Parallel Near-Field Photolithography with Metal-Coated Elastomeric

Dec 30, 2014 - masks with subwavelength apertures created at the apexes were compatible with mask aligners, allowing for the production of wafer-scale...
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Parallel Near-Field Photolithography with Metal-Coated Elastomeric Masks Jin Wu,† Cheng-han Yu,‡ Shaozhou Li,†,§ Binghua Zou,†,∥ Yayuan Liu,† Xiaoqun Zhu,† Yuanyuan Guo,† Hongbo Xu,† Weina Zhang,† Liping Zhang,†,# Bin Liu,# Danbi Tian,⊥ Wei Huang,∥ Michael P. Sheetz,‡ and Fengwei Huo*,†,∥ ∥

Key Laboratory of Flexible Electronic (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergistic Innovation Center for Advanced Materials (SICAM) and ⊥College of Science, Nanjing Technological University, Nanjing 211816, China † School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore ‡ Mechanobiology Institute, National University of Singapore, Singapore 117411, Singapore # School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637457, Singapore § Key Laboratory for Organic Electronics & Information Displays (KLOEID), Nanjing University of Posts and Telecommunications, Nanjing 210046, China S Supporting Information *

ABSTRACT: Developing a cost-effective nanolithography strategy that enables the production of subwavelength features with various shapes over large areas is a long-standing goal in the nanotechnology community. Herein, an inexpensive nanolithographic technique that combines the waferscale production capability of photolithography with the subwavelength feature size controllability of near-field photolithography was developed to fabricate centimeter-scale up to wafer-scale sub-100-nm variously shaped nanopatterns on surfaces. The wafer-scale elastomeric trench-based photomasks with subwavelength apertures created at the apexes were compatible with mask aligners, allowing for the production of wafer-scale subwavelength nanopatterns with adjustable feature sizes, shapes, and periodicities. The smallest feature sizes of 50 and 80 nm were achieved on positive tone and negative tone photoresist surfaces, respectively, which could be ascribed to a near-field optical effect. The fabricated centimeter-scale nanopatterns were functionalized to study cell−matrix adhesion and migration. Compared to currently developed nanolithographic methods that approach similar functionalities, this facile nanolithographic strategy combines the merits of low cost, subwavelength feature size, high throughput, and varied feature shapes, making it an affordable approach to be used in academic research for researchers at most institutions.

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INTRODUCTION Lithography is in great demand in the fabrication of integrated circuits (IC), microelectromechanical systems (MEMS), compound semiconductors, and digital storage devices in industry.1−6 High-resolution nanolithography is also a great engine for promoting many research areas in academia.7−19 Giving an example of molecular biology, the high-resolution nanopatterns build up a new bridge for molecular biologists to explore and understand cell behavior on the molecular level.7−11 However, the slow fabrication process or complicated procedures of currently employed nanolithography strategies cannot meet the high demand for research in this field.7−11 Currently developed lithographic strategies impose trade-offs among cost, feature size, scalability, and feature shape.3 For example, electron beam lithography (EBL)20 and serial scanning probe based lithography (SPL)21−23 can provide high resolution (below 10 and 30 nm for EBL and SPL, respectively), whereas their serial nature limits their throughput.24 For nanoimprint lithography, the high cost of imprint © 2014 American Chemical Society

templates, template wear, and the requirement of leveling between the imprint template and the substrates during the printing process in order to achieve uniform imprint results are major concerns of this method.25,26 Interference lithography27 and block copolymer lithography10,28 are capable of producing high-resolution regular patterns over wide areas. Nevertheless, arbitrarily shaped patterns are precluded. Photolithography is the most successful workhorse for the massive production of microstructures and nanostructures, whereas the resolution is diffraction-limited for traditional far-field photolithography.3,4,24 State-of-the-art photolithography tools used in industry can achieve a resolution below 30 nm by utilizing deep ultraviolet (DUV),1,5 whereas the increasingly high cost limits its accessibility to most academic researchers.5 Near-field photolithography is a promising strategy for produce subwavelength Received: October 28, 2014 Revised: December 25, 2014 Published: December 30, 2014 1210

