A pH-UV Dual-Responsive Photoresist for Nanoimprint Lithography

Article pubs.acs.org/JPCC

A pH-UV Dual-Responsive Photoresist for Nanoimprint Lithography That Improves Mold Release Chengyang Zhao,† Chenchen Shao,† Xiaowei Yu,† Dian Yang,§ and Jie Wei*,†,‡ †

College of Materials Science and Engineering, Beijing University of Chemical Technology, No. 15, Bei San Huan East Road, Chao Yang District, Beijing 100029, P.R. China ‡ Beijing Engineering Research Center for the Synthesis and Applications of Waterborne Polymers, Beijing 100029, P. R. China § Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States S Supporting Information *

ABSTRACT: We have developed a degradable photoresist that is responsive to pH and ultraviolet light (UV). This dual-responsive resist consists of 5,7-diacryloyloxy-4-methylcoumarin (fluorescent monomer), acrylic anhydride, and 3,6-dioxa-1,8-dithiooctane. It can be photocured using thiol−acrylate polymerization and photodimerization of coumarin moieties under 365 nm UV light exposure. The cured resist is degradable in aqueous solutions with pH > 7. The degradation process can be characterized by the change of fluorescence intensity in the aqueous solution. In this study, we have analyzed the properties of the degradation of the resist by changing the pH of the solution and its exposure time under 254 nm UV light. This UV exposure can induce photocleavage of the coumarin dimers. We then used these materials to fabricate micropatterns through nanoimprint lithography (NIL) process. Compared with other conventional degradable materials capable of NIL, the dual-responsive resist can help to clean the NIL mold easily at room temperature. This resist is also more environmentally friendly, is relatively low cost, can be faster to degrade, and is easier to characterize. It also has low volume shrinkage, which may have a valuable and positive effect on the development of NIL.

1. INTRODUCTION Nanoimprint lithography (NIL) as a next-generation lithography technique has exhibited many advantages over previous lithography techniques. Advantages include low cost, high resolution, and high efficiency. It can be applied to many fields, ranging from medical device production to printing of integrated circuitry.1−6 In the NIL process, the imprint mold/ master and the resist are the most important parts. The mold, however, is very expensive and easily damaged by the residual resist, which will impact the integrity of patterns.6−9 There are two methods to prevent the damage introduced by conventional resists through partially adhering to molds or stamps during the demolding process: (1) improving the compositions of resists with low surface energy components such as fluorinated molecules and (2) coating fluorinated antiadhesion layers onto the surface of mold.10−18 Furthermore, there are some similar methods such as treating the surface of resists film or molds with fluorinated plasma.19,20 But these conventional solutions do not solve the problem well because the effects are limited, and they often dramatically increase the cost. Many other research groups have worked on this problem, where they mainly focused on fabricating degradable resists to decrease the impact of residual resists. For example, Nelson’s group synthesized a series of aliphatic polycarbonate polymers via ring-opening polymerization of furanyl and maleimido© XXXX American Chemical Society

bearing cyclic carbonate monomers, where the side chains undergo thermally induced Diels−Alder reactions to form cross-linked films. Furthermore, the Diels−Alder reactions of maleimido and furanyl are reversible when triggered thermally.21 Ge and co-workers have developed a UV-cured and acid-catalytic thermal degradable material for soft nanoimprint lithography, by using 2,10-diacryloyloxymethy-1,4,9,12tetraoxaspiro[4,2,4,2]tetradecane as the acid-degradable crosslinker.22−24 The cured material can be decomposed into linear chains through the cleavage of acid-liable ketal links, and dissolves in an organic solvent when heated in an acid solution. Additionally, Yin developed a new kind of resist named photoreversible resists with coumarin moieties as the crosslinkers. The photodimerization of coumarin moieties and polymerization of acrylate groups can be induced by 365 nm UV light irradiation. The photoreversible linkages can be cleaved by a 254 nm UV light, making the network transfer into a linear structure. This separation could improve the solubility of cured resist in organic solvent.25 All these works to fabricate degradable materials, however, use particular cross-linkers to transfer the cross-linked polymer into the linear polymer for Received: March 6, 2017 Revised: May 12, 2017 Published: May 15, 2017 A

DOI: 10.1021/acs.jpcc.7b02117 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

the adhered curing resist at room temperature, and to make the mold easily cleaned.

