| In this thesis we studied, both experimentally and theoretically, the thermomechanical behavior of multilayer thin film microstructures using MEMS-scale samples (lateral dimensions on the order of hundreds of microns) fabricated using a surface micromachining process. The experimental efforts focus on microstructures consisting of gold/polysilicon multilayers, however the general results are much more broadly applicable. We observed deformation behavior can be generally characterized by the average curvature versus temperature in three regimes: linear, geometric nonlinear, and nonlinear bifurcation. Measurements agree well with predictions based on both an analytical calculation assuming a constant curvature deformation mode, and finite element calculations that removed this restriction. Our studies included the effect of the patterning of the films. Patterning of the film in lines substantially alters the curvature of the system, and the stress state in the lines, both along and across the direction of the lines.; While geometric nonlinearity can be of importance when multilayer thin film microstructures are subjected to thermal loading, material nonlinearity and the interaction between these two is of equal importance in MEMS applications. This is studied in the context of inelastic deformation of multilayer microstructures subjected to cyclic thermal loading and isothermal loading where the combined effects of creep, stress relaxation, and microstructural evolution operate. We studied how to stabilize the microstructures over a temperature range and render the subsequent deformation controllable. Inelastic deformation is often an undesirable phenomenon and our results show that it can greatly affect the deformation and compromise device performance. Calculations with simple power-law creep agree reasonably well with the measurements. We also showed that inelastic deformation during the isothermal holds was greatly reduced for microstructures coated with nanometer-thick alumina layer. However the simple power-law creep model cannot predict this, suggesting that the fundamental deformation mechanisms are altered by the nanoscale coating.; The basic understanding of the thermomechanical behavior was used to design a vertical electrostatic actuator with an extended digital range of motion. By tailoring the material distribution of the actuator, the nonlinear electrostatic restoring force was balanced by the nonlinear mechanical force so that a desired, for example linear, digital output motion can be achieved. |