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Figure 1. Wafer-scale production of subwavelength nanopatterns with metal-coated PDMS photomasks in near-field photolithography. (A) Schematic illustration of the fabrication process of the metal-coated trench array with nanoscopic apertures created at the apexes and the application of such photomasks in near-field photolithography. (B) Optical image of a 5-in., 60-nm-thick gold-coated PDMS photomask with PDMS trenches of various shapes, which were fitted in a mask aligner for photolithography. (C) Optical image of sub-100-nm positive photoresist features fabricated on a 4-in. silicon wafer by using the photomask in (B). (D) Optical image of large-area 80-nm-wide Cr lines with a periodicity of 3 μm fabricated from the photoresist patterns in (C). (E) SEM image of a selective area of the gold dot array in (F) with a dot diameter of 90 nm. (F) Optical image of a large-area gold dot array fabricated from the photoresist patterns in (C). (G) SEM image of a selected area of 80 nm-wide Cr lines in (D).

features by circumventing the diffraction limit.3 For example, phase-shifting edge photolithography was developed by employing transparent elastomeric stamps as photomasks to generate sub-100-nm nanostructures over large areas.12,29−33 However, only the positive photoresist below the edges of polydimethylsiloxane (PDMS) relief nanostructures was left.29,33 Recently, a parallel scanning probe-based lithographic strategy termed beam pen lithography (BPL) has been developed by using a metal-coated pyramidal array with nanoscopic apertures created at the apex of each tip as the optical beam pens to “write” various nanopatterns directly on the photoresist surface.3,4,34,35 However, the requirement of a costly scanning probe microscope (SPM) platform to fabricate variously shaped patterns makes BPL difficult to access by many researchers. Moreover, compared to wafer-scale photolithography on mask aligners with single or several exposures, multiple exposures make BPL slow because beam pens have to be used to scan serially at different places to expose many times in order to fabricate the patterns of complicated shapes. Thus far, a fast lithography strategy that enables the production of various wafer-scale, sub-100-nm features affordable to most academic researchers is still under development. Herein, a strategy was developed to enable wafer-scale subwavelength nanopatterns with various shapes in a low-cost and high-speed manner by utilizing wafer-scale metal-coated elastomeric photomasks for near-field photolithography. Nanoscopic apertures created at the apexes of a metal-coated PDMS

trench array together with the intimate contact between the apertures and the substrate surfaces produce a near-field optical effect. Consequently, the diffraction limit has been broken. Rather than utilizing dynamic beam pens to write variously shaped patterns on surfaces in BPL, herein the PDMS trench array itself was fabricated as the desired various shapes and used as the photomasks in static photolithography to produce the nanopatterns of corresponding shapes directly. Consequently, wafer-scale sub-100-nm nanopatterns could be rapidly produced by single exposure, which was much faster than mutiple-exposure-based BPL in fabricating centimeter-scale nanopatterns. By application of the mask aligner patterning platform, the nanopatterned areas were enlarged to wafer-scale areas and the nanolithography time was shortened to several seconds. The fabricated 100 nm chrome lines on glass substrates were selectively functionalized with RGD peptides and utilized to investigate the cell matrix adhesion formation and migration behaviors.

