Size-Dependent Filling Behavior of UV-Curable Di(meth)acrylate

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Size-Dependent Filling Behavior of UV-Curable Di(meth)acrylate Resins into Carbon-Coated Anodic Aluminum Oxide Pores of around 20 nm Masaru Nakagawa,* Akifumi Nakaya, Yasuto Hoshikawa, Shunya Ito, Nobuya Hiroshiba, and Takashi Kyotani Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan ABSTRACT: Ultraviolet (UV) nanoimprint lithography is a promising nanofabrication technology with cost efficiency and high throughput for sub-20 nm size semiconductor, data storage, and optical devices. To test formability of organic resist mask patterns, we investigated whether the type of polymerizable di(meth)acrylate monomer affected the fabrication of cured resin nanopillars by UV nanoimprinting using molds with pores of around 20 nm. We used carbon-coated, porous, anodic aluminum oxide (AAO) films prepared by electrochemical oxidation and thermal chemical vapor deposition as molds, because the pore diameter distribution in the range of 10−40 nm was suitable for combinatorial testing to investigate whether UV-curable resins comprising each monomer were filled into the mold recesses in UV nanoimprinting. Although the UV-curable resins, except for a bisphenol A-based one, detached from the molds without pull-out defects after radical photopolymerization under UV light, the number of cured resin nanopillars was independent of the viscosity of the monomer(s) in each resin. The number of resin nanopillars increased and their diameter decreased as the number of hydroxy groups in the aliphatic diacrylate monomers increased. It was concluded that the filling of the carbon-coated pores having diameters of around 20 nm with UVcurable resins was promoted by the presence of hydroxy groups in the aliphatic di(meth)acrylate monomers. KEYWORDS: nanopore, resin filling, carbon coating, anodic aluminum oxide, nanoimprint

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INTRODUCTION Ultraviolet nanoimprint lithography (UV-NIL) is a lithographic technique used in micro/nano fabrication that attracts much attention as a next-generation lithography approach because of its high resolution and simplicity.1−6 The 2015 edition of the International Technology Roadmap for Semiconductors 2.0 (ITRS 2.0)7 states that to achieve a “big data” society, the nanofabrication of dynamic random access memory with a half pitch of 18 nm by 2019 and the reduction of the half pitch to 9 nm by 2027 are required. Although several studies on pattern transfer from molds to cured resin patterns by imprinting have been reported,8−15 the suitable monomers in UV-curable resins to achieve fine pattern transfer remain unclear. We recently used fluorescence microscopy to show that the pollution of a fluorinated silica mold was suppressed using the dimethacrylate monomer bisphenol A glycerolate dimethacrylate (BPAGDM) with hydroxy, aromatic phenyl, and methacryloyl moieties.16,17 Line-and-space resist patterns with a width of 32 nm were successful fabricated by UV nanoimprinting and transferred to a silicon surface by fluorine-based dry etching. In contrast, 22 nm-wide resist patterns could not be prepared because of incomplete filling of the UV-curable resin into mold recesses. Kurihara et al.18−20 conducted resonance shear measurements that showed that the viscosity of the water confined between silica surfaces increased when their separation was less than 8 nm because of the strong intermolecular interactions among © XXXX American Chemical Society

water molecules that were called as water structuring. Shimazaki and co-workers revealed that a gap of approximately 20 nm between mica surfaces confined the fluidity of a UV-curable resin composed of urethane-containing acrylate monomers.21 Considering these pioneering studies, we anticipated that the filling of UV-curable resins into mold recesses with smaller diameters than 20 nm might be different because of the increased resin viscosity and/or monomer structuring in comparison to those with larger diameters than 30 nm. Imprint molds with different pore diameters controlled within a few nanometers are needed to investigate resin filling into mold cavities with sub-30 nm sizes. In general, fused silica and silicon molds for imprinting are fabricated by electronbeam lithography. However, it is not practical to fabricate such precise molds by electron-beam lithography. Therefore, we noticed anodic aluminum oxide (AAO) films with perpendicularly aligned pores for the following reasons. First, AAO films with a nominal pore diameter have been fabricated over large areas by electrochemical oxidation, and their pore diameter can be tuned in the range of 6−200 nm by selecting suitable oxidation voltage and acidic electrolyte.22−24 In particular, AAO films with a pore diameter of approximately 100 nm are used as Received: August 23, 2016 Accepted: October 21, 2016 Published: October 21, 2016 A