nanoimprint lithography. These methods still face challenges such as requiring specific thermal conditions, low degradation extent, low efficiency, and pollution of organic solvent. To overcome some of these challenges, Shipp and coworkers have explored a photocurable, degradable polyanhydride cross-linked elastomer that can be used as a stamp in the nanoimprint lithography process. The polymerization was based on thiol−acrylate “click reaction” that was fast and insensitive to oxygen.26−28 They fabricated the degradable stamps on a master by curing the monomers 4-pentenoic anhydride and pentaerythritol tetrakis(3-mercaptopropionate) as sacrificial templates during fabrication of the replicas of the master. The stamps could be easily degraded from the replica in water to avoid residual resists. In our study, we designed a new molecule, 5,7-diacryloyloxy4-methylcoumarin (DAMC), as the photoreversible crosslinker, and a new kind of dual-responsive and degradable material for NIL. It consists of DAMC, acrylic anhydride (ALA), and 3,6-dioxa-1,8-dithiooctane (EGDT). These materials were cured quickly under 365 nm UV light via the polymerization of thiol−acrylate and photodimerization of DAMC. In the network structure, the backbones were made of anhydride groups that could be degraded in alkaline aqueous solution, and photoreversible DAMC. Under 254 nm UV light, the reversible linkages could be cleaved to transfer the crosslinked structure into a linear structure, as shown in Scheme 1.

2. MATERIALS AND METHODS 2.1. Materials. m-Trihydroxybenzene was purchased from Aladdin. Ethyl acetoacetate was purchased from J&K. Acryloyl chloride was bought from Energy Chemical. Acrylic anhydride (ALA) was purchased from HWRK Chem. 3,6-Dioxa-1,8dithiooctane (EGDT) and photoinitiator 2,2-dimethoxy-1,2diphenylethanone (DMPA) were obtained from Beijing InnoChem Science & Technology Co., Ltd. Other chemicals were obtained from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). All the reagents were used as received except as noted. 2.2. Synthesis and Characterization of DHMC. Ethyl acetoacetate (0.965 g) was added dropwise to a solution of mtrihydroxybenzene (0.945 g) and phosphoric acid (10 mL). The mixture was stirred for 2 h at 60 °C. Then the pale yellow powder was filtrated by washing with deionized water. The product, 5,7-dihydroxy-4-methylcoumarin (DHMC), was dried under vacuum at 70 °C overnight. The dry DHMC was used to synthesize DAMC later. FTIR (KBr): ν = 3424, 3153,2 932, 2842, 1720, 1670, 1623, 1586, 1550, 1389, 1303, 1238, 1160, 1096, 832, 761 cm−1. 1H NMR (400 MHz, DMSO-d6, δ): 10.52 (s, 1H, −OH), 10.29 (s, 1H, −OH), 6.27 (d, J = 4 Hz, 1H, Ar−H), 6.18 (d, J = 4 Hz, 1H, Ar−H), 5.86 (d, J = 4 Hz, 1H, Ar−H), 2.50 (d, J = 4 Hz, 3H, −CH3). 13C NMR (400 MHz, DMSO-d6, δ): 161.02, 160.03, 157.89, 156.46, 154.92, 108.79, 102.05, 99.03, 94.47, 23.40. EI-MS:193 (M+). Anal. Calcd for C10H8O4: C, 62.01; H, 4.16. Found: C, 62.5; H, 4.17. 2.3. Synthesis and Characterization of DAMC. A solution of DHMC (0.432 g), triethylamine (0.456 g), and tetrahydrofuran (10 mL) was maintained at 60 °C by stirring for 30 min. Then acryloyl chloride (0.600 g) was added dropwise to the solution at 0 °C by stirring for 1.5 h. The white powders precipitated out when mixed with deionized water and were filtrated after washing several times, and then the products were dried under vacuum at 70 °C overnight to get DAMC. FTIR (KBr): ν = 2920, 2852, 1742, 1622 cm−1. 1H NMR (400 MHz, acetone-d6, δ): 7.23 (d, J = 2 Hz, 1H, Ar−H), 7.11 (d, J = 2.4 Hz, 1H, Ar−H), 6.69 (dd, J = 1.2 Hz, J = 17.2 Hz, 1H, CC−H), 6.63 (dd, J = 1.2 Hz, J = 17.2 Hz, 1H, CC− H), 6.50 (dd, J = 10.4 Hz, J = 17.2 Hz, 1H, CC−H), 6.41 (dd, J = 10.4 Hz, J = 17.2 Hz, 1H, CC−H), 6.30 (q, J = 1.2 Hz, 1H, Ar−H), 6.26 (dd, J = 1.2 Hz, J = 10.4 Hz, 1H, CC− H), 6.18 (dd, J = 1.2 Hz, J = 10.4 Hz, 1H, CC−H), 2.52 (d, J = 1.2 Hz, 3H, CH3). 13C NMR (400 MHz, acetone-d6, δ): 164.61, 164.05, 159.33, 156.12, 153.24, 151.60, 149.40, 134.97, 134.24, 128.23, 128.19, 116.95, 114.72, 112.67, 109.29, 22.78. EI-MS: 301 (M+). Anal. Calcd for C10H8O4: C, 60.02; H, 4.19. Found: C, 60.00; H, 4.03. 2.4. Preparation and pH-Dependent Degradation of Dual-Responsive Resist. The dual-responsive resist was composed of DAMC (24.5 wt %), EGDT (48.9 wt %), ALA (23.6 wt %), and photoinitiator DMPA (3.0 wt %), as shown in Scheme 2. They were mixed and stirred to uniformity in a dark environment. The mixture was purged with N2 gas, then dropped into plastic cylinder molds (8 mm diameter, 4 mm height), and placed under the 365 nm UV light source (23 mW/cm2) for 3 min. After curing, the samples (0.220−0.230 g) were removed from the molds. In the end, the samples were immersed in aqueous solutions with various pHs (12, 9, 7.4) to

