Selection of Di(meth)acrylate Monomers for Low Pollution of

Mar 20, 2015 - We used fluorescence microscopy to show that low adsorption of resin components by a mold surface was necessary for continuous ultravio...
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Selection of Di(meth)acrylate Monomers for Low Pollution of Fluorinated Mold Surfaces in Ultraviolet Nanoimprint Lithography Masaru Nakagawa,*,†,‡ Kei Kobayashi,† Azusa N. Hattori,§ Shunya Ito,† Nobuya Hiroshiba,† Shoichi Kubo,† and Hidekazu Tanaka§ †

Polymer • Hybrid Materials Research Center, Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ‡ Japan Science and Technology Agency (JST), Core Research Evolutional Science and Technology (CREST), 7 Gobancho, Chiyoda, Tokyo 102-0076, Japan § Institute of Scientific and Industrial Research (ISIR), Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan ABSTRACT: We used fluorescence microscopy to show that low adsorption of resin components by a mold surface was necessary for continuous ultraviolet (UV) nanoimprinting, as well as generation of a low release energy on detachment of a cured resin from a template mold. This is because with low mold pollution, fracture on demolding occurred at the interface between the mold and cured resin surfaces rather than at the outermost part of the cured resin. To achieve low mold pollution, we investigated the radical photopolymerization behaviors of fluorescent UV-curable resins and the mechanical properties (fracture toughness, surface hardness, and release energy) of the cured resin films for six types of di(meth)acrylate-based monomers with similar chemical structures, in which polar hydroxy and aromatic bulky bisphenol moieties and methacryloyl or acryloyl reactive groups were present or absent. As a result, we selected bisphenol A glycerolate dimethacrylate (BPAGDM), which contains hydroxy, bisphenol, and methacryloyl moieties, which give good mechanical properties, monomer bulkiness, and mild reactivity, respectively, as a suitable base monomer for UV nanoimprinting under an easily condensable alternative chlorofluorocarbon (HFC-245fa) atmosphere. The fluorescent UV-curable BPAGDM resin was used for UV nanoimprinting and lithographic reactive ion etching of a silicon surface with 32 nm line-and-space patterns without a hard metal layer.

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INTRODUCTION Nanoimprint lithography1−3 is used industrially for nanofabrication, and has been used for semiconductors,4−6 patterned media,6−9 and optical devices such as light-emitting diodes,10 flat panel displays,11 photonic crystals,12 and solar cells.13 Nanoimprint lithography, which uses molded fluid resins cured by exposure to ultraviolet (UV) light, is referred to as UV nanoimprint lithography.14 It is considered to be a highthroughput method for fabricating semiconductors at 11 nm and smaller nodes, as well as photolithography with extreme UV light and immersion ArF photolithography involving multiple patterning techniques. Cost-effective and on-demand nanofabrication at less than 100 nm for a range of practical applications is being developed. Methods for prolonging the mold lifetime are essential for cost reduction in UV nanoimprint lithography. Pull-out defects occur unexpectedly in nanoimprinted resist patterns on substrates with increase number of imprint cycles for a single mold. Pull-out defects are line and dot breaks that cause convex resist pieces to be stuck to the mold surface and left in mold recesses. Pull-out defects are a serious problem because step and repeat UV nanoimprinting at high throughput becomes © 2015 American Chemical Society

impossible. Three strategies are adopted to prevent generation of such resist pattern defects: the addition of fluorinated surfactants to UV-curable resins, the modification of mold surfaces with antisticking agents, and the combined use of antisticking agents and fluorinated surfactants.15−17 The use of fluorinated surfactants lowers the surface free energy of the cured resin surface by segregation of the surfactants at the resin surface. However, a large amount of surfactants reduces the etching durability in subsequent dry etching of substrates after UV nanoimprinting. Kim et al.18 reported that the etching rate of a resist with fluorinated surfactants under an oxygen plasma increased with increasing surfactant content because of the decrease in the cross-linking density and molecular interactions, although the adhesion force of the cured resist with a silica mold was lower than that of a pure resist without surfactants. The modification of a silica mold surface with fluorinated antisticking agents also effectively lowers the adhesion force between a cured resin and a mold surface. The performance of Received: January 26, 2015 Revised: March 15, 2015 Published: March 20, 2015 4188