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EXPERIMENTAL SECTION

Metal-Coated Elastomeric Photomask Fabrication. The obtained PDMS trench array was treated with oxygen plasma (Plasma Cleaner, PDC-32G, Harrick), followed by coating with 60-nm-thick gold to make them opaque. After spin-coating with PMMA (495 c7, MicroChem Inc., USA) to cover the whole trench array surface (1000 rpm for 30 s) followed by baking the samples on a hot plate at 150 °C for 5 min, oxygen plasma with an oxygen flow rate of 300 mTorr was 1211

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Figure 2. Three-dimensional AFM topographical images and SEM images of variously shaped PDMS trench arrays demonstrating the V-shaped trench profiles. (A, C, D, G, I, J, and L) 3D AFM topographical images of a PDMS trench array with the shapes of a straight trench, a connected ellipse, multiple squares, a grid, multiple circles, a rectangle, and a branch, respectively, in the horizontal direction. (B, E, F, H, and K) AFM height profiles of lines across the PDMS trenches in (A), (D), (C), (G), and (J), respectively. (M) Cross-sectional SEM image of the PDMS trenches. (N) SEM image of PDMS pyramids with nanoscale apertures created at the end of the tips. (O) SEM image of PDMS trenches with the shapes of the miniaturized letters and number of “WIDTH-1” in the horizontal direction. employed to etch away the PMMA layer by layer until the apexes of the trenches were exposed (typically 7 min). Subsequently, the samples were placed in gold etching solution (Gold Etchant TFA/ H2O (1:40 v/v)) to etch away the gold at the apexes selectively. Finally, the residual PMMA on the trenches was removed by rinsing the trenches in acetone. Lithography on a Positive-Tone Photoresist Surface. The cleaned glass substrates were soaked in piranha solution (H2SO4/ H2O2 (70/30)) at 90 °C for 1 h, rinsed with water, and dried with

nitrogen. Subsequently, the glass slides were kept in a desiccator with 14 μL of hexamethyldisilazane, purchased from Sigma-Aldrich, for 10 min before applying the photoresist. The Shipley1805 (MicroChem) photoresist was prediluted with propylene glycol monomethyl ether acetate (MicroChem). The 20% (v/v) diluted photoresist was spin coated onto Si/SiO2 (100) wafers, glass slides, or 100-nm-thick goldcoated Si/SiO2(100) slides at 3000 rpm for 30 s to obtain a 40-nmthick photoresist layer. The 40% (v/v) diluted photoresist was spin coated onto the Si/SiO2(100) wafers at 1500 rpm for 30 s to obtain a 1212

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Langmuir 200-nm-thick photoresist layer. The undiluted photoresist was spin coated onto the Si/SiO2 (100) wafers at 2000 rpm for 30 s to obtain a 450-nm-thick photoresist layer. The resulting photoresist-coated substrates were baked on hot plate at 110 °C for 5 min before being used for photolithography. For wafer-scale near-field photolithography on a mask aligner, a 5 in. gold-coated PDMS photomask was integrated into the mask aligner of SUSS MJB4 with a typical exposure time of 1 s to produce subwavelength features, followed by photoresist development in MF319 (MicroChem) for 40 s. For centimeter-scale subwavelength photolithography with a portable halogen light source, a halogen light source (Fiber-lite Illuminators MI150, Dolan-Jenner) with a maximal power of 250 mw/cm2 was used to expose the photoresist. The typical exposure time for a 400 nm halogen lamp was 5 s. Typically, 7 nm Cr or 5 nm Cr/10 nm Au was evaporated on the developed photoresist patterns by using electron beam evaporation, followed by overnight immersion in Remove PG (MicroChem Inc., USA). Finally, the samples were sonicated in Remove PG and acetone, allowing us to visualize the metal features by AFM (Park Systems Co.), optical microscopy (Olympus), and SEM (JEOL JSM-7600). Functionalization of Nanopatterns. Glass substrates with Cr nanopatterns were cleaned with deep UV/ozone (184−254 nm) for 30 min. Small lipid vesicles of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) were then added to the substrates to form supported lipid membranes over glass surfaces. Detailed preparation methods were previously described.36 Cascade Blue neutravidin (0.1 μg/mL, Life Technologies, Grand Island, NY, USA) was added to bind selectively to nanopatterned metal lines for 30 min at room temperature. Excess neutravidin was removed by serial solvent exchange with 25 mL of phosphate-buffered saline (PBS). Next, 1 μg/mL biotinylated RGD, cyclo[Arg-Gly-Asp-D-Phe-Lys(Biotin-PEGPEG)] (Peptides International Inc., Louisville, KY, USA), was added and incubated for 30 min at room temperature. Excess RGD was then removed by serial solvent exchange with 25 mL of PBS. Neutravidin serves as the link between the nanopatterned metal surface and the biotinylated RGD peptide. Live cells were then added to RGDfunctionalized nanopatterns within 2 h after preparation. Cell Culture and Fluorescence Microscopy. RPTPα+/+ mouse embryonic fibroblasts37 were used in this study and cultured in DMEM media with heat-inactivated fetal bovine serum. EGFP-paxillin and Ruby-lifeact plasmids were transiently transfected into cells by electroporation (Neon Transfection System, Life Technologies, Grand Island, NY, USA). Fluorescent images of live cells were taken by a spinning-disk confocal inverted microscope (PerkinElmer UltraVIEW VoX, Waltham, MA, USA) with a 10× air lens, a 100× oil-immersion lens (1.40 NA, UPlanSApo 100×, Olympus, Center Valley, PA, USA), and a cooled EMCCD camera (C9100-13, Hamamatsu Photonics, Hamamatsu, Japan). An environmental chamber (37 °C, 5% CO2) was attached to the microscope body for time-lapse imaging.