Scheme 1. Process of Degradation

Therefore, not only can the cured materials be degraded via hydrolysis of anhydride groups from backbones, the crosslinking density also can be regulated by UV light with different wavelengths, which had great effects on the degradation rate and mechanical property.27 Furthermore, the degree of degradation of cured material can be easier to analyze by the change of fluorescence intensity of coumarin in alkaline aqueous solution compared to the conventional method of mass loss. In this study, we report a novel, environmentally friendly, low-viscosity, dual-responsive, degradable resist system for UV nanoimprint lithography (UV-NIL) that helps to release B


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Figure 2. The FTIR spectra [Figure 2a] demonstrates the change of thiol group absorption peak at 2570 cm−1 and

Scheme 2. Chemical Structure of Each Component of the Dual-Responsive Resist

analyze the degradation rate using mass loss and fluorescence spectrum. 2.5. Photoreversibility of Dual-Responsive Resist. We chose the same cured samples as above to analyze the influence of photoreversibility of resist on its degradation. The samples were placed under 254 nm UV light (14.4 mW/cm2) source and treated with different exposure time. They were then immersed in a NaOH aqueous solution with pH = 9. The degradation time was recorded. 2.6. Fabrication of Micropatterning Replicas. The silicon substrates were cleaned in a 1:3 mixed solution (30% H2O2: 98% H2SO4) at 100 °C for 4 h. After the substrates were removed from the solution, they were cleaned thoroughly with fresh acetone in an ultrasonic bath and dried in nitrogen gas. The components of dual-responsive resist are shown in Scheme 2. The mixture was filtrated through a 0.2 μm filter before spin-coating. The resist was then spin-coated on the silicon substrate at 2800 rpm for 20 s to become a flat and thin film. The resist layer was covered by the mold under a small pressure of 0.3 bar, and the sandwich sample was put in a vacuum chamber to expel the trapped air bubbles. Then it was exposed to 365 nm UV light (23 mW/cm2) for 2 min. After curing, the sample was removed from the nanoimprinter, and the mold was detached to get the micropatterns on the substrate as shown in Figure 1a. The mold detached from the cured resist was exposed under 254 nm UV light source for 30 min, then soaked in a NaOH aqueous solution for about 10 min, and dried after washing with fresh water to get refreshed mold as shown in Figure 1b.