DOI: 10.1021/acsami.6b10561 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. (a−e) Top-view and (f−j) cross-sectional FE-SEM images of (a, f) an unmodified AAO film and (b−e, g−j) ones modified after thermal CVD with a C2H2/N2 gas mixture for (b, d, g, i) 2, (e, j) 4, and (c, h) 5 h. The volume ratio of C2H2/N2 was (b, g, c, h) 5/95 and (d, i, e, j) 20/80 under a total flow rate of 500 mL/min. (k−n) Pore diameter distributions observed for (k) an unmodified AAO film and (l−n) ones modified after thermal carbon CVD. The CVD was carried out for (l, n) 2 and (m) 5 h with a C2H2/N2 gas mixture [C2H2/N2 (v/v) = (l, m) 5/95 and (n) 20/80].

carbon source and the deposition period on carbon deposition onto the outermost and side-wall surfaces of AAO films are studied. Five kinds of di(meth)acrylate monomers are selected, and their respective UV-curable resins are prepared. The cured resin patterns obtained after UV nanoimprinting are investigated by field-emission scanning electron microscopy (FESEM) and scanning probe microscopy (SPM) to reveal whether the resin filling into mold cavities with diameters of around 20 nm depends on the type of di(meth)acrylate monomer.

UV nanoimprint molds to fabricate optical antireflection polymer films with nanopillars.25,26 Second, thermal chemical vapor deposition (CVD) using acetylene gas as a carbon source produces a ultrathin carbon film on AAO surfaces. For example, an incompletely crystallized carbon film with a thickness of less than 10 nm was deposited all over the outermost and side-wall surfaces of AAO films with a pore diameter of 100 nm and then used as nanoscale carbon tubes.27,28 We anticipated that this carbon film could function as an antisticking layer for UV-cured resins, because glassy carbon molds are used for pattern transfer to fused silica surfaces by thermal nanoimprinting29−33 and the surface free energy of nonpolar carbon films is estimated to be as small as 6.5 mJ m−2.34 Third, the size distribution of the pore diameter of AAO films is within approximately 15 nm. We thought that AAO molds with such a diameter distribution would be suitable as molds for combinatorial testing to study the filling behavior of UV-curable resins composed of different monomers into nanoscale mold cavities. In this study, we investigate to fabricate carbon-coated AAO molds having an approximate diameter of 20 nm and depth of 50 nm by electrochemical oxidization followed by thermal CVD. In particular, the effects of the concentration of the

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RESULTS AND DISCUSSION Preparation of Carbon-Coated Porous AAO Molds by Electrochemical Oxidation and Thermal CVD. Aluminum (Al) films with a thickness of 50 nm were deposited on conductive silicon substrates to decrease the depth of AAO pores and to obtain films suitable for use as molds in UV nanoimprinting. This is because large aspect ratios of pore depth to pore diameter usually cause pull-out defects of cured resin patterns after demolding. The electrochemical oxidation of Al films to form an AAO pore diameter of 20 nm was performed in acidic aqueous H2SO4 solution at an applied B