DOI: 10.1021/acs.langmuir.5b00325 Langmuir 2015, 31, 4188−4195

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Langmuir the release layer gradually deteriorates after repeated imprints. Ogawa et al. reported that the use of an antisticking agent and a reactive internal additive improves the release performance of the imprint cycle.19 The problem of intermittent pattern breaks in the demolding process still needs to be overcome. We developed fluorescent UV-curable resins to visualize what occurred at the interface between a mold and a cured resin surface.20,21 Fluorescent UVcurable resins enable the mold surfaces and resist patterns on substrates to be examined nondestructively by simple fluorescence microscopy. We showed that variations in the imprint atmosphere and antisticking agent affected pollution of fluorinated silica mold surfaces by surface adsorption of resin components.22 We found that an increase in the amount of resin components adsorbed by the mold surface made demolding difficult. We also showed that fluorescent UVcurable resins are useful for determining the thickness of a residual layer by measuring the fluorescence intensity;20 resist pattern defects21,23 can be investigated at a resolution limit of a space width of 100 nm for line and space patterns.24 We used fluorescence microscopy to select a suitable di(meth)acrylate monomer causing radical photopolymerization for reducing the pollution of fluorinated silica mold surfaces in UV nanoimprinting. The relationships between mold pollution caused by adsorbed resin components and the mechanical properties and demolding energies of the cured resins were investigated. We also achieved fabrication of 32 nm line and space (LS) resist patterns and transfer to silicon substrates by reactive ion etching with a fluorocarbon gas.

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Figure 1. Chemical structures of monomers: (a) AMP, (b) GDM, (c) BDM, (d) BPAGDA, (e) BPAGDM, and (f) BPAPDA. nanoimprinting. A polished surface of a 4 in. silicon wafer (Matsuzaki Seisakusyo) was cleaned using UV/ozone (Sen Lights PL16-110) and modified with (3-sulfanylpropyl)trimethoxysilane (Tokyo Chemical) by chemical vapor surface modification (CVSM) at 150 °C for 1 h to form an adhesive monolayer. A flat silica plate (10 × 10 × 0.6 mm3; NTT-AT) was cleaned by exposure to UV/ozone and modified with tridecafluoro-1,1,2,2-tetrahydrooctyltrimethoxysilane (FAS13) by CVSM to form a fluorinated release layer. The details are given in our previous paper.16 Briefly, CVSM was performed as follows. A cleaned silica plate and a 3 mL glass vessel containing FAS13 (0.1 mL) were placed in a 120 mL poly(tetrafluoroethylene) container. The container was sealed under nitrogen and placed in an oven at 150 °C for 1 h. The sealed container was opened immediately before cooling. The release energy was measured under nitrogen using a UV nanoimprint stepper (Meisho Kiko NM0801) equipped with a load cell (Kyowa LUR-A-200NSA1) and determined from the area of the recorded stroke−force curve. The release energy per area was calculated by dividing a release energy by a surface area of cured resin in contact with a fluorinated flat silica superstrate. Fluorescent UV-curable resins prepared using low-viscosity monomers without a bisphenol moiety were spin-coated directly onto a modified silicon substrate. The others prepared from high-viscosity monomers were diluted with PGME and spin-coated. A spin-coated film was prebaked at 80 °C for 2 min using a hot stage. The thickness of the spin-coated uncured film was adjusted to approximately 1.0 μm. An adhesive tape (Nitta C-4210K10 Intelimer) was used to fix a fluorinated silica superstrate on a silica exposure window in the imprint stepper. The superstrate surface was lowered to contact the resin film on a substrate. Applied pressure was steadily increased to 1.0 MPa for 10 s, and the position for load adjustment was held in 10 s. A resin film between the superstrate and substrate was cured by exposure to UV light for 20 s at an intensity of 100 mW cm−2 monitored at 365 nm. UV light of wavelength greater than 350 nm was obtained by passing the light emitted from a 200 W Hg−Xe lamp through a cutoff filter to the exposure window using a quartz optical fiber. The superstrate was released at a speed of 1.00 mm min−1. Generated forces on detachment of the superstrate from the cured resin film were recorded every 0.1 s. The release energy was measured five times and averaged. Fracture toughness of cured films was measured using a scratch tester (Rhesca CRS-2000) with a diamond stylus (radius, 5 μm; spring constant, 100 g mm−1) at a scratching speed of 10 μm s−1 under a loading rate of 50 mN min−1. Nanoindentations of cured films were