Figure 2. FTIR spectrum of the resist during 365 nm UV light irradiation (a) and conversion curves of the thiol−acrylate system (b).

acrylate group absorption peak at 1618−1623 cm−1 under 365 nm UV light (23 mW/cm2) exposure for different periods of time (0, 30, 60, 90, and 120 s). Both absorption peaks attenuated rapidly and almost disappeared in 120 s, which means the resist cured quickly in air and was no longer sensitive to oxygen. Meanwhile, the real-time FTIR spectra [Figure 2b] shows the conversion of acrylate groups and thiol groups could reach 96% and 82%, respectively. The polymerization of thiol− acrylate occurs via both “step-growth” and “chain-growth” mechanisms: a competition exists between chain growth, homopolymerization of the acrylate, and step growth, mechanism of a thiol−acrylate polymerization. We note that the kinetic constant of propagation of the acrylates is greater than the kinetic constant of chain transfer from an acrylic radical to a thiol functional group by hydrogen abstraction.29 3.2. Transparency and Volume Shrinkage of the Resist. To investigate the influence of UV exposure time and film thickness on the UV transparency of the resist, the materials were coated onto a wall of a quartz cell and then exposed to 365 nm UV light source for different periods of time, as shown in the UV−vis transmission spectra [Figure 3a]. The UV transparency of a 100 μm thick sample [thickness (1)] did not change with the increased UV exposure time (nearly 100% over 365 nm), which means the cross-linking density did not impact the transparency. When the film thickness was

3. RESULTS AND DISCUSSION 3.1. Reaction Kinetics of Thiol−Acrylate Click Chemistry. In this system, the main reaction is the “click” reaction between thiol and acrylate groups. Therefore, we studied its reaction kinetics by FTIR and real-time FTIR, as shown in

Figure 1. Process of (a) UV-NIL and (b) refreshing mold. C


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Figure 3. (a) UV−vis transmission spectra of the resist with different exposure times and thicknesses; (b) volume shrinkage curve of the resist.

Figure 4. (a) Tg of the three different systems measured by DSC; (b) thermal gravity curves of the three different systems measured by TG.

increased to 300 μm [thickness (2)] and 500 μm [thickness (3)], the UV transparence was still high enough to meet the requirement for UV curing (more than 70% over 365 nm). The high UV transparency could promote the high degree of UV curing of every part. Moreover, the volume shrinkage of the resist was measured by the laser displacement method, as shown in Figure 3b. The result demonstrates low volume shrinkage of about 8.45% after UV curing, which is due to the rigid structure of DAMC and “click chemistry” polymerization. The low shrinkage can enhance the quality of the film formed and the resolution of patterns. 3.3. Thermal and Mechanical Properties of the Resist. To investigate the influence of the addition of DAMC on thermal and mechanical properties of cured resist, the glass transition temperature (Tg) and thermal decomposition temperature of three systems with different components were measured respectively by DSC and TG after UV curing, as shown in Figure 4a,b. Tg of the ALA/EGDT/DAMC system was about 22 °C higher than that of the ALA/EGDT system (linear structure) and close to that of the ALA system (highly cross-linked structure). Besides, the thermal decomposition temperature (about 310 °C) of the former system was also higher than that of the ALA/EGDT system, and close to that of the ALA system. This is because the DAMC is a rigid molecule containing benzene structure. The dimerization of DAMC (Scheme 3) could increase the cross-linking density and limit the mobility of the main chain to improve the thermal and mechanical properties of the cured resist. 3.4. pH-Dependent Degradation and Fluorescence Characteristics. The polymerization based on thiol−acrylate chemistry and the degradation of polyanhydride have been studied for several decades.26−32 The step-growth thiol− acrylate polymerization occurs through radical intermediates and displays low shrinkage, and high monomer conversions. The polyanhydride was mainly applied to biodegradable materials that were placed in an in vivo environment with pH = 7.4.33,34 There were, however, a few works that analyzed its degradation in a solution with pH > 7.4.

