Mechanical reliability of epipoly MEMS structures under shock load

The design, fabrication and characterization of epi-polysilicon test structures for dynamic tensile tests under shock loads ranging from 2,500 g to 30,000 g are reported. In accordance with published data for static fracture strength, their dynamic failure rate obeys a Weibull distribution model. The fracture surfaces suggest that failure criteria based on maximum principal stress are effective. Stresses resulting from shock load were obtained using finite element simulations. It was shown that even slight deviations from optimum loading conditions (i.e., out-of-plane components) can considerably increase the maximum experienced stress. Therefore, the usage of simplistic models to estimate maximum stress levels from shock loads is greatly discouraged. An out-of-plane component was identified in the experimental setup used. Based on an assessment of the magnitude of deviation, the failure stresses are estimated to fall below the quasi-static fracture strength by ca. 50%.