EXPERIMENTAL SECTION

Fluorescent UV-Curable Resins. Six structurally related (meth)acrylate monomers were used as base monomers: 1,3-butanediol dimethacrylate (BDM; Tokyo Chemical, CAS No. 1189-08-8), 1,3glyceryl dimethacrylate (GDM; Shin-Nakamura Chemical, CAS No. 1189-08-8), 1-(acryloyloxy)-3-(methacryloyloxy)-2-propanol (AMP; Tokyo Chemical, CAS No. 1709-71-3), bisphenol A glycerolate (1glycerol/phenol) diacrylate (BPAGDA; Sigma-Aldrich, CAS No. 468794-9), bisphenol A glycerolate dimethacrylate (BPAGDM; SigmaAldrich, CAS No. 1565-94-2), and bisphenol A propoxylate diacrylate (BPAPDA; Sigma-Aldrich, CAS No. 67952-50-5). The chemical structures are shown in Figure 1. 2-Methyl-1-[4-(methylthio)phenyl]2-morpholio-1-propanone (Irgacure 907; BASF Japan) was used as a photoinitiator for radical photopolymerization. 2-[6-(Ethylamino)-3(ethylimino)-2,7-dimethyl-3H-xanthen-9-yl]benzoic acid ethyl ester tetrafluoroborate (rhodamine 6G tetrafluoroborate; Indeco) was used as a fluorescent dye.20 The monomer, photoinitiator, and fluorescent dye were mixed at molar ratios of 100:4:0.04 to prepare fluorescent UV-curable resins. When highly viscous bisphenol-based monomers were used, a solution of 0.01 wt % fluorescent dye in methoxy-2propanol (PGME) was used to obtain homogeneous UV-curable resins. Photo Differential Scanning Calorimetry (Photo DSC). Resin curing was initiated by exposure to UV light at wavelengths greater than 350 nm; a photopolymerization reaction was monitored using a calorimeter (SII X-DSC7000) equipped with a UV light source (Hayashi LA-410UV). Light intensity was set at 0.10 W cm−2 and monitored at 365 nm using an optical power meter (Hamamatsu Photonics C-6080-03). A sample (approximately 0.5 mg) of each fluorescent UV-curable resin was placed in an Al pan. The sample was weighed, and photo DSC was performed at 25 °C under a nitrogen flow of 30 mL min−1 for an exposure period of 60 s; an empty Al pan was used as a reference. Release Energy Measurements. A fluorinated flat silica plate and a modified silicon wafer were used as the superstrate and substrate, respectively, for monitoring the release energy in step and repeat UV 4189

