Characterization of mechanical properties of MEMS materials

(1)我们开发了一种新型的拉伸测试方法来表征MEMS结构材料薄膜。在所提出的“片上拉伸试验方法”中，在同一芯片上包含加载机构和拉伸试样的硅片上进行试验。开发的系统允许准静态和动态疲劳测试。(2)我们设计并制造了不同薄膜材料的测试芯片，如(a)三种不同取向的单晶硅，(b) SiO2, (c) Si3N4, (d)多晶硅。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为5%的弹性应变。这比通过弯曲试验从大块硅中得到的值高得多。我们还观察到试样对断裂应变的尺寸效应。采用片上法测试时，较小的试样显示出较大的断裂应变。(4)循环加载对单晶硅断裂的影响。随着应变峰值的增加，硅断裂的循环次数减少。断裂机制正在讨论中。(5)利用附着在原子力显微镜探针尖端的碳纳米管，原位观察了硅试样的裂纹扩展情况。从裂纹轮廓的变化来看，塑性变形可能引入到试样中。这与硅在室温下不表现出任何可塑性的常识相反。需要进一步调查。(6)对0.1微米厚的Si3N4薄膜进行了拉伸试验。我们进一步设计了一个测试系统，允许对0.01微米厚的薄膜进行拉伸测试。我们期望通过减小厚度对薄膜性能产生表面效应。

(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.