Synthesis of Ordered Nanoporous Silica Film With High Structural Stability
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Synthesis of ordered nanoporous silica film with high structural stability Norikazu Nishiyama1, Shunsuke Tanaka1, Yoshiyuki Egashira1, Yoshiaki Oku2, Akira Kamisawa3 and Korekazu Ueyama1 1 Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan 2 MIRAI project, AIST Tsukuba Central 2, Tsukuba, Ibaraki 305-8568, Japan 3 Process Technology Division, Semiconductor Research and Development Headquarters, Rohm Co., Ltd., 21 Saiin Mizosaki-cho, Ukyo-ku, Kyoto 615-8585, Japan ABSTRACT A mesoporous silica film was prepared on a silicon substrate using a spin-coating process followed by a tetraethyl orthosilicate (TEOS) vapor treatment. The stability of a formed silica network before TEOS treatment is thought to be insufficient because the rate of the condensation reaction is not high at temperatures below 453 K. The density of silica wall surrounding surfactant assembly could be low, resulting in the structural contraction with the formation of a silica network. On the other hand, the TEOS-treated mesoporous silica film did not contract during calcination, showing high structural stability. In the TEOS treatment, TEOS molecules penetrate into an originally deposited silicate film and react with silanol groups. The densified silica wall has high structural stability and hardly contracts under a calcination process. A flat mesoporous silica film about 250 nm thick was grown from the silicon substrate. A periodic hexagonal porous structure was observed in the FE-SEM image of the cross section of the TEOS-treated film. This indicates that the channels run predominantly parallel to the surface of the silicon substrate. The developed film is a promising material such as chemical sensors, low-k films and other optoelectronic devices.
INTRODUCTION
A new family of ordered mesoporous molecular sieves designated as M41S was discovered in 1992 [1,2]. The M41S molecular sieves include members having uniform pore structures of hexagonal, cubic and lamellar symmetry. In this method, surfactant liquid-crystal structures serve as an organic template for the polymerization of silicates. Recently, self-supporting thin films made of mesoporous materials with unidimensional pore structures have been prepared at air/water [3-6] and oil/water [7] interfaces. Supported mesoporous silica films have been reported by several research groups [8-15]. The mesoporous silica films were grown under acidic conditions at a variety of interfaces including water/mica [8,9], water/graphite [8,10] and water/silica [8] by hydrothermal synthesis. A simpler way to synthesize mesoporous silica films has been developed by a spin-coating [11-13] and dip-coating [14,15] methods. The spin-coating products [12] were transparent films of the hexagonal, cubic and lamellar phases. These solvent-evaporation techniques have been utilized for the coating on glass substrates [11,12] and on silicon wafers [13-15]. B5.2.1 Downloaded from https:/www.cambridge.org/core. University of Arizona, on
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