Subcritical crack growth in silicon MEMS

Abstract

New experimental techniques need to be developed to address fundamental materials issues in MEMS. Experimental protocols developed for macroscale testing are not necessarily applicable, and an understanding of the behavior of macroscale specimens cannot necessarily be relied upon to predict the behavior of microscale MEMS structures. An experimental protocol for studying slow crack growth in MEMS materials has been developed, and this protocol has been used to show that polycrystalline silicon (polysilicon) MEMS are susceptible to stress corrosion cracking. Using a model of the nonlinear dynamics of a specimen allowed an estimation of crack length and crack closure from the frequency response of the specimen. The procedure can resolve 1-nm crack extensions and crack growth rates below 10/sup -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 and stress. In contrast to macroscale stress corrosion cracking behavior, a clear relationship between crack growth rate, stress intensity and humidity has not been found. Micrographs suggest that the crack path is transgranular.