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Langmuir measured at a peak load of 15 μN using a nanoindenter (Elionix ENT2100). Fluorescence Microscopy. Superstrates detached during release energy measurements were examined using fluorescence microscopy to investigate the amounts of resin components adsorbed by a fluorinated silica surface. An optical microscope (Olympus BX51) equipped with a fluorescence cube (Olympus U-MWIGA3; excitation wavelength, 530−550 nm; detection wavelength, 575−625 nm), an objective lens (UPlanSApo 10 × /0.40), and a charge-coupled device camera (Hamamatsu Photonics ORCA-R2) was used. As shown in Figure 2a, nine observation positions (P1−P9, areas 835 × 668 μm2)

emissions from the fluorinated silica plate after detachment of cured films comprising different monomers, 0.9 μm thick cured films were prepared on silica plates using a similar method. The fluorescence intensities measured in an acquisition period of 100 ms were standardized to the film thickness to remove the influence of the fluorescence intensity on the type of cured film. UV Nanoimprint Lithography. UV nanoimprinting was performed under an easily condensable gas, that is, 1,1,1,3,3pentafluoropropane (HFC-245fa),25,26 which effectively reduces nonfill defects caused by air bubbles, using a UV nanoimprint stepper (Sanmei ImpFlex Essential) with a 10 × 10 mm2 silica mold (HOYA test mold-A). A silica mold contained 32 or 22 nm LS patterns in 100 × 100 μm2 squares, and was modified with an antisticking agent, namely FAS13, by CVSM in the same manner as the silica superstrate described above. A film of thickness 0.03 μm was prepared by spincoating a fluorescent BPAGDM-based UV-curable resin onto a 4 in. silicon wafer and prebaked at 80 °C for 2 min. An imprint cycle consisting of the following five steps was performed: a mold surface was moved toward the resin surface at a speed of 25 μm s−1; applied pressure was increased to 0.5 MPa; load adjustment position was held in 10 s; molded resin was cured by exposure to UV light (intensity at 365 nm, 5.0 mW cm−2; exposure period, 20 s); and demolding was performed at a speed of 4000 μm s−1. One shot took approximately 50 s. Dry etching of silicon with a nanoimprinted resist mask was performed using a reactive etching system (Samco RIE-10NR); the conditions were a gas mixture consisting of SF6/CHF3/O2 = 36/11/20, a flow rate of 67 sccm, a pressure of 1.5 Pa, and a bias power of 60 W. Field-emission scanning electron microscopy (FE-SEM) images were obtained using Hitachi S-4800 and SU-9000, and JEOL JSM-7001 instruments. Line edge roughness (LER) and line width roughness (LWR) values of cured resin pattern were evaluated by critical dimension scanning electron microscopy (CD-SEM) using an Advantest E3630 instrument. Cross-sectional FE-SEM images were obtained using a Hitachi SU-9000 instrument for manual cleaving of etched silicon patterns. Transmission electron microscopy (TEM) images of cross sections prepared by Ar ion milling were obtained using a JEOL EM-3000F instrument.

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RESULTS AND DISCUSSION Radical Photopolymerization and Mechanical Properties. Six types of di(meth)acrylate monomer with similar chemical structures (Figure 1) were used. Radical photopolymerization was initiated by UV light in the presence of a photoinitiator and the reactions were monitored using photo DSC. Figure 3 shows the photo DSC curves of fluorescent UVcurable resins consisting of BDM, GDM, AMP, BPAGDA, BPAGDM, or BPAPDA monomers. Figure 3 shows that generation of the reaction heats caused by radical photo-

Figure 2. (a) Used silica plate without patterns. The respective fluorescence microscopic images after resin detachment are shown in (b). Observation positions P1−P9, 835 × 668 mm2, were used for fluorescence microscopic detection of resin adsorbed on fluorinated silica plate surfaces. in a fluorinated silica plate were observed after five detachments in an acquisition period of 1000 ms. Captured images were analyzed using WinROOF software (Mitani). To compare fluorescence intensities of

Figure 3. Photo DSC curves of fluorescent UV-curable resins consisting mainly of BDM, GDM, AMP, BPAGDA, BPAGDM, or BPAPDA. 4190