DOI: 10.1021/acsami.6b10561 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces voltage of 20 V with an electrolyte temperature of 1 °C according to the literature.22 Figure 1a and f show top-view and cross-sectional FE-SEM images of a fabricated AAO film on a silicon substrate. A porous AAO film with approximately a pore diameter of 20 nm and pore depth of 50 nm was fabricated by the electrochemical oxidation process. Subsequently, thermal CVD was performed by pyrolysis with acetylene (C2H2) as a carbon source at 600 °C to form a carbon antisticking layer on the AAO surface. Figure 1 presents top-view and cross-sectional FE-SEM images and diameter distributions of porous AAO films subjected to thermal CVD in which the C2H2 volume fraction and deposition period were changed. The AAO films used in these experiments had an average pore diameter of 22 nm with a diameter distribution in the range of 10−38 nm (Figure 1k). Thermal CVD with a 20 vol % C2H2 mixture decreased the average diameter to 16 nm (Figure 1d, n). As illustrated in Figure 1i, the carbon layer deposited on the outermost surface of the AAO film was thicker than that deposited on the sidewall surface of the AAO film. It is thought that the supply of C2H2 was sufficient, so the outermost surface preferentially reacted with C2H2 to cause carbon deposition rather than the side-wall surface. Prolonged CVD for 4 h caused the AAO pores to be capped by deposited carbon with a thickness of approximately 10 nm (Figure 1e). The concentration of C2H2 was decreased to promote carbon deposition on the side-wall surface. It was thought that thermal CVD of carbon proceeded with 5 vol % C2H2 for 2 h because the gray AAO film on the silicon substrate darkened. However, the FE-SEM image of the AAO film after CVD (Figure 1b) was not obviously different from that of the AAO film before CVD (Figure 1a). The average pore diameter slightly decreased to 21 nm, and the size distribution decreased in the range of 8−36 nm. Part of the alumina was selectively dissolved by immersion in aqueous NaOH solution and the residue was observed by transmission electron microscopy (TEM) to confirm carbon deposition on the AAO surfaces. Figure 2 revealed that a carbon film with a thickness of approximately 2 nm was formed, and the AAO surfaces were completely covered with a uniform carbon film in accordance with previous reports.27,28 In the tubular carbon films, many short and wrinkled (002) lattices were observed, as reported previously.28 This indicates that the crystallinity of the carbon film was low. Prolonged CVD for 5 h decreased the average pore diameter to 15 nm. We confirmed that thermal CVD with a 5 vol % C2H2 gas mixture for 2 h enabled uniform coverage of the outermost and side-wall AAO surfaces with a carbon film. These carbon-coated AAO films were then used to investigate the filling of UV-curable resins into carbon-coated mold cavities in UV nanoimprinting, as described in the next section. Effect of the Type of Di(meth)acrylate Monomer on Resin Filling. Five kinds of di(meth)acrylate monomers with a wide range of viscosities from 10 to >400,000 mPa·s, as summarized in Table 1, were selected. UV-curable resins composed of each di(meth)acrylate monomer were prepared by addition of the photoinitiator Irgacure 907. First, we compared the performance of a carbon-coated AAO mold with that of a bare AAO mold (without thermal CVD) in UV nanoimprinting using a glycerol 1,3-diglycerolate diacrylate (GDD)-based UV-curable resin to investigate whether the deposited carbon layer functions as an antisticking layer. Figure 3a, b shows photographs of bare and carbon-coated AAO molds, respectively, after contacting them with the GDD-based

Figure 2. (a, b) TEM images of carbon thin films prepared by thermal CVD with a C2H2/N2 gas mixture [C2H2/N2 (v/v) = 5/95; total flow rate = 500 mL min−1] on AAO surfaces for 2 h.

UV-curable resin, curing the resin by exposure to UV light, and detaching them from the cured resin film. Cured resin was pulled out from a silica plate and entirely stuck onto the bare AAO film, whereas the carbon-coated AAO film readily detached from the cured resin patterns on a silica plate. This confirms that the deposited carbon layer with a thickness of approximately 2 nm functioned as an antisticking layer. Figure 4 shows AFM topographic images of imprint resin patterns fabricated with the five UV-curable resins using carbon-coated AAO molds. The UV-curable resins composed of hydroxy-containing glycerol dimethacrylate (GDM), dipropanoic acid (1-methyl-1,2-ethanediyl)bis[oxy(2-hydroxy-3,1propanediyl)] ester (70PA), or GDD monomers gave nanopillar patterns similar to the pore structures of the modified AAO molds, as illustrated in Figure 4a, b and c, respectively. The average pillar heights were 53 ± 6 nm for GDM (Figure 4f), 45 ± 5 nm for 70PA (Figure 4g), and 44 ± 5 nm for GDD (Figure 4h), which were almost identical to the pore depth of 50 nm. Note that the number of imprinted nanopillars appeared to increase with the number of hydroxy groups independent of their increasing viscosity in the range of 40− 12,950 mPa·s. Figure 4d and i reveal that no pillar pattern formed when the hydroxy-free 1,10-decanediol diacrylate (AC10) monomer with a low viscosity of 10 mPa·s was used. Similarly, no imprinted pillar pattern was obtained using the resin with the high-viscosity BPAGDM monomer (>400 000 mPa s) with two hydroxy groups and an aromatic bisphenol A moiety, as shown in Figure 4e, j. The monomer molecules are much smaller than the AAO pores, so we wondered why the UV-curable resins composed of BPAGDM or AC10 were not molded by UV nanoimprinting. We doubted that molded and cured resin nanopillars were pulled out during demolding and stuck to the carbon-coated mold surfaces. Figure 5 shows topC