length features due to a near-field optical effect (step 5). It is worth noticing that the elastomeric photomasks are compatible with mask aligners to produce wafer-scale subwavelength nanopatterns and can be put on the photoresist surface directly by hand to fabricate centimeter-scale nanopatterns using a flashlight for exposure. After the fabrication of variously shaped V-shaped silicon masters via the method of combining traditional photolithography and KOH anisotropic etching (Figure S1 in Supporting Information), various V-shaped PDMS trench arrays could be replicated from these molds (Figure 2).6 The scanning electron microscopy (SEM) images and 3D atomic force microscopy (AFM) topographical images show that all of the PDMS trenches exhibit a V-shaped profile vertically with sharp apexes at the ends of the trenches regardless of the shapes in the horizontal direction (Figure 2). Until now, several strategies have been reported to open the nanoscopic apertures at the apexes of PDMS pyramids after opaque metal coating, such as wet etching after dry etching,3 electrochemical etching,34 direct peel-off using sticky substrates,4 and focused ion beam (FIB) lithography.4 The size of apertures created by these strategies ranged from 50 nm to 5 μm. Herein, we chose the etching method to construct the apertures at the apexes of trenches with controllable size (Figure 3A and Figure S2 in

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Figure 3. Fabrication of sub-60-nm and larger features by using the near-field optical effect in photolithography. (A) SEM image showing 50-nm-wide apertures created at the apexes of gold-coated PDMS trenches. (B) SEM image of the developed positive photoresist nanopatterns with a line width of 50 nm, which were produced by utilizing the photomask in (A) via near-field photolithography. (C) SEM image of 55-nm-wide Cr lines obtained from the photoresist patterns in (B). The inset is one of the Cr lines with a width of 55 nm. (D) SEM image of the developed positive photoresist patterns with a line width of 90 nm produced at the pressure-applied position. (E) SEM image of the developed positive photoresist patterns 0.5 mm away from the pressure-applied position, which had a line width of 200 nm. (F) SEM image of the positive photoresist patterns 3 mm away from the pressure-applied position, which had a line width of 410 nm. The underexposed positive photoresist trenches did not reach the substrate after development. (G and H) Cross-sectional SEM images of the developed positive photoresist trenches with widths of 95 and 300 nm, respectively, which were produced by using the PDMS photomask of 90-nm-wide apertures with exposure times of 1 and 3 s, respectively. It was demonstrated that the photoresist trenches had vertical sidewalls. (I) Plot of the width of obtained Cr lines vs the exposure time.

