Formation of silica coating films from spin-on polysilazane at room temperature and their stability in hot water
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Eisuke Tadaoka Research & Development Department, Hitachi Housetec Co., Ltd., 1250 Shimoezure, Shimodate-shi, Ibaraki 308-8523, Japan
Hiromitsu Kozuka Department of Materials Science and Engineering, Kansai University, 3-3-35 Yamate-cho, Suita, 564-8680, Japan (Received 4 August 2003; accepted 17 November 2003)
Spin-coating was conducted on single-crystal silicon substrates using a xylene solution of perhydropolysilazane (PHPS) to obtain 0.14-m-thick PHPS films. The PHPS films obtained were suspended over various acidic and basic solutions at room temperature so that they were exposed to the vapors from the solutions. PHPS-to-silica film conversion was studied by infrared absorption spectroscopy, and the stability of the exposed films in hot water was examined by soaking the films in 80 °C water. The as-deposited PHPS films were found to be dissolved in water. When the PHPS films were suspended over water, the films were partially converted into silica, but the resultant film had low stability in hot water. Higher degrees of PHPS-to-silica conversion and higher stabilities in hot water were found on exposing the PHPS films to basic or acidic vapors. Exposure to highly basic vapors from aqueous solutions of ammonia and ethylamine was significantly effective in PHPS-to-silica film conversion, which provided silica thin films with high stability in hot water.
I. INTRODUCTION
Silica coating films have a variety of applications as electric insulating coatings,1 protective barriers against oxidation, corrosion, and scratching for metallic materials,2 passivation films for semiconductors,3 coatings on glass for preventing alkali-dissolution,4 and antireflection coatings.5 Various techniques have been used thus far in preparing silica coating films, such as vacuum deposition,6 sputtering,7 chemical vapor deposition (CVD),8 sol-gel method using alkoxides,9 liquid phase deposition (LPD),10 and electrophoresis.11 Recently, perhydropolysilazane (PHPS) has attracted much attention as the precursor for silica thin films. PHPS is an inorganic polymer with Si–N skeleton and is often used as the precursor for silicon nitride.12 Matsuo and Yamada13 deposited PHPS films on single-crystal silicon substrates and heated them at 450 °C, obtaining silica thin films of density 2.1–2.2 g cm−3, refractive index 1.45–1.46, resistivity about 1015 ⍀ cm, and dielectric constant 4.2—all similar to those of silica glass. Kamiya et al.14 studied the structural changes in PHPS films on heat treatment by infrared absorption spectroscopy and x-ray radial distribution analysis, finding that the J. Mater. Res., Vol. 19, No. 2, Feb 2004
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fourfold siloxane rings are converted into sixfold rings at 400–500 °C in PHPS, whereas it occurs at 600–800 °C in alkoxide-derived gels, which they thought the reason for the formation of nonporous silica film from PHPS at low temperatures. Matsuo and Yamada also reported that PHPS films can be converted into silica films at lower temperatures by some post-tre
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