Characterization of mechanical properties of MEMS materials

(1)针对作为MEMS结构材料的薄膜材料，我们开发了一种新型的拉伸测试方法。在所提出的“片上拉伸测试方法”中，在同一芯片上对含有加载机构的硅片和拉伸样品进行了测试。(2)针对不同的薄膜材料，如(A)三种不同取向的单晶硅、(B)SiO_2、(C)Si3N_4、(D)多晶硅，设计并制作了测试芯片。得到了上述材料的断裂应力应变和杨氏模量。(3)单晶硅在断裂前表现出5%的弹性应变。这远远高于体硅弯曲试验所得的值。我们还观察到了试件对断裂应变的尺寸效应。较小的试件显示出较大的断裂应变，虽然两者都是通过芯片上的方法测试的。(4)循环加载对单晶硅的断裂有影响。随着施加应变峰值的增加，硅片断裂的循环次数减少。利用原子力显微镜探头上的碳纳米管原位观察了硅试件的裂纹扩展情况。从裂纹轮廓的变化来看，可能会在试件中引入塑性变形。这违背了硅在室温下不表现出任何可塑性的常识。我们成功地对0.1微米厚的Si3N4薄膜进行了拉伸测试。我们进一步设计了一种测试系统，可以对0.01微米厚的薄膜进行拉伸测试。我们预计，通过减小厚度，薄膜的性能会出现表面效应。

(1) We developed a new type of tensile testing method to characterize thin films utilized as structural materials for MEMS.In the proposed "on-chip tensile testing method", test is carried out on a silicon chip containing a loading mechanism and a tensile specimen on a same chip. The developed system allows both quasi-static and dynamic fatigue testing.(2) We designed and fabricated test chips for different film materials such as (a) single crystal silicon in three different orientations, (b) SiO2, (c) Si3N4, (d) poly-crystalline silicon. We obtained fracture stress and strain, and Young's modulus of above-mentioned materials.(3) Single crystal silicon showed 5% elastic strain until fracture. This is much higher than values obtained from bulk silicon by bending test. We also observed a size-effect of the specimen on fracture strain. Smaller specimen showed larger fracture strain though both are tested by on-chip method.(4) Cyclic loading influenced on the fracture of single crystal silicon. Number of cycles when silicon fractured decreased according to the increase in applied peak value of strain. The fracture mechanism is under discussion.(5) Crack growth of the silicon specimen was observed in-situ by using a carbon-nano-tube attached on an AFM probe tip. From a change in crack profile, it was suggested plastic deformation might be introduced to the specimen. This is against the common knowledge that silicon does not show any plasticity at room temperature. Further investigation is needed.(6) We succeeded in tensile test for Si3N4 film 0.1 micron thick. We further designed a test system allowing a tensile test for films 0.01 micron thick. We expect a surface effect appear on the film property by decreasing the thickness.