Rapid Micropatterning of Mesoporous Silica Film by Site-Selective

Aug 27, 2008 - National Institute of Advanced Industrial Science & Technology ... Rapid microfabrication of mesoporous silica film at low temperature ...
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Langmuir 2008, 24, 11141-11146

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Rapid Micropatterning of Mesoporous Silica Film by Site-Selective Low-Energy Electron Beam Irradiation Atsushi Hozumi* and Tatsuo Kimura National Institute of AdVanced Industrial Science & Technology (AIST), Shimoshidami, Moriyama-ku, Nagoya 463-8560, Japan ReceiVed May 23, 2008. ReVised Manuscript ReceiVed June 30, 2008 Rapid microfabrication of mesoporous silica film at low temperature was achieved with low-energy electron beam (LEEB) irradiation. A mesostructured film (thickness ∼ 200 nm), which was prepared through hydrolysis and condensation of tetramethoxysilane in the presence of hexadecyltrimethylammonium chloride, was irradiated with LEEB at 25 kV and 300 µA under pressures of 10 and 1000 Pa. The surfactant molecules can be eliminated completely at temperatures less than 40 °C after only 10 min (10 Pa) and 5 min (1000 Pa) of irradiation, resulting in conversion to a highly ordered mesoporous silica film without cracking. The LEEB-irradiated film also showed reasonable chemical resistance toward dilute hydrofluoric acid solution due to sufficient consolidation by cross-linking of silicate networks during the irradiation. The unirradiated regions were etched away preferentially to the irradiated areas; therefore, rapid micropatterning of the mesoporous silica film was possible by area-selective LEEB irradiation followed by chemical etching.

Introduction Surfactant templated materials are the subject of growing interest not only as catalysts and adsorbents but also as insulating layers, chemical and gas sensors, and optical and electrical devices.1-11 By taking advantage of the large internal surface area, well-ordered mesospaces, and high stability of the framework, various guest species, such as metallic nanoparticles, ions, carbon sources, semiconducting nanocrystals and polymers, and dye molecules, have been successfully doped into mesopores.3,8-29 In order to fabricate microdevices based on such functionalized mesostructured and well-ordered materials, control * To whom correspondence should be addressed. Tel.: +81-52-7367252. Fax: +81-52-736-7406. E-mail: [email protected]. (1) Yanagisawa, T.; Shimizu, T.; Kuroda, K.; Kato, C. Bull. Chem. Soc. Jpn. 1990, 63, 988. (2) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (3) Cortial, G.; Siutkowski, M.; Goettmann, F.; Moores, A.; Boissie`re, C.; Grosso, D.; Floch, P. L.; Sanchez, C. Small 2006, 2, 1042. (4) Baskaran, S.; Liu, J.; Domansky, K.; Kohler, N.; Li, X. H.; Coyle, C.; Fryxell, G. E.; Thevuthasan, S.; Williford, R. E. AdV. Mater. 2000, 12, 291. (5) Cho, A. T.; Pan, F. M.; Chao, K. J.; Liu, P. H.; Chen, J. Y. Thin Solid Films 2005, 483, 283. (6) Yamada, T.; Zhou, H. S.; Uchida, H.; Tomita, M.; Ueno, Y.; Ichino, T.; Honma, I.; Asai, K.; Katsube, T. AdV. Mater. 2002, 14, 812. (7) Yantasee, W.; Lin, Y.; Li, X.; Fryxell, G. E.; Zemanian, T. S.; Viswanathan, V. V. Analyst 2003, 128, 899. (8) Hua, Z. L.; Shi, J. L.; Zhang, L. X.; Ruan, M. L.; Yan, J. N. AdV. Mater. 2002, 14, 830. (9) Goettmann, F.; Moores, A.; Boissie`re, C.; Floch, P. Le.; Sanchez, C. Small 2005, 1, 636. (10) Gu, J. L.; Shi, J. L.; You, G. J.; Xiong, L. M.; Qian, S. X.; Hua, Z. L.; Chen, H. R. AdV. Mater. 2005, 17, 557. (11) Zhao, D.; Seo, S. J.; Bae, B. S. AdV. Mater. 2007, 19, 3473. (12) Fukuoka, A.; Araki, H.; Sakamoto, Y.; Sugimoto, N.; Tsukada, H.; Kumai, Y.; Akimoto, Y.; Ichikawa, M. Nano Lett. 2002, 2, 793. (13) Zhang, Z.; Pan, Z.; Mahurin, S. M.; Dai, S. Chem. Commun. 2003, 2584. (14) Besson, S.; Gacoin, T.; Ricolleau, C.; Boilot, J. P. Chem. Commun. 2003, 360. (15) Ryoo, R.; Joo, S. H.; Jun, S. J. Phys. Chem. B 1999, 103, 7743. (16) Pang, J.; Li, X.; Wang, D.; Wu, Z.; John, V. T.; Yang, Z.; Lu, Y. AdV. Mater. 2004, 16, 884. (17) Tamura, M.; Kemmochi, Y.; Murakami, Y.; Chino, N.; Ogura, M.; Naik, S. P.; Takai, M.; Tsuji, Y.; Maruyama, S.; Okubo, T. Appl. Phys. A: Mater. Sci. Process. 2006, 84, 247. ¨ .; Ozin, G. A.; Yang, H.; Reber, C.; Bussie`re, G. AdV. Mater. 1999, (18) Dag, O 11, 474. (19) Besson, S.; Gacoin, T.; Ricolleau, C.; Jacquiod, C.; Boilot, J. P. Nano Lett. 2002, 2, 409. (20) Wu, J. J.; Gross, A. F.; Tolbert, S. H. J. Phys. Chem. B 1999, 103, 2374.

