Strength Evaluation of Single Crystal Silicon Microelements and Related Micromaterials

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In order to develop a reliable micromachine in a service operation, much care must be taken to micro mechanical evaluation, i.e., mechanical properties of μm-sized microelements. However, the evaluation method has not been well established. In this research project, the fracture behavior is investigated in the micromaterials for the micromachines and micro-electro mechanical system, including single crystal silicon. This will help us establish design criteria and life assessment methods for micromachines. The materials investigated were single-crystal silicon, poly-silicon film, and high-purity vitreous silica fiber. In this investigation, micro-mechanical testing machine is newly developed, and tensile testing of micro samples is thereby possible. The tensile strength of the poly-silicon films is about 0.9 GPa for the width of 10 μm, whereas the strength of the microsample with 5 μm is about 3.7 GPa, which are very strong compared with the structural materials for the ordinary sized machines and structures. Single-crystal silicon is immune to fatigue fracture and stress corrosion in laboratory air. However, the fracture behavior is sensitive to the water environment, and the synergistic effects of water environment and the dynamic stress caused a decrease in strength. Nanoscopic observation of the crack initiation site shows that the nanometric crack is formed on the surface owing to water and the dynamic loading, and the crack nucleation mechanisms are discussed. On the other hand, the influence of water environments on the fracture behavior is investigated in a high-purity vitreous silica fiber. Silica fiber is reported to be sensitive to water, and the static strength is decreases when the tests are conducted in water. However, when the impurity elements are decreased, the static strength is not influenced by water environment, indicating that the resistance to water environment is increased by a decrease in the impurity element in the vitreous silica fiber.