In-situ Observation of Formation Processes of Anodic Porous Alumina on a Si Substrate Using Infrared Absorption Spectros
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In-situ Observation of Formation Processes of Anodic Porous Alumina on a Si Substrate Using Infrared Absorption Spectroscopy Yasuo Kimura1,2, Hirokazu Shiraki1, Hisao Ishii1,2, Sachiko Ono3, Kingo Itaya2,4, and Michio Niwano1,2 1 Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Phone: +81-22-217-5502 E-mail: [email protected] 2 CREST, Japan Science and Technology Corporation, Aoba-yama 04, Sendai, 980-8579, Japan 3 Department of Applied Chemistry, Faculty of Engineering, Kogakuin University, 1-24-2 Nishi Shinjuku, Shinjuku-ku, Tokyo 163-8677, Japan 4 Depeartment of Applied Chemistry, Graduate School of Engineering, Tohoku University, Aobayama 04, Sendai, 980-8579, Japan ABSTRACT We investigated formation processes of a porous anodic alumina film on a p-type silicon (Si) substrate using infrared absorption spectroscopy in the multiple internal reflection geometry (MIR-IRAS). We observed drastic IR spectral changes when porous anodic alumina film approached interfaces between an aluminum (Al) layer and a Si substrate. The intensity of the IR absorption peaks due to water (H2O) molecules and silicon oxides (SiO2) increased simultaneously with a spike of anodic current density. The IR spectral changes indicated that the penetration of electrolytes brought about inhomogeneous oxidation of a Si substrate surface. We observed that the arrangement of the SiO2 nanodots closely reflected that of pores of a porous anodic alumina film. IR absorption peaks due to porous anodic alumina finally disappeared. The formation of SiO2 nanodots on a Si substrate promoted penetration of electrolytes to peel the porous anodic alumina film off it. INTRODUCTION Recently, nanodevices utilizing quantum effects are widely studied. It is necessary to form ordered nanostructures on semiconductors for fabricating and integrating nanodevices with uniform properties. In particular, it is important to fabricate nanodevices on a silicon (Si) substrate in terms of hybridization with a traditional silicon technology such as a complementary metal - oxide - semiconductor (CMOS) technology. Electrochemical etching technique is one of methods of forming nanostructures. Porous anodic alumina has attracted as a key material for the fabrication of various nanostructures [1-4] because ordered nanohole arrays are self-organized by the two-step method [5]. It has useful structural features that cannot be obtained by conventional lithographic techniques, e.g. uniform pore sizes, a high pore density, and a high aspect ratio. The size and the period of pores are readily controlled by electrolytes and an anodic potential [6]. Porous anodic alumina is usually obtained by anodizing bulk aluminum (Al) plates or foils at a constant voltage in various acidic electrolytes such as oxalic acid. Recently, anodization processes of aluminum films deposited on metal or semiconductor substrates are widely studied to fabricate nanostructures on the substrates using the porous anodic alumina film as a
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