of patterned morphologies and geometries at the nano- to micrometer scale is key. Several techniques, including spin- and dip-coating, heterogeneous nucleation at solid/liquid interfaces, vapor infiltration, and evaporation-induced self-assembly (EISA), have been reported to obtain thin films.30-45 In addition to these techniques, micropatterning is another fundamental aspect for (21) Nguyen, T. Q.; Wu, J. J.; Doan, V.; Schwartz, B. J.; Tolbert, S. H. Science 2000, 288, 652. (22) Fowler, C. E.; Lebeau, B.; Mann, S. Chem. Commun. 1998, 1825. (23) Marlow, F.; McGehee, M. D.; Zhao, D.; Chmelka, B. F.; Stucky, G. D. AdV. Mater. 1999, 11, 632. (24) Yang, P.; Wirnsberger, G.; Huang, H. C.; Cordero, S. R.; McGehee, M. D.; Scotto, B.; Deng, T.; Whitesides, G. M.; Chmelka, B. F.; Buratto, S. K.; Stucky, G. D. Science 2000, 287, 465. (25) Scott, B. J.; Wirnsberger, G.; Stucky, G. D. Chem. Mater. 2001, 13, 3140. (26) Scott, B. J.; Wirnsberger, G.; McGehee, M. D.; Chmelka, B. F.; Stucky, G. D. AdV. Mater. 2001, 13, 1231. (27) Wirnsberger, G.; Yang, P.; Huang, H. C.; Scott, B.; Deng, T.; Whitesides, G. M.; Chmelka, B. F.; Stucky, G. D. J. Phys. Chem. B 2001, 105, 6307. (28) Yoshikawa, T.; Nakamura, T.; Kuroda, K.; Ogawa, M. Bull. Chem. Soc. Jpn. 2002, 75, 2589. (29) Fukuoka, A.; Miyata, H.; Kuroda, K. Chem. Commun. 2003, 284. (30) Ogawa, M. J. Am. Chem. Soc. 1994, 116, 7941. (31) Lu, Y.; Ganguli, R.; Drewien, C. A.; Anderson, M. T.; Brinker, J. C.; Gong, W.; Guo, Y.; Soyez, H.; Dunn, B.; Huang, M. H.; Zink, J. I. Nature 1997, 389, 364. (32) Zhao, D.; Yang, P.; Melosh, N.; Feng, J.; Chmelka, B. F.; Stucky, G. D. AdV. Mater. 1998, 10, 1380. (33) Yang, H.; Kuperman, A.; Coombs, N.; Mamiche-Afara, S.; Ozin, G. A. Nature 1996, 379, 703. (34) Aksay, I. A.; Trau, M.; Manne, S.; Honma, I.; Yao, N.; Zhou, L.; Fenter, P.; Eisenberger, P. M.; Gruner, S. M. Science 1996, 273, 892. (35) Miyata, H.; Kuroda, K. J. Am. Chem. Soc. 1999, 121, 7621. (36) Suzuki, T.; Miyata, H.; Watanabe, M.; Kuroda, K. Chem. Mater. 2006, 18, 4888. (37) Nishiyama, N.; Tanaka, S.; Egashira, Y.; Oku, Y.; Ueyama, K. Chem. Mater. 2002, 14, 4229. (38) Nishiyama, N.; Tanaka, S.; Egashira, Y.; Oku, Y.; Ueyama, K. Chem. Mater. 2003, 15, 1006. (39) Zhao, D.; Yang, P.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Chem. Commun. 1998, 2499. (40) Brinker, C. J.; Lu, Y. F.; Sellinger, A.; Fan, H. Y. AdV. Mater. 1999, 11, 579. (41) Besson, S.; Gacoin, T.; Jacquiod, C.; Ricolleau, C.; Babonneau, D.; Boilot, J.-P. J. Mater. Chem. 2000, 10, 1331. (42) Cagnol, F.; Grosso, D.; Soler-Illia, G. J.; de, A. A.; Crepaldi, E. L.; Babonneau, F.; Amenitsch, H.; Sanchez, C. J. Mater. Chem. 2003, 13, 61. (43) Hayward, R. C.; Alberius, P. C. A.; Kramer, E. J.; Chmelka, B. F. Langmuir 2004, 20, 5998. (44) Yamauchi, Y.; Sawada, M.; Noma, T.; Ito, H.; Furumi, S.; Sakka, Y.; Kuroda, K. J. Mater. Chem. 2005, 15, 1137. (45) Miyata, H.; Kawashima, Y.; Itoh, M.; Watanabe, M. Chem. Mater. 2005, 17, 5323.