DOI: 10.1021/acsami.6b10561 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Table 1. Chemical Structure, Viscosity, And Number of Hydroxyl Groups Per Molecule of Di(meth)acrylate Monomers in UVCurable Resins Used in This Study

moieties. Therefore, the reason for the difference between the number of imprinted resin nanopillars of the UV-curable resins was mainly attributed to the different filling behavior of the resins into the carbon-coated AAO pores with an average diameter of 21 nm. It was considered that for the BPAGDMbased resin, strong π−π interactions between the aromatic ringcontaining monomer and the graphene-like (002) lattice carbon layer caused the cured resin to stick onto the carbon film surface, as observed in Figure 5i. To confirm whether the number of imprinted resin nanopillars increased with the number of hydroxy groups in the aliphatic monomers of GDM, 70PA, and GDD, we selected 70PA and GDD with two and three hydroxy groups, respectively, because of their similar chemical structures. Two UV-curable resins containing both 70PA and GDD monomers with weight ratios of 1:2 and 2:1 were prepared, in addition to the UV-curable resins containing only 70PA or GDD monomers. Top-view FE-SEM images of imprinted resin

Figure 3. Photographs of (a) unmodified and (b) carbon-coated AAO surfaces after UV nanoimprinting with GDD-based UV-curable resin.

view and cross-sectional FE-SEM images of carbon-coated AAO molds after UV nanoimprinting with each UV-curable resin. These images confirm that such pull-out defects did not occur during demolding of the UV-curable resins except for the BPAGDM-based resin containing aromatic bisphenol A

Figure 4. AFM (a−e) topological images and (f−j) height profiles of cured resin patterns composed of (a, f) GDM, (b, g) 70PA, (c, h) GDD, (d, i) AC10, and (e, j) BPAGDM fabricated by UV nanoimprinting with carbon-coated AAO molds. D

DOI: 10.1021/acsami.6b10561 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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nanopillars prepared with UV curable resins with GDD-to70PA weight ratios of 1:0 (Figure 6a), 2:1 (Figure 6b), 1:2 (Figure 6c), and 0:1 (Figure 6d) were obtained. These images confirm that the number of resin nanopillars increased with the content of high-viscosity GDD monomer with three hydroxy groups in the UV-curable resins. Therefore, the filling of the carbon-coated AAO pores having diameters of around 20 nm with hydroxy-containing aliphatic diacrylate monomers was assisted by the hydroxy groups of the aliphatic monomers. Figure 6d displays the diameter distributions of resin nanopillars prepared with the UV-curable resins determined from the FE-SEM images. An increased content of GDD in the UV-curable resin resulted in not only an increased number of molded nanopillars, but also decreased pillar diameters. Unfortunately, we could not compare the diameter distributions between the mold AAO pores and imprinted resin nanopillars because the nanopillars required Pt/Pd sputtering before FE-SEM observation, whereas the carbon-coated AAO mold pore could be observed without sputtering. We believe that the larger diameters of the resin nanopillars relative to those of the AAO pores were probably caused by an increase in diameter induced by Pt/Pd deposition on the resin surface and by the influence of the charge-up process during FE-SEM observation.