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The surface hardness of the hydroxy-containing cured thin films was investigated using a nanoindenter. Figure 4b shows the hardness of the cured films. The hardness values of the AMP- and GDM-based films, without a bisphenol group, were higher than those of the BPAGDM- and BPAGDA-based films. The presence of a bisphenol moiety also decreased the surface hardness of the cured films as a result of lack of chemical crosslinking among hydroxy groups via hydrogen bonds. The AMPand BPAGDA-based films, with an acryloyl moiety, had slightly lower hardness values than the GDM- and BPAGDM-based films, with a methacryloyl moiety. Mold Pollution and Release Energy. Fluorescence microscopic observations of mold surfaces enable visualization of mold pollution caused by surface adsorption of resin components in different imprint atmospheres and with various antisticking agents.22 In this study, we used this method to select monomers suitable for UV nanoimprinting. We showed that adsorption of large amounts of resin components on mold surfaces caused the resin to stick, which made eventual release of the cured resin films difficult; therefore, the adsorption of resin components on mold surfaces should be minimized. The fluorescence emission behavior of the used fluorescent dye is affected by the solvent polarity; therefore, we investigated the fluorescence intensities at detection wavelengths of 575−625 nm using UV-cured films of thickness 0.9 μm for comparison of the monomers. When the fluorescence intensity detected for the BDM-based film was unity, the fluorescence intensities of the other monomer-based films were 1.7 for AMP, 1.6 for GDM, and 1.1 for BPAPDA, BPAGDA, and BPAGDM, as shown in Figure 5a. The coefficient values were used to normalize the fluorescence intensities detected for fluorinated silica surfaces after resin detachment. Figure 5b shows the fluorescence intensities of the fluorinated silica surfaces after five detachments from the respective fluorescent cured resins. Figure 5c shows the fluorescence intensities normalized by the coefficient values determined from Figure 5a. It is clear that the hydroxy-free BDM- and BPAPDA-based resins caused larger resin adsorption on the fluorinated silica surface than the hydroxy-containing resins did. This confirmed that the hydroxy group in the monomer structure caused high fracture toughness as a result of hydrogen bonding, and suppressed resin adsorption on the fluorinated silica surface. Among the hydroxy-containing monomers, the BPAGDA- and BPAGDM-based resins, with a bisphenol moiety, caused smaller resin adsorption than did the AMP- and GDM-based resins, without a bisphenol moiety. The cured films of the BPAGDA- and BPAGDM-based resins showed smaller fracture toughness and surface hardness than the AMP- and GDMbased resins did. These results indicate that the presence of a bulky bisphenol moiety suppressed resin adsorption on the fluorinated silica surface. The fluorinated silica surface consists of a self-assembled adsorbed monolayer formed from FAS13. The rigid fluoroalkyl groups are tilted in the packing structures, and this causes orientation defects. The small AMP and GDM monomers could enter the orientation defects in the release layer more easily than the bulky BPAGDA and BPAGDM monomers could, leading to greater resin adsorption on the FAS13-modified mold surface. The release energies were monitored on every detachment of the fluorinated silica plates from the respective UV-cured resins. The average release energies of the monomers without a bisphenol moiety were 1.4 J m−2 for BDM, 95.4 J m−2 for AMP, and 35.8 J m−2 for GDM; those for the bisphenol-containing

polymerization started at a point of 60 s when the resins were exposed to UV light. The slopes of the generated reaction heats are related to the reaction rate. Among the monomers without a bisphenol moiety, the reaction rate of AMP, which has a hydroxy group, was slightly slower than that of GDM and faster than that of BDM which does not have a hydroxy group. The reaction behaviors were in accordance with a previous report that the reactivity of a hydroxy-containing monomer is higher than that of a monomer without a hydroxy group, because the hydroxy group easily forms intermolecular hydrogen bonds.27 In contrast, for the monomers with a bisphenol moiety, the presence of hydroxy groups hardly affected the difference in reaction rate between hydroxy-free BPAPDA and hydroxycontaining BPAGDM. It was implied that the presence of bisphenol moiety would prevent the formation of hydrogen bonds between intermolecular hydroxy groups. BPAGDA, with two acryloyl groups, reacted faster than BPAGDM, with two methacryloyl groups. Radical polymerization occurred readily, and the order of the photopolymerization rates was AMP > GDM ≫ BDM, without a bisphenol moiety, and BPAPDA > BPAGDA ≫ BPAGDM, with a bisphenol group. The fracture toughness of the UV-cured thin films was investigated by a scratch test, using films of thickness approximately 1.1 μm. As shown in Figure 4a, the critical

