1. Advanced Research Institute for Science and Engineering, Waseda University, Shinjuku‐ku, Tokyo 169, Japan
2. Department of Applied Chemistry, School of Science and Engineering, Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, Tokyo 169, Japan
3. Microsystem Technology Laboratory, Oki Electric Industry Company, Limited, Tokyo 193, Japan
4. Institute for Materials Research, Tohoku University, Sendai 980-77, Japan