Effect of Surface Oxide Layer on Mechanical Properties of Single Crystalline Silicon

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Effect of Surface Oxide Layer on Mechanical Properties of Single Crystalline Silicon Kenji Miyamoto, Koji Sugano, Toshiyuki Tsuchiya, and Osamu Tabata Microengineering, Kyoto University, Yoshidahonmachi, Sakyo-ku, Kyoto, 606-8501, Japan ABSTRACT This paper reports on the tensile testing of single crystal silicon (SCS), whose specimen surface was intentionally oxidized, and the effect of the oxide thickness on the mechanical properties in order to investigate the fatigue fracture mechanism under cyclic loading. SCS specimens were fabricated from silicon-on-insulator (SOI) wafer with 3-µm -thick device layer and oxide layer were grown to the specimens using thermal dry oxidation at 1100 ºC. The specimen test part was 120 or 600 µm long and 4 µm wide. Quasi-static tensile testing of SCS specimen without oxide layer, with 100-nm-thick oxide, and with 200-nm-thick oxide was performed. As the results, the fracture origin location changed from the surface of the specimen of SCS without oxide to inside of silicon of oxidized specimen. This change may be caused by the smoothing of the surface and formation of oxide precipitation defects in silicon during oxidation. The estimated radius of the defects in specimen with 100 -nm-thick oxide and with 200-nm-thick oxide was 26 nm and 45 nm, respectively, which is well agreed with the fractureinitiating crack sizes calculated from the measured strengths. INTRODUCTION Single crystal silicon (SCS) is widely used for MEMS devices as structural material, due to its excellent mechanical properties and its compatibility to the microfabrication process. Since silicon is a brittle material, the mechanical reliability, especially the fatigue fractures under long term cyclic loading, is being concerned for and the fatigue properties and mechanisms are being investigated widely [1,2,3]. Three features were reported in the previous works on the fatigue of SCS. One is the relationship between the fatigue life and applied stress. Like metals, as the applied stress increases, the fatigue life decreases. Another is the necessity of cyclic loading for fatigue failure. Unlike glass, fatigue fracture doesn’t occur under static loading. The other is the effect of the humidity of the test environment. In higher humidity environment, the fatigue life becomes shorter [3]. From these features Muhlstein et al. proposed reaction layer fatigue theory as a fatigue mechanism of silicon [1]. In this theory, fatigue process of silicon is the combination of the surface oxide layer growth and the crack propagation within it under cyclic loading, which is enhanced by the surrounding moisture. However, the role of the surface oxide layer is not understood. The goal of our research is to reveal the effect of surface oxidation on the mechanical properties, such as tensile strength, Young’s modulus, and fatigue fracture. For this purpose, SCS specimens covered with oxide layer of known-thickness were prepared. This paper reports on quasi static tensile testing of the SCS specimens to evaluate the fracture