RESULTS AND DISCUSSION Figure 1A illustrates the procedures for fabricating various nanostructures with metal-coated V-shaped elastomeric photomasks. The V-shaped PDMS trench array was fabricated by casting PDMS monomer and cross-linker in V-shaped silicon molds followed by peeling the cured PDMS off of these molds (step 1 in Figure 1A). The PDMS trenches (up to a 5 in. wafer surface until now) were then coated with a thin layer of opaque metal such as gold used here (around 60 nm in thickness) by electron beam evaporation (step 2) to block the transmission of light during the photolithography process. After removing the metal layer at the apexes of trenches (step 3), we placed the PDMS trench array into intimate contact with the photoresistcoated substrates on the mask aligner or directly by hand for photolithography (step 4). The opaque metal coating outside the trenches allowed the incident light to pass strictly from the subwavelength apertures created at the apexes to expose the underlying photoresist, producing variously shaped subwave1213

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The capability of generating subwavelength features is attributed to the near-field optical effect over the contact areas between the nanoscopic apertures and photoresist.4 For example, the developed photoresist patterns with a line width of 50 ± 10 nm were obtained on a 40-nm-thick positive photoresist (Shipley1805, MicroChem) by using the PDMS photomask with an aperture width of 50 ± 10 nm with 400 nm light exposure in a typical photolithography process (Figure 3A,B). Subsequently, a thin layer of Cr was deposited on the photoresist patterns, and the Cr lines with a width of 55 ± 10 nm were obtained after the photoresist lift-off in acetone (Figure 3). To investigate the near-field optical effect between subwavelength apertures and underlying photoresist, the impact of the gap size between them on the sizes and shapes of the produced features was explored (Figure 3D−F). After placing the glass-supported elastomeric photomask in contact with the photoresist-coated silicon surface, we applied a force of 0.4 N on one side of the 1 cm2 PDMS trench array with an aperture width of 90 nm, but no force was applied on the other side of the trench array during the exposure process. As such, the PDMS trenches below the pressure-applied position could be in intimate contact with underlying photoresist, producing 90-nmwidth photoresist patterns at these places owing to the nearfield optical effect (Figure 3D). However, the increase in the gap between the apertures and underlying photoresist at position far away from the pressure-applied position resulted in increased feature size at these places because of light diffraction (Figure 3E). With further increases in the gap, the photoresist could not be exposed completely because the effective exposure dose decreased gradually (Figure 3F). This indicated that the intimate contact between subwavelength apertures and the photoresist surface was crucial in leading to the near-field optical effect in producing subwavelength feature sizes. In particular, this experiment could be utilized to fabricate nanopatterns with a gradient in line width from 150 to 625 nm over 3 mm2 areas (Figure S8 in Supporting Information), which demonstrated one of the important methods in fabricating gradient patterns.38,39 The gradient patterns are useful in the rapid and systematic screening of stem cell adhesion.40 Importantly, the feature size can be controlled by tailoring the exposure dose. For example, when the exposure time increased from 1 to 7 s, the width of the obtained Cr lines increased linearly from 90 ± 10 to 655 ± 44 nm (Figure 3G−I and Figure S9 in Supporting Information), demonstrating the controllability of feature size in this approach. Furthermore, the spacing of obtained metal lines could be conveniently tuned by adjusting the pitch of PDMS trenches. For example, line patterns with different pitches of 2, 3, and 5 μm and similar line widths (90 ± 10 nm) could be fabricated by using the corresponding PDMS trench array with the same aperture size of 90 nm but with different periodicities (Figure S10A−C in Supporting Information). The AFM height profile of one line across the Cr lines demonstrated that the height of the Cr lines was around 7 nm (Figure S10I in Supporting Information). The PDMS trench base size was not found to lead to a variation in feature size because the incident light was allowed to pass only from the nanoscopic apertures at the end of the trenches (Figure S11 in Supporting Information). It is worth noticing that the developed photoresist trenches have vertical side walls that are favorable in fabricating useful devices because these patterns can be easily transferred to functional materials with

