Characteristics of a commercially available silicon-on-insulator MEMS material

Design of microelectromechanical systems (MEMS) requires an understanding of the material characteristics inherent to the specific processing technologies. In this study, several design-critical characteristics of a silicon-on-insulator (SOI) MEMS fabrication technology (SOIMUMPs) have been examined and compared to a surface micromachined MEMS fabrication technology (polyMUMPs), both produced by MEMSCAP, Inc. Diagnostic structures were utilized to evaluate material characteristics including: minimum achievable feature size, deviation of feature size from nominal design dimensions, through-thickness residual strain gradient, in-plane residual strain, resistivity, surface roughness, and fracture strength. In general, most of the measured characteristics were similar for SOIMUMPs and polyMUMPs, in spite of their very different fundamental process constraints. For example, the characteristic fracture strength was 1.97 and 1.43 GPa, with associated Weibull moduli of 8.9 and 14.0, respectively. These values are similar given the very different sources of silicon: mechanically thinned single crystal silicon wafers versus low-pressure chemical vapor deposition for polycrystalline films. Fractography suggests that fracture strength for the both technologies is controlled by surface topography features: individual sidewall etching defects drive SOIMUMPs failures whereas top-surface grain boundary crevices drive polyMUMPs failures. Discussion focuses on the ramifications of the process-controlled material characteristics in SOIMUMPS and polyMUMPs to the design of MEMS components.