Evaluation of mechanical properties and strain criterion for realibility design of micromachined polysilicon thin films

In order to accomplish reliable mechanical design of MEMS, the method for bending strength measurements by beam bending using a nanoindenter has been demonstrated to evaluate the bending strength of microfabricated polysilicon beams with dimensions of 16-34 mum long, 10-50 mum wide, and 2.4 mum thick. Also, a new microtensile test device using a magnetic-solenoid force actuator was developed to evaluate the mechanical properties of microfabricated polysilicon thin films with dimensions of 100 -660 mum long, 20 -200 mum wide, and 2.4 mum thick, It was found that the bending strength of polysilicon microstructures shows size effect on the size of the specimens. Statistical analysis of bending strength for various specimen sizes shows that the average bending strength of polysilicon microcantilever beams is 2.885 +/- 0.408 GPa. The measured average value of Young's modulus, 164 GPa +/- 1.2 GPa, falls within the theoretical bounds. The average fracture tensile strength is 1.36 GPa with a standard deviation of 0.14 GPa, and the Weibull modulus is 10.4-11.7, respectively. The test data accounts for the uncertainties in mechanical properties and may be used in the future reliability design of polysilicon MEMS. The tensile testing of 40 specimens to failure results in a recommendation for design that the nominal strain be maintained below 0.0057.