10.1021/la801575t CCC: $40.75  2008 American Chemical Society Published on Web 08/27/2008

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the realization of practical devices. Ozin and co-workers reported for the first time the microfabrication of a mesostructured silica/ surfactant composite using a micropatterned self-assembled monolayer (SAM).46 They demonstrated spatially regulated growth of the mesocomposites on hydrophobic SAM surface regions. Sugimura et al. also reported site-selective nucleation of continuous mesostructured silica/surfactant composite films based on a modification of Ozin’s approach using micropatterned hydrophobic SAM/hydrophilic oxide (SiO2, Al2O3, and TiO2) surfaces.47,48 However, with this template, undesirable deposition of composites was frequently observed on the hydrophilic oxide regions. In addition, site-selectivity on this type of template varies greatly depending on the pattern’s geometry.47 To overcome such shortcomings, one of the authors designed dual SAM surfaces composed of positively charged amino-terminated and hydrophobic fluoroalkyl-terminated SAMs with different pattern interval distances.49 Because adsorption of the cationic surfactant molecules was effectively suppressed through electrostatic repulsion between the amino-terminated regions and the surfactant molecules, well-ordered microlines of the continuous mesostructured silica film could be obtained on a relatively large area independent of surface geometries.49 Mesoporous and mesostructured materials can be fabricated into micropatterned thin films using various techniques such as micromolding, micropen lithography, ink-jet printing, dip-coating of patterned SAMs, and site-selective UV irradiation.50-64 Wu et al. recently reported the fabrication of patterned mesoporous thin films based on electron-beam lithography.65 They successfully fabricated large area patterns of continuous mesoporous SiO2 and TiO2 thin films in the range of 0.25-50 µm.65 After micropatterning, the surfactant molecules must be removed from the micropatterned mesocomposite films without distorting the well-ordered mesostructure. In general, organic components such as surfactants are removed by “calcination” at temperatures higher than 250 °C. However, conventional calcination is not always suitable for thin films and temperature-sensitive substrates, such (46) Yang, H.; Coombs, N.; Ozin, G. A. AdV. Mater. 1997, 9, 811. (47) Sugimura, H.; Hozumi, A.; Kameyama, T.; Takai, O. AdV. Mater. 2001, 13, 667. (48) Hozumi, A.; Sugimura, H.; Hiraku, K.; Kameyama, T.; Takai, O. Nano Lett. 2001, 1, 395. (49) Hozumi, A.; Kojima, S.; Nagano, S.; Seki, T.; Shirahata, N.; Kameyama, T. Langmuir 2007, 23, 3265. (50) Trau, M.; Yao, N.; Kim, E.; Xia, Y.; Whitesides, G. M.; Aksay, I. A. Nature 1997, 390, 674. (51) Huo, Q.; Zhao, D.; Feng, J.; Weston, K.; Buratto, S. K.; Stucky, G. D.; Schacht, S.; Schu¨th, F. AdV. Mater. 1997, 9, 974. (52) Yang, P.; Deng, T.; Zhao, D.; Feng, P.; Pine, D.; Chmelka, B. F.; Whitesides, G. M.; Stucky, G. D. Science 1998, 282, 2244. (53) Yang, P.; Rizvi, A. H.; Messer, B.; Chmelka, B. F.; Whitesides, G. M.; Stucky, G. D. AdV. Mater. 2001, 13, 427. (54) Yang, P.; Wirnsberger, G.; Huang, H. C.; Cordero, S. R.; McGehee, M. D.; Scotto, B.; Deng, T.; Whitesides, G. M.; Chmelka, B. F.; Buratto, S. K.; Stucky, G. D. Science 2000, 287, 465. (55) Scott, B. J.; Wirnsberger, G.; Stucky, G. D. Chem. Mater. 2001, 13, 3140. (56) Scott, B. J.; Wirnsberger, G.; McGehee, M. D.; Chmelka, B. F.; Stucky, G. D. AdV. Mater. 2001, 13, 1231. (57) Wirnsberger, G.; Yang, P.; Huang, H. C.; Scott, B.; Deng, T.; Whitesides, G. M.; Chmelka, B. F.; Stucky, G. D. J. Phys. Chem. B 2001, 105, 6307. (58) Chang, S. C.; Liu, J.; Bharathan, J.; Yang, Y.; Onohara, J.; Kido, J. AdV. Mater. 1999, 11, 734. (59) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S.; Mirkin, C. A. Science 1999, 283, 661. (60) Fan, H.; Lu, Y.; Stump, A.; Reed, S. T.; Baer, T.; Schunk, R.; Perez-Luna, V.; Lo´pez, G. P.; Brinker, C. J. Nature 2000, 405, 56. (61) Fan, H.; Reed, S. T.; Baer, T.; Schunk, R.; Lo´pez, G. P.; Brinker, C. J. Microporous Mesoporous Mater. 2001, 44(45), 625. (62) Doshi, D. A.; Huesing, N. K.; Lu, M.; Fan, H.; Lu, Y.; Simmons-Potter, K.; Potter, B. G.; Hurd, A. J.; Brinker, C. J. Science 2000, 290, 10. (63) Dattelbaum, A. M.; Amweg, M. L.; Ecke, L. E.; Yee, C. K.; Shreve, A. P.; Parikh, A. N. Nano Lett. 2003, 3, 719. (64) Kim, H. C.; Wallraff, G.; Kreller, C. R.; Angelos, S.; Lee, V. Y.; Volksen, W.; Miller, R. D. Nano Lett. 2004, 4, 1169. (65) Wu, C-W.; Aoki, T.; Kuwabara, M. Nanotechnology 2004, 15, 1886.

