Thermal cycling response of layered gold/polysilicon MEMS structures

Multi-layer material structures in micro-electromechanical systems (MEMS) differ considerably from their micro-electronic counterparts since the thicknesses of the film and substrate are comparable. Due to thermal expansion coefficient mismatch between metallic and polysilicon materials, thermal fluctuations in multi-layer MEMS structures may lead to out of plane displacements several times the structure thickness. In the present study we examine the deformation of a series of gold/polysilicon test structures produced using the commercial multi-user MEMS Process. The test structures are multi-layer plates with a gold layer that is 0.5 μm thick on a polysilicon layer with a thickness that is either 1.5 or 3.5 μm. The structures have varying in-plane dimensions on the order of 50-300 μm. Using an interferometric microscope, we measure the full-field deformed shapes of the Au-Si structures subjected to thermal cycles or hold periods. Depending on the Au-Si plate geometry, the structures deform in a non-linear manner and buckle when the temperature change is large enough. When subjected to temperature holds, the curvature of the plates relaxes. Beam-like plates subjected to repeated thermal cycles demonstrate a cyclic response that depends strongly on the mean (or maximum) temperature. Samples cycled at lower mean temperatures show significant ratcheting toward lower curvatures as the number of cycles approaches 10,000. Samples cycled at higher mean temperatures undergo degradation of surface reflectivity as the number of cycles approaches 1000. © 2003 Elsevier Ltd. All rights reserved.