Scheme 3. Synthesis of DAMC

In our work, the cured samples are composed of DAMC, EGDT, and ALA, which we made to have nearly the same dimension and effective surface area, so as to control the effect of dimension and effective surface area in the degradation behavior. The samples were immersed in an aqueous solution with various pHs (7.4, 9, and 12). The degradation rate was measured by two methods: mass loss and the change of fluorescence intensity in degradation solutions, as shown in Figure 5a and the video of degradation in electronic form Supporting Information. The 0.1 mL degradation solution was taken out to mix with 2 mL of deionized water every hour. Then the mixture was characterized using a fluorescence spectrometer (EX, 360 nm; EM, 455 nm). As shown in Figure 5b, the fluorescence intensity at 455 nm in degradation solution (pH = 7.4) increases with time, which means the degree of degradation was increasing. Therefore, the rate of degradation could be counted by integrating the peaks of the fluorescence spectrum around 455 nm using the following equation: w= D

A f − At × 100% Af DOI: 10.1021/acs.jpcc.7b02117 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 6b shows the degradation curves of samples measured by mass loss; the samples were taken out from the solutions and dried in vacuum. Then their weight was measured every hour. In comparison to the method of the fluorescence spectrum, the overall trend of degradation was similar, but there were still some different parts among them. For example, there was a small increase in mass during the initial 3 h when the pH was 7.4. The gain in mass can be attributed to water uptake and swelling, and this gain may be eliminated by increasing the drying time.28 Furthermore, we can find the accuracy of the method of the fluorescence spectrum is higher than the method of mass loss because the samples could not be measured by mass loss when they were very small, and it is also more efficient than the method of mass loss. Therefore, the degradation of these samples with fluorescent groups can be analyzed by fluorescence spectrum easily, efficiently, and accurately. And the degradation rate increases sharply with the pH increasing. 3.5. Relationship between Photoreversibility and Degradation. The DAMC as a cross-linker contains coumarin structure, which is capable of undergoing reversible photodimerization. The photodimerization reaction proceeds by the [2π + 2π]-cycloaddition mechanism, which can be reversed upon applying an appropriate wavelength of light. Besides, DAMC can produce strong UV absorption at a certain wavelength because of the electronic conjugation that can be destroyed by the [2π + 2π]-cycloaddition reaction between two alkenes to form a cyclobutane dimer, which can be restored by cyclobutane cleavage. Therefore, this process can be monitored by ultraviolet−visible spectroscopy (UV−vis).36 The UV absorption spectra of DAMC, ALA, EGDT, photoinitiator DMPA, and their mixture are shown in Figure 7. The

Figure 5. (a) Fluorescent pictures of the sample in water with pH = 7.4 at different times; (b) fluorescence spectrum of degradation solution with pH = 7.4 at different times.

where w is the undegraded mass fraction, Af is the final integral area and At is the integral area at time t. As shown in Figure 6a,

Figure 7. UV spectra of the resist and each component.

absorption maximum at 282 nm was the characteristic absorption of DAMC. The photoreversible process of DAMC in absolute ethyl alcohol solution was studied by exposure to 365 nm (23 mW/cm2) and 254 nm (14.4 mW/cm2) UV light sequentially. The degree of dimerization of DAMC can be calculated using the following equation,

Figure 6. Degradation curves of the samples in water with different pHs (7.4, 9, and 12) measured by fluorescence spectrum (a) and mass loss (b).

degree of dimerization = (1 − A t / A 0) × 100%

where At denotes the absorbance at 282 nm at time t and A0 is the original absorbance at 282 nm prior to 365 nm exposure. As shown in Figure 8a, the absorption peak around 282 nm decreased visibly with an increase in the exposure time under 365 nm UV light, which means photodimerization occurred and the electronic conjugation was destroyed. After irradiation of 254 nm UV light for about 2 h, the absorption peak around 282 nm increased gradually [Figure 8b], which indicates the photocleavage of cyclobutane dimers and the restoration of

comparing the rates of degradation of three samples with each other, we can find that it took 6 h for the samples to be thoroughly degraded at pH = 12, which was almost half the time to do so at pH = 7.4. When the pH was 7.4, the process of degradation was very slow in the first 5 hthis is consistent with some former studies.34,35 This initial slow degradation did not occur when the pH was 12 and 9. This may be due to the high concentration of OH− that could promote the diffusion of molecules on the surface. E


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Figure 8. UV spectra of DAMC in absolute ethyl alcohol solution during exposure to 365 nm UV light (a) and 254 nm UV light (b); UV spectra of the dual-responsive resist during exposure to 365 nm UV light (c) and 254 nm UV light (d).