Hozumi and Kimura

as gold, copper, and polymers. Long-term calcination at high temperature leads to distortion and breakage of films because of the different thermal expansion coefficients of mesoporous silica films and substrates. In addition, calcination has a serious disadvantage for practical device application of mesoporous silica films as a low-k material.66 It is reported that high-temperature treatment promotes the agglomeration of narrow copper interconnects and copper diffusion, leading to serious problems in copper/low-k interconnect integration.66 Consequently, lowtemperature processes for removing organic fractions have strong potential utility. There have been several alternative strategies to remove surfactants from the mesostructured precursor films requiring neither organic solvents nor thermal energy. Novel approaches based on photochemistry have been reported by us67,68 and Clark et al.69 The former method, referred to as “photocalcination”, is based on photochemical reactions induced under irradiation with vacuum ultraviolet (UV) light of 172 nm wavelength, while Clark et al. employed oxidation with photochemically generated ozone molecules. These photochemical approaches are promising, since they can treat a relatively large sample area once at room temperature and mesostructural distortion can be remarkably suppressed compared with conventional calcination. In another approach, the use of argon or oxygen plasma has been proposed to eliminate surfactants.70-72 Gomez et al. have demonstrated that an amphiphilic triblock copolymer could be quickly eliminated using oxygen plasma.70 Although this approach succeeded in effectively eliminating the organic templates at low temperature, it promoted the distortion of the periodic mesostructures. The damage and distortion was suppressed relative to calcination, but they were much larger than that demonstrated by photocalcination. Cracking was also observed on resulting samples.70 Considering practical applications of mesoporous silica films, a low-temperature process that enables simultaneous micropatterning and the complete elimination of surfactants should be developed. An interesting method to form a patterned mesoporous silica film was reported by Doshi et al. using selective UV exposure (256 nm) on photosensitive mesocomposite silica films.62 However, after preferential chemical etching of uncondensed regions, treatment for 3 h at 450 °C was required to condense siloxane networks. This site-selective UV irradiation method has been modified and developed by Dattelbaum et al.63 They have successfully fabricated mesoporous silica micropatterns without photosensitizing substances using deep-UV light (187-254 nm).63 By taking advantage of the great difference in chemical durability between the UV-irradiated and the masked regions, the latter was etched away using dilute NaOH solution. Although this process does not require additional postthermal treatment and allows both elimination of the organic phases and enhancement of silica framework simultaneously, more than 2 h is required to obtain sufficient cross-linking of siloxane networks. (66) Donation, R. A.; Coenegrachts, B.; Maenhoudt, M. Microelectron. Eng. 2001, 55, 277. (67) Hozumi, A.; Yokogawa, Y.; Kameyama, T.; Hiraku, K.; Sugimura, H.; Takai, O. AdV. Mater. 2000, 12, 985. (68) Hozumi, A.; Sugimura, H.; Hiraku, K.; Kameyama, T.; Takai, O. Chem. Mater. 2000, 12, 3842. (69) Clark, T., Jr.; Ruiz, J. D.; Fan, H.; Brinker, C. J.; Swanson, B. I.; Parikh, A. N. Chem. Mater. 2000, 12, 3879. (70) Gomez-Vega, J. M.; Teshima, K.; Hozumi, A.; Sugimura, H.; Takai, O. Surf. Coat. Technol. 2003, 169(170), 504. (71) Zhang, J.; Palaniappan, A.; Su, X.; Tay, F. E. H. Appl. Surf. Sci. 2005, 245, 304. (72) Palaniappan, A.; Zhang, J.; Su, X.; Tay, F. E. H. Chem. Phys. Lett. 2004, 395, 70.

