Transferability of weakest link model parameters of polysilicon from indentation fracture to failure of MEMS structures

Abstract

Polysilicon weakest link model parameters obtained from indentation fracture are used to predict failure in MEMS structures. Plain and notched polysilicon micro-cantilevers 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 such as 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, were 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 was generated using weakest link parameters from indentation fracture. Failure loads, thus predicted, were validated with experimental data.