Strength Evaluation of Single Crystal Silicon Microelements and Related Micromaterials

为了在使用中开发出可靠的微机械，必须对微机械性能进行评估，即μm尺寸微元件的力学性能。然而，评价方法还没有很好地建立起来。在本研究项目中，研究了用于微机械和微电子机械系统的微材料，包括单晶硅的断裂行为。这将有助于我们建立微型机械的设计标准和寿命评估方法。所研究的材料为单晶硅、多晶硅薄膜和高纯玻璃二氧化硅纤维。在本研究中，新研制了微机械试验机，从而使微小试件的拉伸试验成为可能。对于宽度为10μm的多晶硅薄膜，其抗拉强度约为0.9 Gpa，而对于宽度为5μm的微样，其抗拉强度约为3.7 Gpa，与普通机械和结构的结构材料相比，这是非常强的。单晶硅对实验室空气中的疲劳断裂和应力腐蚀具有免疫力。但断裂行为对水环境敏感，水环境与动应力的协同作用导致强度下降。对裂纹萌生部位的纳米观察表明，由于水和动载的作用，表面形成了纳米级的裂纹，并对裂纹的形核机理进行了讨论。另一方面，研究了水环境对高纯玻璃石英纤维断裂行为的影响。据报道，石英纤维对水很敏感，在水中进行测试时，静态强度会降低。然而，当杂质元素减少时，玻璃纤维的静强度不受水环境的影响，表明玻璃石英纤维中杂质元素的减少提高了抗水环境的能力。

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.