Rapid Micropatterning of Mesoporous Silica Film

Among the numerous high-energy sources available, lowenergy EB (LEEB) of ∼60 kV is one of the most promising for curing, surface modification, and patterning of polymeric materials, SAMs, and sol-gel films.73-82 The characteristic penetration depth of UV light and plasma into materials is only several hundreds of nanometers (100 kV), can be suppressed.83 In the present study, we reported the rapid fabrication and micropatterning of mesoporous silica film at low temperature using LEEB irradiation (accelerating voltage of 25 kV). Since LEEB irradiation is under vacuum, surfactants can be decomposed and eliminated rapidly from the precursor film without distorting mesostructures, resulting in the formation of crack-free mesoporous silica films. It is crucial to elucidate properties of the LEEB-irradiated mesocomposite films to understand the reactions, which include decomposition of organic fractions and crosslinking of siloxane networks during LEEB irradiation. The LEEBirradiated film showed high chemical durability to dilute acidic solution, suggesting that the LEEB irradiation can promote the cross-linking of silicate networks. Accordingly, we succeeded in the micropatterning of mesoporous silica film by site-selective LEEB irradiation and subsequent chemical etching.

Experimental Section Substrates (10 × 10 × 0.5 mm3) cut from n-type Si(100) wafers (Shinetsu Handoutai) were photochemically cleaned up using UV/ ozone treatment. A clear precursor solution was prepared by reaction of tetramethoxysilane (TMOS, Tokyo Kasei Organic Chemicals) in acidic aqueous hexadecyltrimethylammonium chloride (C16TMACl, Tokyo Kasei Organic Chemicals). C16TMACl (3.2 g) was dissolved in 50 mL of Milli Q water. After hydrochloric acid was added to the surfactant solution to adjust the pH to 2.0, 12.18 g of TMOS was added and the mixture stirred for 20 min at room temperature. The precursor solution was then spin-coated onto the UV/ozone-cleaned Si substrates that were subsequently annealed at 100 °C in ambient air for several hours. Each sample was placed in a vacuum chamber evacuated by a rotary pump. The pressure in the chamber was maintained at 10 or 1000 Pa by introducing air through a variable leak valve. The film was then irradiated with LEEB at 25 kV (accelerating voltage) and 300 µA (tube current) for 1-30 min generated from a LEEBirradiation device (Ushio Inc., Mini-EB)83 at a distance of 20 mm between the LEEB transmission window and the substrate surface. The dose of the LEEB for the sample surfaces at 10 and 1000 Pa was estimated by a digital radiachromic reader (Far West Technology, Inc., FWT-92D) to be ca. 314 and 214 kGy min-1, respectively. For (73) Vasilopoulou, M.; Boyatzis, S.; Raptis, I.; Dimotikalli, D.; Argitis, P. J. Mater. Chem. 2004, 14, 3312. (74) Liu, P. T.; Tsai, T. M.; Chang, T. C. Appl. Phys. Lett. 2005, 86, 182903. (75) Harnett, C. K.; Satyalakshmi, K. M.; Craighead, H. G. Appl. Phys. Lett. 2000, 76, 2466. (76) Maeng, I. S.; Park, J. W. Langmuir 2003, 19, 4519. (77) Chang, T. C.; Tsai, T. M.; Liu, P. T.; Chen, C. W.; Tseng, T. Y. Thin Solid Films 2004, 469(470), 383. (78) Nathawat, R.; Kumar, A.; Kulshrestha, V.; Singh, M.; Ganesan, V.; Phase, D. M.; Vijay, Y. K. App. Surf. Sci. 2007, 253, 5985. (79) Giustina, G. D.; Brusatin, G.; Guglielmi, M.; Romanato, F. Mater. Sci. Eng. 2007, C27, 1382. (80) Holla¨nder, A.; Klemberg-Sapieha, J. E.; Wertheimer, M. R. J. Polym. Sci., Part A 1995, 33, 2013. (81) Cho, S. O.; Lee, E. J.; Lee, H. M.; Kim, J. G.; Kim, Y. J. AdV. Mater. 2006, 18, 60. (82) Lee, E. J.; Lee, H. M.; Li, Y.; Hong, L. Y.; Kim, D. P.; Cho, S. O. Macromol. Rapid Commun. 2007, 28, 246. (83) http://www.ushio co jp/en/.