Supporting Information). The poly(methyl methacrylate) (PMMA) resist was spin-coated onto the PDMS trenches to cover the whole trench surface. Because of the fluidity of liquid PMMA, the resist at the apexes was thinner than that over other areas. Subsequently, oxygen plasma was employed to remove the top layer of PMMA to expose the metal exclusively at the apexes of the trenches. Then the samples were placed in gold etching solution to etch away the gold selectively at the apexes, generating nanoscopic apertures. By controlling the gold etching time from 2 to 10 min, the aperture size could be well controlled from 50 to 550 nm (Figure S2 in Supporting Information).The sub-100-nm apertures could also be constructed by utilizing other methods, such as a direct peel-off method using sticky substrates (Figure S3A−F in Supporting Information) and chemical lift-off lithography (Figure S3G−I in Supporting Information). Interestingly, the apertures exhibit self-healing behavior under electron beam illumination. Under SEM characterization, the initial 50-nm-wide apertures shrank gradually toward sub-10-nm apertures until being fused together in the end (Figure S4 in Supporting Information). It is difficult to achieve a resolution below the diffraction limit using traditional far-field photolithography (for chromebased photomasks on common mask aligners, the common resolution is 1 μm for the hard contact mode, such as the mask aligner used here, SUSS MJB4 UV400 from SUSS MicroTec company32), but mask aligners enable wafer-scale patterning easily. Herein, 4-in. wafer-scale sub-100-nm nanopatterns were obtained by using metal-coated elastomeric photomasks fitted into the mask aligner without changing its setup and standard operating procedures (Figure 1B−G and Figures S5−S7 in Supporting Information). A wafer-scale gold-coated PDMS trench array on 5-in. soda lime or quartz substrates was used as the photomask (Figure 1B). The gold-coated PDMS photomasks are compatible with mask aligners because the elastomeric PDMS trenches with nanoscopic apertures at the apexes can be in intimate and simultaneous contact with underlying substrates after alignment. Consequently, waferscale sub-100-nm features can be produced (Figure 1C−G). In checking the uniformity of feature size over wafer-scale areas, it was found that there was a width variation of 15% for the produced 90-nm-wide photoresist trenches (Figure S5 in Supporting Information). In contrast to traditional Cr-based photomasks in producing micrometer feature sizes, the PDMS photomasks generated wafer-scale features with the same shapes but with sub-100-nm feature sizes (Figures S6 and S7 in Supporting Information). For conventional chrome-based hard photomasks, although pressure and vacuum can be used to reduce the gap between photomasks and the photoresist in order to enhance the resolving ability, the intimate contact between them is still challenging because of the rigidity of both the photomasks and underlying substrates.19 Herein, the elastomeric nature of PDMS ensured the intimate contact between nanoscopic apertures and the photoresist and therefore alleviated light diffraction significantly. As such, this simple PDMS-based photomask can be used as a good complement to traditional Cr-based photomasks in producing wafer-scale subwavelength nanopatterns based on the maskaligner patterning platform. It is worth mentioning that the PDMS photomasks in this approach can be used repeatedly. After utilizing the same elastomeric photomasks 30 times, we found that the aperture size on the trenches did not show a significant change (Figure S2A,B,H,I in Supporting Information). 1214

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Langmuir fidelity (Figure 3G,H and Figure S12 in Supporting Information).32 In addition to line patterns, other large-area nanopatterns with various shapes and subwavelength feature sizes could be achieved by utilizing the PDMS elastomeric trenches of corresponding aperture shapes. In a proof-of-concept experiment, mutiple-circle nanopatterns with a line width of 155 nm were fabricated by utilizing corresponding gold-coated PDMS photomasks with 150-nm-wide circular apertures at the apexes (Figure 4A,B). Similarly, grid patterns with a line width of 180

