Microelectromechanical Systems (MEMS) devices are being extensively used in diverse applications such as biomedical devices, automobiles, aerospace, optical displays, wireless
and optical communications. The structural layer in many surface micro-machined MEMS devices is polysilicon which is a brittle material. Strength statistics of polysilicon is strongly affected by the presence of defects that are inherent to the manufacturing processes. For safe operation and life assessment of these devices, it is of importance to incorporate the statistical nature of the fracture properties of MEMS materials in the design process.

A combined experimental and computational approach is taken to develop a significantly simpler methodology, compared to the existing ones, to characterize the statistical nature of fracture properties of polysilicon by probing the unexposed volume using indentation fracture. Micro-indentation fracture data was generated for a 3 _m thick polysilicon film on a silicon substrate and the average fracture toughness was evaluated using conventional approach, that ignores the effect of other layers, to be 0:7MPapm. The radial cracks, depending on the load applied, were either in the polysilicon layer or extended beyond into the oxide layer or further into the silicon substrate. Weakest link theory (WLT) based local criterion was applied to an equivalent model of a center loaded penny crack incorporating the effect of different layers on the stress field at the crack front to estimate the parameters for the Weibull strength distribution for the polysilicon thin film. The obtained parameters fall within the range reported in existing literature. Further, using the WLT with these parameters on standard fracture test geometries, the scatter in the fracture toughness reported in literature is well reproduced.

Polysilicon weakest link model parameters obtained from indentation fracture are used to predict failure in MEMS structures. Plain and notched polysilicon microcantilevers were fabricated using surface micro-machining and bending tests were carried out till the structure failed catastrophically. To estimate the failure probability of fabricated samples, numerical studies were carried out on models closer to the actual experimental condition, by estimating and incorporating fabrication details like undercut and roughness. Initially, the undercut inherent to fabrication process was estimated on plain cantilevers by comparing the elastic behavior with experiments. The geometry of open cavities in the sidewall, formed as a consequence of etching, was characterized using SEM micrographs. The stress field obtained from 3D FEM analysis of the notched geometries including these geometric details showed great sensitivity to the positioning of cavities in the sidewall relative to the top surface. Applying weakest link theory based local criteria, the probability distribution of failure load of tested structures were generated using weakest link parameters from indentation fracture. Failure loads, thus predicted were validated with experimental data.