Figure 4. (a) Critical peel forces of UV-cured films consisting of BDM, AMP, GDM, BPAPDA, BPAGDA, or BPAGDM on silicon substrates. (b) Hardness of UV-cured films consisting of AMP, GDM, BPAGDA, or BPAGDM.

peel forces were 29.9 mN for AMP, 26.0 mN for GDM, and 16.3 mN for BDM, which do not have a bisphenol group, and 15.2 mN for BPAGDA, 14.5 mN for BPAGDM, and 10.2 mN for BPAPDA, which have a bisphenol group. The presence of a hydroxy group gave UV-cured films with high fracture toughness. It was indicated that the improved mechanical properties are due to hydrogen bonding among monomers. As Figure 4a shows, the bisphenol-containing monomers had smaller critical peel forces than those without a bisphenol group. 4191

DOI: 10.1021/acs.langmuir.5b00325 Langmuir 2015, 31, 4188−4195

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A comparison of the release energies of the hydroxycontaining monomers showed that the methacryloyl-containing monomers, that is, GDM and BPAGDM, had lower release energies than the acryloyl-containing monomers, that is, AMP and BPAGDA. In general, the reactivity of a free radical generated from an acryloyl group is higher than that generated from a methacryloyl group. Truffier-Bountry et al. used electron spin resonance analysis to determine the stabilities of free radicals generated in acrylate monomers used for UV nanoimprinting.28 The concentrations of radical species decrease in the presence of silica particles modified by immersion in solutions of the fluorinated antisticking agents OPTOOL DSX and FAS13. They suggest that the free radicals generated in acrylate monomers induced interactions with the antisticking agents; this contributed to the deterioration in the release properties of the release layer. A comparison of acryloylcontaining BPAGDA with methacryloyl-containing BPAGDM showed that BPAGDA had a higher release energy and resin adsorption than BPAGDM, as shown in Figures 6 and 5c. This may be because of the higher reactivity of the acryloyl group. UV-curable resins with low release energies after curing are considered to be preferable for UV nanoimprinting, because the higher release energy causes greater stress on mold detachment, leading to generation of pull-out defects in resist patterns. According to our study of release properties, based on fluorescence microscopy and mechanical measurements, the UV-curable resin with the lowest mold pollution by resin adsorption and the lowest release energy was best for causing fracture at the interface between the fluorinated mold and cured resin surfaces. The methacryloyl group, which is less reactive than the acryloyl group, was better for causing fracture at the interface. The hydroxy-containing monomers were better than those without a hydroxy group, and the bisphenol-containing monomers were better than those without a bisphenol moiety. We therefore selected BPAGDM as a suitable monomer for UV nanoimprinting with FAS13-modified silica molds. Demonstration of UV Nanoimprint Lithography at 32 nm. Figure 7a shows an FE-SEM image of 32 nm LS patterns

Figure 5. (a) Fluorescence intensities of fluorescent cured films of thickness 0.9 μm. (b) Measured and (c) normalized fluorescence intensities of fluorinated silica surfaces after detachment from fluorescent cured films consisting of BDM, AMP, GDM, BPAPDA, BPAGDA, or BPAGDM.

monomers were 10.2 J m−2 for BPAPDA, 161.3 J m−2 for BPAGDA, and 5.8 J m−2 for BPAGDM. Figure 6 shows that the

Figure 6. Release energy detected upon detachment of fluorinated silica plates from fluorescent cured films consisting of BDM, AMP, GDM, BPAPDA, BPAGDA, or BPAGDM.