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CONCLUSION Porous AAO films with diameters of around 20 nm and a depth of 50 nm were prepared by electrochemical oxidation and successfully used as molds in UV nanoimprinting for combinatorial testing to study the filling behavior of resins into mold recesses with distributed pore diameters. Thermal CVD at 600 °C with C2H2 as a carbon source entirely covered the outermost and side-wall surfaces of the AAO films with a 2 nm-thick carbon film that functioned as an antisticking layer in UV nanoimprinting. UV-curable resins composed of hydroxycontaining aliphatic di(meth)acrylate monomers GDM, 70PA, and GDD could be molded by the carbon-coated AAO molds, whereas a UV-curable resin composed of a hydroxy-free aliphatic monomer, AC10, hardly filled the mold cavities having diameters of around 20 nm under an applied pressure of 1.6 MPa, which is higher than that usually used in UV

Figure 5. (a−d) Top-view and (e−i) cross-sectional FE-SEM images of carbon-coated AAO molds after UV nanoimprinting of respective UV-curable resins composed of (a, e) GDM, (b, f) 70PA, (c, g) GDD, (d, h) AC10, and (i) BPAGDM.

Figure 6. (a−d) Top-view FE-SEM images of cured resin patterns composed of GDD and 70PA monomers fabricated by UV nanoimprinting with carbon-coated AAO molds. The weight ratio of GDD to 70PA in the UV-curable resins was (a) 1:0, (b) 2:1, (c) 1:2, and (d) 1:0. (e) Nanopillar diameter distributions observed for the cured resin patterns composed of GDD and 70PA monomers. E

DOI: 10.1021/acsami.6b10561 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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an intensity of 100 mW cm−2 for 20 s. UV light was obtained from a UV light-emitting-diode (Hamamatsu Photonics, L11921−400). The light intensity at 365 nm was monitored at the same position as a substrate using an optical power meter (Hamamatsu Photonics, C6080−03). The carbon-coated AAO mold was then detached from the cured resin patterns at a rate of 4000 μm−1. Characterization. The morphology of AAO molds and convex dot resin patterns was observed by FE-SEM (Hitachi High-Technologies, S-4800). AAO molds before and after carbon coating were observed without sputtering. Cured resin patterns were observed after Pt/Pd sputtering at an applied voltage of 2.5 kV for 20 s using ion sputtering equipment (Hitachi, E-1010). FE-SEM images were analyzed using software (Mitani, WinRoof). The surface morphology of imprinted resin patterns was studied in terms of pattern height by SPM (SII, Simage) in dynamic force mode with a microcantilever (Olympus, OMCL-AC200TS-R3). A carbon antisticking layer on an AAO surface was observed using TEM (JEOL, JEM-2010) at an accelerating voltage of 200 kV. The specimen was prepared as follows. An AAO film after thermal CVD of carbon was broken into pieces with an approximate area of 25 × 8 mm. The pieces were immersed in an aqueous solution of NaOH (3 mol dm−3) for 3 min under ultrasonic vibration to dissolve AAO. Carbon residue on the Si substrate was rinsed thoroughly with distilled water and floated by dropping ethanol onto the substrate surface. The carbon residue floating on ethanol was transferred onto a copper grid for TEM observation.

nanoimprinting. We demonstrated that the number and diameter of imprinted resin nanopillars were affected by the number of hydroxy groups present in the aliphatic di(meth)acrylate monomers. Monomer viscosity near the interface between the solid mold and fluid resin should be studied to fully comprehend the filling behavior of resins into nanoscale cavities. This study suggested that the combination of mold surface and resin monomer is important to realize sub-20 nm fabrication by UV nanoimprinting.