conformable contact with photoresist-coated surfaces are placed directly by hand, the apexes of PDMS trenches with nanoscopic apertures are in intimate contact with the underlying photoresist. Subsequently, by employing UV light from a portable halogen lamp to expose the underlying photoresist through the elastomeric photomasks, centimeterscale sub-100 nm nanopatterns could be produced (Figures S10 and S18 in Supporting Information). As such, large-area nanopatterns can also be enabled without the application of mask aligners. By applying negative photoresist SU-8 on Si/SiO2 or glass surfaces and performing photolithography using the metalcoated elastomeric photomasks, negative photoresist nanopatterns with a line width as small as 80 nm could be obtained (Figure 5). Contrary to the generation of recessed trench

Figure 4. Large-area variously shaped nanopatterns. (A) SEM image showing 150-nm-wide circular apertures created at the apexes of goldcoated PDMS circular trenches. (B) SEM image of multiple-circle Cr nanopatterns with a line width of 155 nm, which were fabricated by using the circular PDMS trench array as photomasks. (C) SEM image showing grid Cr patterns with an average line width of 180 nm fabricated by utilizing the PDMS grid photomask. (D) SEM image of prismatic Cr nanopatterns obtained by rotating the PDMS straight trench photomask by 45° in the second exposure. (E and F) SEM images of gold nanopatterns with the shapes of a connected ellipse and nanorod, respectively, which were fabricated by employing correspondingly shaped PDMS photomasks. (G−I) SEM images of Cr nanopatterns with shapes of multiple squares, a ladder, and a cup, respectively.

Figure 5. Variously shaped negative photoresist SU-8 nanopatterns fabricated by utilizing corresponding PDMS photomasks. (A−C) SEM images of SU-8 lines with pitches of 2, 5, and 7 μm, respectively, fabricated on silicon substrates. (D) Three-dimensional AFM topographical image of SU-8 lines with a pitch of 5 μm. (E) AFM height profile of one line across the SU-8 line patterns in (D), suggesting that the SU-8 patterns had a uniform height of 100 nm. (F) SEM image of SU-8 grid patterns with a pitch of 4 μm. Inset: SEM image of one SU-8 grid with a line width of 200 nm. (G) SEM image of an SU-8 dot array with a pitch of 3 μm. Inset: Fourier transform of this SEM image. (H) SEM image of an SU-8 dot array with a pitch of 5 μm. Inset: SEM image of a single dot with a diameter of 250 nm. (I) SEM image of a connected ellipse array with a line width of 200 nm.

nm could be produced by using PDMS grid trenches as the photomask (Figures 2G,H and 4C). By using the PDMS pyramids with triangular apertures as photomasks, triangular features could be produced as well (Figure S13 in Supporting Information). In addition, other variously shaped features with subwavelength line width, such as prismatic, multiple square, connected ellipse, ladder, cup, cross, rod, and branch arrays, could be fabricated by using correspondingly shaped PDMS trenches as photomasks (Figure 4D−I and Figures S14 and S15 in Supporting Information). In principle, this strategy could be extended to produce any other desired feature geometries simply by employing correspondingly shaped PDMS photomasks. The obtained subwavelength photoresist nanostructures could also be transferred onto other substrate surfaces such as metal thin films and silica surfaces to form nanoscale apertures and trench arrays, respectively (Figures S16 and S17 in Supporting Information). Besides achieving wafer-scale sub-100-nm features on a mask aligner, centimeter-scale sub-100 nm features can be conveniently obtained by using a portable flashlight for direct exposure. Specifically, after the PDMS-based photomasks in

nanostructures on a positive photoresist surface, photolithography on a negative photoresist surface produced raised nanostructures (Figure 5D,E). Large-area SU-8 nanopatterns with various shapes could also be produced by utilizing correspondingly shaped PDMS trenches as photomasks (Figure 5 and Figure S19 in Supporting Information). Because the SU8 photoresist is stable in most organic solvents, these fabricated nanopatterns can be favorable to useful device fabrication.30 Large-area nanopatterns can be utilized to study cell matrix adhesion formation and migration behaviors. Previously, adhesive RGD peptides, derived from the fibronectin repeat III 10th domain, were used to activate the integrin-mediated focal adhesion formation.9 Here, as a proof of concept, we selectively functionalize a 100-nm-wide Cr line array with RGD peptides and passivate the underlying glass substrates with nonadhesive supported lipid membrane. Mouse embryonic 1215

