Temperature dependence of viscoelasticity in thin gold films and its impact on the restoring force in RF MEMS capacitive switches

The temperature dependence of viscoelasticity in 1.0 mum Au films has been studied through stress relaxation measurements using gas pressure bulge testing. Tests were conducted at temperatures ranging from 20 to 80 °C. Stress relaxation was followed for three hours at a constant applied strain of 0.1% after a rapid ramp to that strain from an initial applied strain of zero. A series of such transients was carried out at each temperature, one per day, with the film maintained at zero strain between them to allow for viscoelastic stress recovery.;The stress relaxation was shown to consist of two components: a fully recoverable viscoelastic component and a non-recoverable plastic (creep) component. The creep component diminished with successive three hour transients until a stable state was reached with constant residual stress; thereafter the transient consisted of only fully recoverable viscoelasticity. By analyzing the temperature dependence of these transients the activation energy of the relaxation process was determined to be approximately 0.25 eV. This low value suggests that the relaxation can not be due to grain boundary diffusion, power-law creep, or grain boundary sliding. Dislocation double-kink nucleation is proposed as a possible mechanism for recoverable relaxation.;In addition, the measurements of viscoelastic relaxation were used in an analytical model to predict the time-dependent loss of restoring force that would be produced in an RF MEMS capacitive switch at various operating temperatures.