BDM- and BPAPDA-based resins, without a hydroxy group, had smaller release energies than the hydroxy-containing resins. However, the resins without a hydroxy group showed larger fluorescence intensities and smaller critical peel forces than did the monomers with a hydroxy group, based on a comparison of monomers with similar chemical structures. The results strongly suggest that fracture occurred at the outermost part of the cured resin film rather than at the interface between the fluorinated silica and cured resin surfaces.

Figure 7. (a, c) FE-SEM and (b) CD-SEM images of BPAGDM cured resin patterns produced using a fluorinated silica mold with (a, b) 32 nm and (c) 22 nm LS patterns. 4192

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Langmuir fabricated by UV nanoimprinting of a 0.03 μm thick BPAGDMbased resin with a FAS13-modified silica mold. The resist pattern at 32 nm was successfully fabricated without pull-out defects. The LER and LWR values were evaluated using CDSEM (Figure 7b). The 32 nm convex lines had LER(3σ) = 2.9 nm and LWR(3σ) = 5.0 nm. However, 22 nm LS patterns could not be fabricated, as shown in Figure 7c. It was difficult for the BPAGDM-based UV-curable resin to fill up the mold cavities of line width 22 nm. Shimazaki et al. used shear resonance measurements to show that a gap of approximately 20 nm between mica surfaces confines the viscous property of a base resin comprising acrylate monomers. The fluidity of the resin is improved by the addition of fluorine-containing acrylates.29 Although the FAS13-modified silica surface in this study was different from the mica surface that they used, it was anticipated that the fluorinated release monolayer interacted too strongly with BPAGDM to hardly fill the 22 nm wide mold cavities. To determine the optimum etching conditions, silicon was etched perpendicularly to a silicon surface with BPAGDMbased cured resin masks, and easily observable large LS patterns were fabricated by UV nanoimprinting of a BPAGDM-based UV-curable resin with a fluorinated silica mold (NTT-AT, NIM-PHL80). Figure 8a−c shows FE-SEM images of the

from CHF3 and SF6 was suppressed, as reported in the literature.30 The flow ratio and bias power were then fixed, and the pressure of 4 Pa was changed to 1.5 Pa (Figure 8d) or 10 Pa (Figure 8e); the side walls became closer to perpendicular with decreasing pressure. Anisotropic etching occurred because the mean free path of the reactive ion species increased. Under a flow of SF6/CHF3/O2 = 36/11/20 and a pressure of 1.5 Pa, larger bias powers of 100 W (Figure 8f) and 150 W (Figure 8g) promoted the formation of facets on the silicon patterns, although a larger bias power generally causes anisotropic etching.31 We used FE-SEM and TEM observations of cross sections of etched silicon patterns as shown in Figure 9a and

Figure 9. Cross-sectional (a) FE-SEM and (b) TEM images of etched silicon patterns.

Figure 9b, respectively. Figure 9b showed that anisotropic etching of silicon occurred on the atomic scale. We used these results to determine the dry etching conditions described in the Experimental Section. Figure 10 shows FE-SEM images of

Figure 8. Cross-sectional FE-SEM images of etched silicon patterns. (a−c) SF6/CHF3/O2 gas mixture at pressure of 4 Pa, bias power of 60 W, total flow rate of 67 sccm, and SF6/CHF3/O2 flow ratios of (a) 41/ 11/15, (b) 36/11/20, and (c) 33/11/23. (d, e) SF6/CHF3/O2 gas mixture at bias power of 60 W, total flow rate of 67 sccm, SF6/CHF3/ O2 flow ratio = 36/11/20, and pressures of (d) 1.5 Pa and (e) 10 Pa. (f, g) SF6/CHF3/O2 gas mixture at pressure of 1.5 Pa, total flow rate of 67 sccm, SF6/CHF3/O2 flow ratio = 36/11/20, and bias powers of (f) 100 W and (g) 150 W.