electronic conjugation. Besides, the maximum of the degree of dimerization could reach 85% and the recovery 58%. The degree of recovery was not 100% because both photocleavage and photo-cross-linking simultaneously took place upon 254 nm irradiation.37,38 The photoreversibility of the resist was studied also by coating the materials onto a wall of a quartz cell. As shown in Figure 8c,d, the absorption peak around 282 nm decreased visibly by 81% after irradiation of 365 nm UV light for 2 min, which means the photodimerization occurred and the crosslinkages were formed by [2π + 2π]-cycloaddition reaction. Then, the degree of dimerization of the cured resist was decreased by cyclobutane cleavage after irradiation of 254 nm UV light, indicating the cross-linking density decreased. It took a relatively long time to recover by 62%, which may be due to the limitation of 254 nm UV light source used in our study and the difficulty for the breakage of the chemical bond. The influence of photoreversibility of resist on its thermal property and degradation was studied as shown in Figure 9. Figure 9a shows the glass transition temperature (Tg) of cured resist decreased from −43.33 to −53.14 °C, indicating the limit of molecular mobility became weak. This is due to the photocleavage reaction under 254 nm UV light exposure. Figure 9b shows the degradation time (the time from immersing the samples in solution to their complete dissolution) decreased visibly from 8 to 5.75 h in pH = 9 solution with an increase of irradiation time (the time for irradiating the samples by the UV light) under 254 nm UV light. This increase is likely due to the increase of molecular mobility. We concluded that the response of resist to both 365 and 254 nm UV light is efficient, and the cross-linking density of cured resist can be regulated to impact its thermal/ mechanical properties and degradation. Moreover, it can take a shorter exposure time of 254 nm UV light to promote the degradation of cured resist visibly by using 254 nm UV light source with higher intensity.

Figure 9. (a) Influence of 254 nm UV light exposure on Tg of the cured resist; (b) influence of 254 nm UV light exposure on the degradation of the cured resist in water at pH = 9.

3.6. Patterns Transfer and NIL-Molds Release. We studied the performance of this dual-responsive resist in fabricating micropatterning replicas in a UV-NIL process. Figure 10a,c are the SEM images of the NIL mold and the replica, respectively, and they have nearly the same features and dimensions from a far view. After increasing the magnification, the equilateral triangles of the NIL mold and the replica also show the nearly same side length, as shown in Figure 10b,d; however, the corners of the replica are more round compared to that of the NIL mold, which may be attributed to the flexibility of the resist. Moreover, the molds with gratings and dots were also used in the UV-NIL process to fabricate the F


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Figure 10. (a) and (c) SEM images of the mold and replica, respectively (scale bar = 30 μm); (b) and (d) SEM images of the triangles of the mold and replica (scale bar = 5 μm).

Figure 11. (a) and (b) SEM images of the mold and replica, respectively (scale bar = 30 μm); (c) and (d) SEM images of the larger versions of the mold and replica (scale bar = 15 μm).

4. CONCLUSIONS

replicas, as shown in Figure 11, which indicates the dualresponsive resist can transfer the patterns of the molds in a variety of shapes and sizes easily. We can find that the resist could perform very well in UV-NIL and no serious problems or distortions existed in the replicas. We also studied the mold release property of the resist, as shown in Figure 12. Some residual cured resist adheres to the mold after demolding. The mold could be cleaned easily without any damages, which can be attributed to the good degradation properties of the dual-responsive resist.

We have successfully developed a novel dual-responsive and degradable resist based on 5,7-diacryloyloxy-4-methylcoumarin and acrylic anhydride for UV nanoimprint lithography. Through the polymerization of thiol−acrylate and photodimerization of DAMC by exposure to 365 nm UV light, the resist can be photocured quickly to generate cross-linked networks, which can then be photocleaved by 254 nm UV light to promote the degradation of backbone in water. This dualresponsive resist is environmentally friendly and cost-effective. It can help to clean UV-NIL mold easily by releasing the G