Langmuir, Vol. 24, No. 19, 2008 11143 comparison, another sample was heated at 400 °C (heating rate of 10 °C min-1) for 60 min in flowing N2 and then calcined in flowing O2 for another 120 min. Data collected by “photocalcination”, which was previously reported by us, are shown in the Supporting Information because mesostructured precursor films used in the previous papers were prepared using a different method involving a hydrothermal process.67,68 The substrate temperature was measured by a K-type thermocouple that was attached to its backside. During both LEEB and vacuum UV irradiation, this temperature was confirmed to be less than 40 °C. After LEEB and vacuum UV irradiation, changes in thickness of the precursor films by shrinkage were estimated using both an ellipsometer (Philips, PZ2000) and a laser microscope (Olympus, OLS3100). X-ray diffraction (XRD) patterns were obtained by using a Rigaku RINT 2100 with monochromated Fe KR radiation. Fourier transform infrared (FT-IR) spectra were collected by using a BioRad Laboratories FTS-175C and a JASCO MICRO-20. Transmission electron microscopic (TEM) images were taken by using a JEOL JEM 2010, operated at 200 kV. Samples for TEM observation were prepared by removing the LEEB-irradiated film from the Si substrate. The chemical durability of the LEEB- and vacuum UV-irradiated films was studied by exposing them to 0.01 vol % HF solution in a Teflon vessel at room temperature. After immersion for 2-8 min, the substrate was removed from the solution and rinsed thoroughly with Milli Q water to remove any residual HF solution and then blown dry with dry N2 gas. In order to fabricate micropatterns, some of the samples were exposed by LEEB or vacuum UV irradiation through a copper grid mesh contacting the precursor film surface and subsequently etched chemically by a method similar to that described above. The microstructures fabricated on the substrates were observed at room temperature using an atomic force microscope (AFM, Seiko Instruments, SPA300HV+SPI3800N) using a Si probe (Seiko Instruments Inc., cantilever; force constant ) 0.02 N m-1) with a response frequency of 0.5 Hz and using an optical microscope (Nikon, ECLIPES, ME600).

Results and Discussion LEEB Irradiation. A mesostructured silica-surfactant composite film with a thickness of 190-200 nm was directly LEEBirradiated under the controlled pressures. Parts A and B of Figure 1 show FT-IR spectra (2800-3000 cm-1) of the films before and after LEEB irradiation at 10 and 1000 Pa, respectively. The C-H stretching vibration of C16TMACl at 2850-2900 cm-1 vanished after LEEB irradiation at 10 Pa for 10 min (spectrum c, exposure dose of ca. 3400 kGy) and at 1000 Pa for 5 min (spectrum e, exposure dose of ca. 1230 kGy), indicating that C16TMACl molecules were eliminated from the precursor composite films. LEEB irradiation was found to be as effective as calcinations for eliminating the organic molecules from the composite films. This is probably due to the decomposition of organic molecules in the precursor film due to the energetic electrons. The decomposed molecules further react with activated oxygen species and converted to volatile and gaseous products such as CO, CO2, and H2O, resulting in the elimination from the film to the vacuum environment.81,82,84 Indeed, when oxygen molecules are present in sufficient concentration (at 1000 Pa), the elimination rate was ca. 2 times faster than that at 10 Pa. Although we cannot compare these results directly, the best rate at 1000 Pa was ∼24 times faster than the Dattelbaum’s previous results obtained with a 187-254 nm deep-UV light source.63 For comparison, photocalcination using a vacuum UV light (172 nm) was also conducted under vacuum by using the composite films (Supporting Information, Figures S-1 and S-2). The (84) Ishikawa, Y.; Yoshima, H.; Hirose, Y. J. Surf. Finish. Soc. Jpn. 1996, 47, 74.

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Figure 1. FT-IR spectra of the samples: (A) (a) precursor film, (b) LEEB-irradiated at 10 Pa for (b) 5 min and (c) 10 min; (B) LEEB-irradiated at 1000 Pa for 3 min (d) and 5 min (e); (C, D) precursor film (f, i), photocalcined at 10 Pa for 15 min (g, j), and LEEB-irradiated at 10 Pa for 10 min (h, k) at 800-1600 cm-1 (C) and at 3000-3600 cm-1 (D).

elimination speed was slower than that conducted by the above LEEB irradiation. Mesostructural variation in the composite films during the removal of C16TMACl was investigated by XRD. As can be seen in trace a in Figure 2A, a typical XRD pattern of the precursor film demonstrates that it had well-ordered mesostructures. One intense (10) peak (d10 ) 4.0 nm) and a weak (20) peak are observed, as well as the typical mesoporous silica film with 2-D hexagonal (p6mm) structure. The (11) and (21) peaks due to the 2-D hexagonal structure were not detected in the XRD pattern, revealing that the channel axis was parallel to the substrate.37,38,85 Due to the LEEB irradiation, a slight decrease in the d10 spacing was observed in both traces a and b in Figure 2B. Differences between the d10 values before and after LEEB irradiation at 10 and 1000 Pa were 0.4 and 0.2 nm, respectively. This indicates that the d10 had decreased by 10 and 5%, respectively. The latter value was identical to that of the calcined film (trace b in Figure 2A). Although the distortion of the mesostructure of the sample treated at 10 Pa was slightly larger than that of the calcined film, the 2-D hexagonal structure still remained, even after elimination of C16TMACl molecules. The result is further supported by TEM of the film obtained by the irradiation at 10 Pa (Figure 3), exhibiting the presence of well-ordered mesopores ca. 3.5 nm (85) Sneh, O.; George, S. M. J. Phys. Chem. 1995, 99, 4639.

