Microstructure and Reliability of Surface Micromachined Polysilicon Used for MEMS

Surface micromachining (SMM) techniques produce complex microscale polysilicon features on the surface of a silicon wafer using a patterned multilayer film deposition process. Failure characteristics of SMM polysilicon obtained from testing series of 2 μm and 4 μm wide ligaments fabricated using standard SMM processing techniques, fit a Weibull distribution, suggesting a behavior governed by a distribution of flaws, similar to brittle ceramic materials. However, positive identification of critical flaws that dictate the failure distributions within the ligaments remains unclear. Likely candidates are flaws associated with surface roughness or grain boundary intersections within the polysilicon microstructure. To address the possible connection between microstructure and failure behavior of SMM polysilicon, templates based on observed polysilicon microstructure were subjected to deformation simulations using polycrystal elasticity modeling. Series of simulations were performed to capture the statistical failure response of polysilicon due to local elastically driven stress concentrations between grains with different crystallographic orientations. Simulated results are presented and discussed in the context of experimental failure data.