Figure 10. FE-SEM images of etched silicon patterns at 32 nm.

etched silicon surfaces produced with a 32 nm LS BPAGDMbased resist mask under the optimum dry etching conditions. As shown in Figure 10a, several silicon line breaks were observed. The number of silicon line breaks clearly increased after dry etching. We think that the physical properties of the masked and unmasked regions in the BPAGDM-based cured resin differed. This is under investigation. The line width of the silicon convex pattern was 28 nm at the outermost surfaces, and the space width was 32 nm.

silicon surfaces etched for 120 s using a mixture of SF6/CHF3/ O2 gases under a pressure of 4 Pa, a bias power of 60 W, a total flow rate of 67 sccm, and SF6/CHF3/O2 flow ratios of (a) 41/ 11/15, (b) 36/11/20, and (c) 33/11/23. As shown in Figure 8a, trapezoid silicon was obtained. When the flow ratio of O2 was increased, the trapezoid silicon pattern became rectangular. The etched walls became perpendicular to the silicon wafer surface at a flow ratio of SF6/CHF3/O2 = 36/11/20, as shown in Figure 8b. This indicated that the increased number of O2 species formed a side-wall protective layer of SiOxFy, and isotropic etching of silicon by reactive ion species generated

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CONCLUSIONS We selected di(meth)acrylate monomers that undergo radical photopolymerization to provide suitable resins for low pollution of silica mold surfaces modified with FAS13 in UV nanoimprinting under a readily condensable gas atmosphere, that is, the alternative chlorofluorocarbon HFC245fa. Mold 4193

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Article

Langmuir

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pollution caused by adsorption of resin components was detected by fluorescence microscopy and correlated with the radical photopolymerization behaviors identified using photo DSC and the mechanical properties, that is, fracture toughness, surface hardness, and release energy of thin UV-cured resin films. Radical photopolymerization occurred readily, in the rate order AMP > GDM ≫ BDM, without a bisphenol moiety, and BPAPDA > BPAGDA ≫ BPAGDM, with a bisphenol moiety. The hydroxy-containing and acryloyl-containing monomers induced faster radical photopolymerization than the hydroxyfree and methacryloyl-containing monomers did, because of monomer association caused by hydrogen bonding and higher monomer reactivity; the bisphenol-containing monomers prevented photopolymerization, because of the strong molecular interactions among the aromatic bisphenol moieties. The presence of hydroxy groups in the monomer increased the fracture toughness and surface hardness of the UV-cured resin films. In contrast, the presence of a bisphenol moiety impaired the mechanical properties. Fluorescence microscopy showed that the presence of hydroxy groups and bisphenol moieties suppressed resin adsorption on the fluorinated silica surface as a result of the enhanced fracture toughness and monomer bulkiness, respectively. A comparison of methacryloyl-containing BPAGDM with acryloyl-containing BPAGDA suggested that the presence of methacryloyl resulted in a low release energy and low mold pollution by adsorption of resin components, because of the less reactivity of the methacryloyl groups. We concluded that a UV-curable resin with a low release energy and low mold pollution was necessary for continuous step-and-repeat UV nanoimprinting, because fracture occurred at the interface between the fluorinated mold and cured resin surfaces rather than at the outermost part of the cured resin. We therefore selected BPAGDM, which has hydroxy, bisphenol, and methacryloyl moieties, as a suitable monomer for UV nanoimprinting with FAS13-modified silica molds. We showed that the BPAGDM-containing UV-curable resin was suitable for UV nanoimprinting of 32 nm LS patterns and lithographic dry etching using a SF6/CHF3/O2 gas without a hard metal layer. These silicon nanopatterns will be useful for directed self-assembly of polystyrene-block-poly(methyl methacrylate) diblock copolymers for further fine patterning by graphoepitaxy.

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AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: +81-22-217-5668. E-mail: [email protected]. ac.jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the Nano-Macro Materials, Devices and System Research Alliance (MEXT). The authors thank Advantest for the assistance of CD-SEM measurement.

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REFERENCES

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DOI: 10.1021/acs.langmuir.5b00325 Langmuir 2015, 31, 4188−4195

Article

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DOI: 10.1021/acs.langmuir.5b00325 Langmuir 2015, 31, 4188−4195