completely fill up the recessed nanoareas of the master mold due to its high viscosity, and the height of the soft stamp nanostructures will thus be lower than the height of the master patterns resulting in a poor inprinted thickness contrast. The decreasing of the chain length in the preparation of the hard-PDMS prepolymer, produce a lowering of its viscosity with a consequent higher ability of hard-PDMS prepolymer to replicate with accuracy the original master mold especially for high dense and small patterns.

The importance to reduce the viscosity of the prepolymer for an accurate replication of the master mold have been reported by other two groups (Kang et al. (2006); Koo et al. (2007)). In this case the viscosity of the PDMS prepolymer was reduced with the introduction of a solvent to the mixture, together with the use of an excessive amount of modulator to delay the cross-linking. The improved stamp replication process resulted in a better more accurated imprinting process, as shown in 8.

In Figure 7 is summarized the procedure for the replication of the bi-layer hard-PDMS/PDMS stamp. The hard-PDMS prepolymer, prepared by following a recepie similar to the one proposed by Schmid (Schmid & Michel (2000)), is spin coated onto a master at 5000 rpm for 30 sec and then degassed in vacuum for 10 min (the thickness of the hard-PDMS is about 5-8 *μm*). A mixture of conventional PDMS (1:10) is then poured on the spin coated hard-PDMS layer curing at 60 ◦C for 24 hours, the bi-layer stamp can be peeled off from the master and it is treated with trichloromethylsilane (TMCS).

10 Will-be-set-by-IN-TECH

Simple PDMS stamps are typically replicated by first mixing two commercial PDMS components: 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 the nanostructured master molds and degassed in a dessicator. 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 due to thermal shrinkage. Longer curing times and higher temperatures allow up to twice the elastic modulus and hardness of the polymer, but can also lead to higher roughness and deformations. The stamps are left to cool to room temperature, carefully peeled off from the master mold and treated with silane based antisticking treatment to further reduce the low PDMS surface energy. These stamps are not suitable for the replication structures at the sub-100 nm scale or with a high aspect ratio because of the low elastic modulous of PDMS. To address this issue, a modified PDMS with a higher elastic modulus was already proposed 10

In 2000, Schmid et al. (Schmid & Michel (2000)) used a modified PDMS with higher elastic modulus (hard-PDMS) to extend the range in which conventional soft lithography could be applied. The hardening of the PDMS was accomplished by decreasing the chain length of the prepolymer. Increasing the elastic modulus of PDMS allows for the replication of smaller features in the 100 nm range, but it results in poor flexibility and an increase in the brittleness of the stamp. To overcome this problem, Odom et al. (Odom et al. (2002)) proposed a composite stamp of hard-PDMS and standard PDMS which combined the advantages of a rigid layer to achieve a high resolution pattern transfer and an elastic support which enabled conformal contact even at a low imprint pressure. Another important property of hard-PDMS is the lower viscosity of its prepolymer in comparison to PDMS. PDMS prepolymer cannot completely fill up the recessed nanoareas of the master mold due to its high viscosity, and the height of the soft stamp nanostructures will thus be lower than the height of the master patterns resulting in a poor inprinted thickness contrast. The decreasing of the chain length in the preparation of the hard-PDMS prepolymer, produce a lowering of its viscosity with a consequent higher ability of hard-PDMS prepolymer to replicate with accuracy the original

The importance to reduce the viscosity of the prepolymer for an accurate replication of the master mold have been reported by other two groups (Kang et al. (2006); Koo et al. (2007)). In this case the viscosity of the PDMS prepolymer was reduced with the introduction of a solvent to the mixture, together with the use of an excessive amount of modulator to delay the cross-linking. The improved stamp replication process resulted in a better more accurated

In Figure 7 is summarized the procedure for the replication of the bi-layer hard-PDMS/PDMS stamp. The hard-PDMS prepolymer, prepared by following a recepie similar to the one proposed by Schmid (Schmid & Michel (2000)), is spin coated onto a master at 5000 rpm for 30 sec and then degassed in vacuum for 10 min (the thickness of the hard-PDMS is about 5-8 *μm*). A mixture of conventional PDMS (1:10) is then poured on the spin coated hard-PDMS layer curing at 60 ◦C for 24 hours, the bi-layer stamp can be peeled off from the master and it

**3.2 Basic PDMS stamp fabrication process**

**3.3 Improved hard-PDMS/PDMS bilayer stamp fabrication process**

master mold especially for high dense and small patterns.

imprinting process, as shown in 8.

is treated with trichloromethylsilane (TMCS).

years ago.

Fig. 6. Left panel: SEM images of (a) a 100 nm line of the master pattern, (b) patterned polymer with the composite stamp prepared by the method proposed by Schmid and Odom and (c) by the improved method with toluene as solvent for hard-PDMS (the scale bar is 500 nm). Reprinted with permission from (Kang et al. (2006)). Copyright 2006 IOPP Publication. Right panel: schematic of the influence of the material viscosity on the penetration depth into the master cavities for nanostructures. Exemplary SEM pictures of an 50 nm dots array imprinted with diluted PDMS molds prepared with toluene concentration of: A: 10 wt%, B: 20 wt%, C: 40 wt% and D: 60 wt% whom correspond to a pattern height increases from 10 nm (A) up to 70 nm (D). Reprinted with permission from (Koo et al. (2007)). Copyright 2007 Elsevier.

### **4. Soft imprinting process**