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EXPERIMENTAL SECTION

Fabrication of AAO Films on Silicon Substrates. Electrochemical oxidation of thick Al films was performed according to the method used by Zhang et al.23 to fabricate AAO films with a diameter of 20 nm and micrometer-scale depth. An Al film with a thickness of 50 nm was deposited from an Al target (Furuuchi Chemical, Tokyo Japan, purity 99.99%) on a boron-doped p-type Si(100) substrate (25 × 25 × 0.52 mm, resistivity 3.72−4.09 Ω cm) by magnetron sputtering (Shibaura Mechatronics, CFS-4ES II). Magnetron sputtering was carried out for 2.0 min using a chamber pressure of 8.0 × 10−4 Pa, sample rotation rate of 20 rpm, Ar-ion gauge pressure of 5.0 × 10−1 Pa, output power of 300 W, and presputtering period of 2.0 min. Anodic oxidization of the thin Al film was carried out at a constant applied voltage of 20 V for 10 min in an aqueous solution of H2SO4 (2.93 wt %) kept at 1 °C using an Al plate (Furuuchi Chemical, 25 × 25 × 0.5 mm) as the cathode and a power supply (Kikusui Electronics, PWR1600L). The oxidized Al film was immersed in an aqueous solution of H3PO4 (5 wt %) at 30 °C for 6 min and then rinsed thoroughly with deionized water to give an AAO film with a pore diameter of 21 nm and pore depth of 50 nm. Thermal CVD of a Carbon Antisticking Layer on an AAO Film. An AAO film prepared on a Si substrate was placed in the center of a synthetic quartz tube (inner diameter 58 mm, length 700 mm) and heated at a rate of 10 °C min−1 to 600 °C under N2 at a flow rate of 500 mL min−1. Thermal CVD of carbon was started by introducing acetylene gas with the N2 gas. The total flow rate of the mixed gas was 500 mL min−1, and the flow rate of acetylene gas was 25 or 100 mL min−1; namely, the concentration of acetylene was 5 or 20 vol %, respectively. UV Nanoimprinting of Thin Films of UV-Curable Resins. Five kinds of di(meth)acrylates were used as monomers to prepare respective UV-curable resins: BPAGDM (Sigma-Aldrich), GDD (Sigma-Aldrich), 70PA (Kyoeisha Chemical, Osaka Japan), GDM (Tokyo Chemical Industry), and AC10 (Tokyo Chemical Industry). The chemical structures and bulk viscosities of the monomers at 25 °C are listed in Table 1. A photoinitiator for radical photopolymerization, 2-methyl-1-[4-(methylthio)phenyl]-2-(4-morpholinyl)-1-propanone (Irgacure 907, BASF Japan), was added to each monomer with a molar ratio of 0.04, and then the mixtures were diluted with 1-methoxy-2propanol (PGME, Sigma-Aldrich) for spin coating. The weight ratios of UV-curable resin to solvent were 12:88 for BPAGDM, 20:80 for GDD, 20:80 for 70PA, 15:85 for GDM, and 25:75 for AC10. UVcurable resins containing both GDD and 70PA were prepared by mixing respective diluted UV-curable resins with weight ratios of 1:2 and 2:1. Fused silica substrates (8 × 8 × 0.3 mm) were cleaned for 15 min by exposure to vacuum UV light using a Xe excimer lamp (Ushio, UER-20−172VA). In the case of UV-curable resins containing GDM or AC10, silica surfaces were modified with 3-(trimethoxysilyl)propyl acrylate at 150 °C for 1 h to prevent dewetting of the spin-coated film. Thin films of UV-curable resins were prepared by spin coating onto silica substrates and then annealed on a hot stage at 50 °C for 2 min to evaporate the solvent. A silica plate coated with UV-curable resin and a carbon-coated AAO mold were positioned as a superstrate and substrate, respectively, in a UV nanoimprint stepper (Sanmei, ImpFlex Essential). The mold was contacted with the resin film under a 1,1,1,3,3-pentafluoropropane atmosphere. The applied pressure was increased to 1.6 MPa. After a holding period of 30 s for load adjustment, the molded resin was cured by exposure to UV light with

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel/Fax: +81-22-2175668. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by a KAKENHI (15H03860) Grant-in-Aid for Scientific Research (B) and the Nano-Macro Materials, Devices and System Research Alliance from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT).

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REFERENCES

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DOI: 10.1021/acsami.6b10561 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX