Size effect on fracture of MEMS materials

Weibull strength distributions of single crystal silicon and polysilicon measured from micro-specimens are analyzed by a simple flaw statistics model. The model can be used to determine important material properties such as the nana- / micro-flaw density and flaw size distribution from the common Weibull parameters. The fracture toughness of single Si should be used for both single crystal Si and polysilicon MEMS structures as it controls fracture initiated form the nano- / micro-flaws. The fracture toughness measured from micro-specimens of polysilicon needs to be treated with caution as it may be subjected to size effect as the microcrack length is comparable to the grain size. The comparison of AFM measurements and predictions of the flaw density based on various toughness results has been used to confirm the size effect.