Figure 2. XRD patterns of (a) the precursor and (b) calcined films at 400 °C for 3 h (A), and (a) LEEB-irradiated films at 10 Pa for 10 min and (b) at 1000 Pa for 5 min (B).

in diameter. Optical microscopy (dark field observation) confirmed that no irradiation damage (e.g., cracking) was observed on the LEEB-irradiated samples. We thus conclude that precursor composite films can be successfully converted to well-ordered

Rapid Micropatterning of Mesoporous Silica Film

Figure 3. Typical TEM image of the precursor film after LEEB irradiation at 10 Pa for 10 min.

Figure 4. Decrease in thickness of the samples after immersion in dilute HF solution: LEEB-irradiated at 10 Pa for 10 min (solid triangles), at 1000 Pa for 5 min (open circles), and at 1000 Pa for 15 min (open squares).

mesoporous silica films without both cracking and large shrinkage by the LEEB irradiation. Difference in Chemical Durability. Judging from FT-IR and XRD, no specific differences were observed in the elimination rate and the resulting mesostructures of the samples after both LEEB and vacuum UV irradiation. However, the actual chemical durability of the samples assessed after exposing them to acidic solution was markedly different. Figure 4 shows the decrease in thickness of the mesoporous silica films (samples identical to Figure 2B) after immersion in dilute HF solution. The untreated precursor film was easily dissolved and completely etched away from the substrate after immersion for 4 min. In contrast, due to the LEEB irradiation, the etching was considerably suppressed. As indicated by closed triangles, with a sample treated at 10 Pa, the decrease in the thickness was ca. 6 nm, even when the sample was immersed for 8 min. In the case of irradiation at 1000 Pa (indicated by the open circles), the thickness decreased ca. 26 nm after 8 min. The etching rate of each was calculated to be ca. 0.01 and 0.05 nm s-1, respectively. This difference is probably due to the exposure dose, because the exposure dose of LEEB for the film surfaces at 10 Pa (ca. 341 kGy min-1) is about 1.38 times larger than that at 1000 Pa (ca. 246 kGy min-1). When the LEEB irradiation at 1000 Pa was extended to more than 10 min, the chemical durability was improved, as indicated by open squares in Figure 4. On the other hand, in the case of photocalcination at 10 and 1000 Pa, the etching rates were 20-30 times faster than that achieved by

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LEEB irradiation at 10 Pa for 10 min (Supporting Information, Figure S-3). This difference in chemical durability observed between the LEEB- and vacuum UV-irradiated films is related to the degree of cross-linking of the siloxane networks. In the case of conventional calcination, not only the thermal decomposition of the organic phases but also dehydration and condensation reactions proceed simultaneously in the silica matrices. Indeed, FT-IR confirmed that Si-OH bonds completely vanished from the calcined films.48 Although organic fractions can also be eliminated by photocalcination without distorting the mesostructures (Supporting Information, Figures S-1 and S-2), siloxane networks in the photocalcined mesoporous silica film are not considered to be formed enough compared with those in the film calcined at high temperature, because the excimer lamp does not generate infrared rays (photocalcination proceeds moderately at low temperature, or even room temperature).48,68,69 Similarly, the LEEB irradiation process allows low-temperature treatment, i.e., less than 40 °C. However, it is well-known that the LEEB has energy several thousand times higher than that of UV light.83 Accordingly, the characteristic penetration depth of LEEB into the polymeric films is much larger than that of vacuum UV light. The latter is only several hundreds of nanometers, while the former is more than 100 times deeper.81-83 Of course, in the present irradiation conditions, i.e., low vacuum conditions (10 and 1000 Pa) and long radiation gap (2 cm), the penetration depth of electrons is considered to be much shallower than that under high vacuum conditions, because the average electron energy might be reduced considerably by the collision of electrons with the residual gases. However, because our precursor film is only ∼200 nm thick, complete penetration of LEEB through the entire film is expected.83 Thus, we believe that the elimination of surfactant molecules and condensation reaction proceeds more effectively than in the case of vacuum UV light. This is also supported by the FT-IR data (800-1600 and 3000-3600 cm-1) shown in Figure 1C,D. The IR absorption band in frame C can be seen at around 1490 cm-1, corresponding to C-H deformation vibration of C16TMACl molecules.86 In addition, this spectrum displays a shoulder at 1150-1200 cm-1, due to the rocking vibration of CH3 in the methoxy groups.86 The spectra also contain peaks at around 910 cm-1 (frame C) and 3000-3500 cm-1 (frame D), and 1050-1070 cm-1 (frame C), corresponding to Si-OH and Si-O-Si bonds, respectively.85 Spectra h and k, and g and j are those of the mesoporous silica films prepared by LEEB irradiation (at 10 Pa for 10 min) and photocalcination (at 10 Pa for 15 min), respectively. Due to both treatments, peaks at around 1490 cm-1 corresponding to C-H deformation vibration have completely disappeared. It is noteworthy that the IR absorption peak intensities corresponding to the Si-OH bond (910 and 3000-3500 cm-1) and the rocking vibration of CH3 in the ethoxysilane (1150-1200 cm-1) in the spectra h and k weakened relatively, compared to the spectra g and j. This clearly indicates that the LEEB irradiation promotes the dehydration and condensation reactions simultaneously in the silica matrices,87 similar to the conventional calcination. Therefore, the mesoporous silica film prepared by LEEB irradiation is only slightly etched away, unlike that prepared by photocalcination. Site-Selective LEEB Irradiation. To obtain micropatterns of the mesoporous silica film, we used the technique of siteselective LEEB irradiation of precursor films through a Cu mesh and subsequent chemical etching. Parts a and b of Figure 5 (86) Socrates, G. Infrared Characteristic Group Frequencies: Tables and Charts, 2nd ed.; John Wiley & Sons, Inc.: Chichester, England, 1994; p 157. (87) Hozumi, A.; Sekoguchi, H.; Takai, O. J. Electrochem. Soc. 1997, 144, 2824.

