Subcritical crack growth in silicon MEMS

New experimental techniques need to be developed to address fundamental mat erials issues in MEMS, Experimental protocols developed for macroscale test ing are not necessarily applicable, and an understanding of the behavior of macroscale specimens cannot necessarily be relied upon to predict the beha vior of microscale MEMS structures. An experimental protocol for studying slow crack growth in MEMS materials h as been developed, and this protocol has been used to show that polycrystal line silicon (polysilicon) MEMS are susceptible to stress corrosion crackin g. Using a model of the nonlinear dynamics of a specimen allowed an estimat ion of crack length and crack closure from the frequency response of the sp ecimen. The procedure can resolve 1-nm crack extensions and crack growth ra tes below 10(-13) m/s, Crack closure, which has a pronounced effect on the dynamics of this nonlinear system, may be associated with the native oxide that grows on the faces of the crack, The data show that subcritical crack growth in polysilicon MEMS is driven by the synergistic effects of water an d stress. In contrast to macroscale stress corrosion cracking behavior, a c lear relationship between crack growth rate, stress intensity, and humidity has not been found. Micrographs suggest that the crack path is transgranul ar.