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Langmuir fibroblasts readily adhere to functionalized nanopatterns within 30 min and form polarized adhesion morphology parallel to the nanopatterned line array in 2 h (Figure 6A). Focal adhesions,

usually fabricated by EBL or nanoimprint lithography,7−9,11 the experimental conditions may vary from one substrate to another. Herein, centimeter-scale and even wafer-scale nanopatterns provide the opportunity to study large populations of cells on the same substrate under the same experimental conditions, which avoids experimental variations. In addition, large-area nanopatterns functionalized with signaling molecules can also be useful in cell biology research, such as stem cell differentiation,41 spatial-temporal cell matrix interaction,9,42 and T cell immunological synapse formation.43

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, creation of subwavelength apertures, electron-beam-induced aperture self-healing, uniformity of feature size, influence of PDMS trench size on the produced feature size, cross-sectional SEM images of produced positive photoresist nanopatterns with different photoresist thicknesses, and facile centimeter-scale subwavelength nanopatterning without using mask aligners. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 6. Guided adhesion formation on RGD-functionalized nanopatterns. (A) Mouse embryonic fibroblasts form polarized adhesion morphology parallel to an RGD-functionalized nanopatterned line array (100 nm line width). (B and C) Focal adhesions, visualized by EGFP-paxillin (green), are found at RGD-functionalized lines (blue) and are interconnected by an actin cytoskeleton, visualized by Ruby-lifeact (red). (D and E) Time-lapse images of new adhesion formation at the leading edge of the cells, as shown in the boxed area in (B). Together with actin lamellipodia protrusions, new adhesions (white arrows) are formed along the nanopatterned line array.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS F.H. acknowledges the financial support of AcRF Tier 2 (RG 17/12) from the Ministry of Education, Singapore and the Singapore National Research Foundation under the Campus for Research Excellence and Technological Enterprise Programmed Nanomaterials for Energy and Water Management and Foundation for Distinguished Young Scholars of Jiangsu Province (BK20140044).

visualized by transiently transfected EGFP-paxillin, are formed at RGD-functionalized adhesive nanopatterns (Figure 6B,C). Polarized actin lamellipodia protrusions, visualized by Rubylifeact, are observed at the leading edge, and new adhesions are subsequently formed over adhesive nanopatterns (Figure 6D,E).

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SUMMARY AND CONCLUSIONS We report a cost-effective, high-throughput nanolithographic strategy that utilizes metal-coated elastomeric photomasks to produce wafer-scale, sub-100-nm variously shaped features via near-field photolithography. This strategy combines the waferscale patterning capability of traditional photolithography on mask aligners with the subwavelength feature size generation capability of near-field photolithography. The intimate contact between subwavelength apertures on the elastomeric trenches and resist surfaces was found to be essential to the near-field optical effect, which led to the generation of features with line widths of as small as 50 and 80 nm on positive and negative tone photoresist surfaces, respectively. The compatibility of the elastomeric photomask with the mask aligner makes it a good complement to the chrome-based hard photomask in producing wafer-scale subwavelength nanopatterns. After the fabrication of elastomeric masks, centimeter-scale sub-100 nanopatterns can also be produced facilely even without the use of mask aligners. The nanoscale feature size controllability and good scalability of this nanolithographic strategy make it potentially capable of completing some work requiring EBL or nanoimprint lithography. This nanolithography strategy provides an affordable nanopatterning platform for academic research, as shown by the example of cell adhesion here. For studying statistical cell behavior using small-area nanopatterns

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