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Hozumi and Kimura

Figure 5. Optical micrograph (a) and AFM image (b) of the microfabricated mesoporous silica films. Panels a and b show a site-selectively LEEB-irradiated film (at 10 Pa for 10 min). The sample was etched for 8 min in dilute HF solution. Panel c shows a cross-sectional view of the micropatterns between the A and B regions on the AFM image in b.

respectively show typical optical micrographic and AFM images of the LEEB-irradiated film obtained at 10 Pa for 10 min, followed by immersion in HF solution for 8 min. The masked regions were selectively etched, while the LEEB-irradiated regions were protected from the etching because of sufficient siloxane condensation. Square-shaped micropatterns composed of 25 × 25 µm2 features can be fabricated over a relatively large area. Judging from the cross-sectional image of these patterns shown in Figure 5c, the height of the micropatterns was measured to be 180-187 nm. Since the original thickness of the precursor film was ∼200 nm, the LEEB-irradiated regions appeared to be etched away by only 6-10%. This corresponds well with the results shown in Figure 4. In addition, the surface of the micropatterns was relatively smooth. Close inspection of Figure 5b,c reveals that the unirradiated regions around the edge of the patterns, within 1-1.5 µm, remained intact on the surface, even after chemical etching. These undissolved regions of each pattern are estimated to be ca. 50 nm in height. This undoubtedly indicates that the cross-linking of the film proceeded in the borders of the masked regions. LEEB under low vacuum conditions is reported to irradiate not perpendicularly but rather randomly to the sample surface, because of the collision of electrons with the residual gases.83 Accordingly, electrons might have penetrated not only unmasked regions but also masked regions, resulting in the crosslinking of the film even around the masked regions. To eliminate such undesirable parts surrounding the LEEB-irradiated regions completely, it required much longer treatment periods of more than 35 min. In spite of careful control of the treatment time, the LEEB-irradiated mesoporous regions further etched away. The resulting decrease in the thickness was 30-35% of the initial value. In contrast, in the case of photocalcination, it was difficult to achieve clear micropatterns (Supporting Information, Figure S-4). Although the vacuum UV-unirradiated regions still remained, even after etching, there was marked distortion of the micropatterns. The achieved micropatterns appear not to be square, but rather, rounded by etching. Some of the edged regions were peeled off from the surface.

The results presented clearly indicate that the LEEB irradiation allows both quick elimination of structure-deciding agents and organic phases and the enhancement of siloxane condensation without the need for the use of photosensitizing substances. The method presented here shows great potential for removing organic fractions from mesostructured films at both low temperature and a relatively high processing speed, which will be applicable to not only silica but also thermally unstable inorganic oxides, such as transition metal oxides and phosphates.

Conclusions A facile strategy for fabricating of micropatterns of mesoporous silica film at low temperature is proposed. Using LEEB irradiation, surfactant molecules can be eliminated from the mesocomposite film after only ∼10 min, resulting in rapid formation of crackfree mesoporous silica films with well-ordered mesostructures. LEEB irradiation is clearly superior to other low-temperature processes using UV light and plasma.63,70 Because LEEB irradiation also promotes the condensation of siloxane networks, the resulting mesoporous silica films show reasonable chemical durability against dilute HF solution, compared to the photocalcined mesoporous silica film. This is a great advantage for site-selective chemical etching of mesostructured and mesoporous films. This LEEB irradiation will be applicable to micropatterning of thermally unstable inorganic oxides and phosphates and may open the path to a wide variety of advanced device and sensor applications. Acknowledgment. This study was supported in part by an Aichi Science and Technology Foundation program and “Nagoya Nano-Technology Cluster of Innovative Production System” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT). Supporting Information Available: Figures S1-S4. This material is available free of charge via the Internet at http://pubs.acs.org. LA801575T