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Figure 12. (a) SEM image of the mold with residual cured resist; (b) SEM image of the mold after refreshing (scale bar = 30 μm). (6) Guo, L. J. Nanoimprint Lithography: Methods and Material Requirements. Adv. Mater. 2007, 19, 495−513. (7) Ro, H. W.; Popova, V.; Chen, L.; Forster, A. M.; Ding, Y.; Alvine, K. J.; Krug, D. J.; Laine, R. M.; Soles, C. L. Cubic Silsesquioxanes as a Green, High-Performance Mold Material for Nanoimprint Lithography. Adv. Mater. 2011, 23, 414−420. (8) Li, B.; Zhang, J.; Ge, H. A Sandwiched Flexible Polymer Mold for Control of Particle-Induced Defects in Nanoimprint Lithography. Appl. Phys. A: Mater. Sci. Process. 2013, 110, 123−128. (9) Higashiki, T.; Nakasugi, T.; Yoneda, I. Nanoimprint Lithography and Future Patterning for Semiconductor Devices. J. Micro/Nanolithogr., MEMS, MOEMS 2011, 10, 043008. (10) Honda, K.; Morita, M.; Takahara, A. Room-Temperature Fabrication of Nanotexture in Crystalline Poly (Fluoroalkyl Acrylate) Thin Film. Soft Matter 2008, 4, 1400−1402. (11) Kim, J. Y.; Choi, D. G.; Jeong, J. H.; Lee, E. S. UV-Curable Nanoimprint Resin with Enhanced Anti-Sticking Property. Appl. Surf. Sci. 2008, 254, 4793−4796. (12) Ito, S.; Kaneko, S.; Yun, C. M.; Kobayashi, K.; Nakagawa, M. Investigation of Fluorinated (Meth) Acrylate Monomers and Macromonomers Suitable for a Hydroxy-Containing Acrylate Monomer in UV Nanoimprinting. Langmuir 2014, 30, 7127−7133. (13) Ito, S.; Yun, C. M.; Kobayashi, K.; Nakagawa, M. Release LayerFree Acrylate Resins with Segregation Auxiliary Agents for Ultraviolet Nanoimprinting. J. Vac. Sci. Technol., B: Nanotechnol. Microelectron.: Mater., Process., Meas., Phenom. 2012, 30, 06FB05. (14) Lin, H.; Wan, X.; Jiang, X.; Wang, Q.; Yin, J. A Nanoimprint Lithography Hybrid Photoresist Based on the Thiol−Ene System. Adv. Funct. Mater. 2011, 21, 2960−2967. (15) Lin, G.; Zhang, F.; Zhang, Q.; Wei, J.; Guo, J. Fluorinated Silsesquioxane-Based Photoresist as an Ideal High-Performance Material for Ultraviolet Nanoimprinting. RSC Adv. 2014, 4, 44073− 44081. (16) Truffier-Boutry, D.; Galand, R.; Beaurain, A.; Francone, A.; Pelissier, B.; Zelsmann, M.; Boussey, J. Mold Cleaning and Fluorinated Anti-Sticking Treatments in Nanoimprint Lithography. Microelectron. Eng. 2009, 86, 669−672. (17) Khang, D. Y.; Lee, H. H. Sub-100nm Patterning with an Amorphous Fluoropolymer Mold. Langmuir 2004, 20, 2445−2448. (18) Jung, G. Y.; Wu, W.; Li, Z.; Chen, Y.; Olynick, D. L.; Wang, S. Y.; Tong, W. M.; Williams, R. S. Vapor-Phase Self-Assembled Monolayer for Improved Mold Release in Nanoimprint Lithography. Langmuir 2005, 21, 1158−1161. (19) Schvartzman, M.; Mathur, A.; Hone, J.; Jahnes, C.; Wind, S. J. Plasma Fluorination of Carbon-Based Materials for Imprint and Molding Lithographic Applications. Appl. Phys. Lett. 2008, 93, 153105. (20) Lee, J.; Lee, J.; Lee, H. W.; Kwon, K. H. Anti-Adhesive Characteristics of CHF3/O2 and C4F8/O2 Plasma-Modified Silicon Molds for Nanoimprint Lithography. Mater. Res. Bull. 2015, 69, 120− 125.

residual cured resist, thus reducing damages to the molds and imprinting patterns. It can also improve the production efficiency and cut the cost of the UV-NIL process. Moreover, it may have some potential applications in fabrication of a replica of the master as a single-use stamp. It may also have potential in manufacturing flexible circuits and microfluidics, controllable release materials for drug delivery, and biosensors.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b02117. NMR spectra, mass spectra, FTIR spectrum, and images of the cured samples (PDF) Video of degradation (MPG)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] ORCID

Jie Wei: 0000-0001-7935-1660 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support by the National Natural Science Foundation (Grant Nos. 51573012 and 51173013).

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REFERENCES

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