Development of environment controlled micro mechanical testing system for thin films

为了在使用中开发出可靠的微机械和微机械系统，必须对微机械性能进行评估，即μm尺寸微元件的力学性能，包括疲劳行为和加载模式的影响，例如拉伸和弯曲加载。然而，评价方法还没有很好地确立。本文研制了一套基于纳米压头和压电台的弯曲测试系统，并对厚度为3.5μm、宽为5μm、长为10μm的多晶硅薄膜微元件进行了弯曲实验。弯曲试验测得的断裂应力为3.9±0.5 Gpa，高于单轴拉伸强度。为了评价加载方式对薄壁微构件力学性能的影响，还研制了单轴拉伸试验系统。该系统由PZT驱动定位平台驱动的微机械拉伸试验机、微操作系统、双视场显微镜辅助的非接触式应变测量系统组成。利用该测试系统，可以对厚度为3 5μm、宽为10μm的多晶硅薄膜微元件在不同的环境、潮湿环境、干燥空气和实验室空气中进行高达10 Hz的应力循环频率的拉伸-拉伸疲劳试验。采用蒙特卡罗方法建立了多晶硅薄膜的有限元模型，分析了多晶硅薄膜的晶向、晶界数目和晶粒度分布对薄膜弹性模量的影响。并将分析结果与实验结果进行了比较。

In order to develop a reliable micro machine and MEMS in a service operation, much care must be taken into micro mechanical evaluation, i.e., mechanical properties of μm-sized microelements including fatigue behavior and effects of loading mode, e.g. tensile and bending loading. However, the evaluation method has not been well established yet. In this study, a bending testing system based on a Nano Indenter with knife-edge type indenter and a piezo stage were developed and the bending tests have been conducted for poly-Si thin microelements with 3.5μm thick, 5μm wide and 10μm long. Fracture stress obtained by the bending test was 3.9 ± 0.5GPa, which was higher than the strength under uni-axial tensile loading. To evaluate the effect of loading mode on the mechanical properties of a thin microelement, the uni-axial tensile testing system has also been developed. The system consists of a micromechanical tensile testing machine actuated by a PZT driven positioning stage, a micro manipulation system, a non-contact strain measurement system with a help of a double field of view microscope. By using this testing system, tension-tension fatigue tests for poly-Si thin microelements with 3.5μm thick and 10μm wide can be carried out up to 10 Hz of stress cycle frequency in various environments ; in wet environment, in dry air and in laboratory air. Moreover to evaluate the influence of the crystalline orientation, the number of grains and the grain size distribution on the elastic modulus of poly-Si thin film, finite element models were developed based on the Monte Carlo method. The analytical results were compared with the experimental ones.