Temperature-Dependent Viscoelasticity in Thin Au Films and Consequences for MEMS Devices

Thin metal films acting as structural components in microelectromechanical systems (MEMS) devices can exhibit viscoelastic mechanical behavior even at small strains, causing time-dependent changes in device performance. In an effort to characterize the temperature dependence of this behavior, stress relaxation experiments using gas pressure bulge testing have been conducted on 1.0-mu m-thick Au films at temperatures between 20 degrees C and 80 degrees C. By fitting a Prony series of saturating exponentials to the resulting relaxation curves, a function for the time-dependent elastic modulus was developed for each temperature. The time-dependent elastic moduli were used in an analytical model to demonstrate the impact of viscoelastic stress relaxation on the restoring forces of two different RF MEMS capacitive switch designs. The implications for testing and performance of a cantilever-type contact switch were also assessed. Finally, using time-temperature superposition, a master curve was generated that may enable prediction of room temperature device performance out to times greater than one year.