An Experimental and Theoretical Investigation of Creep in Ultrafine Crystalline Nickel RF-MEMS Devices

The creep behavior of ultrafine crystalline RF micro-electromechanical systems (RF-MEMS) devices is investigated in this paper. The RF-MEMS devices are characterized through a highly accurate capacitance-sensing setup, under a special bi-state bias condition. In particular, the devices are kept continuously biased (on-state) for up to 1400 h, but the bias voltage is momentarily removed for 1 min every hour in order to record the off-state capacitance. Measurements at two bias voltages of 20 and 40V are reported. The capacitance measurement uncertainty is less than 200 aF and the long-term stability is better than 4 fF throughout the measurement period. Furthermore, we report the creep behavior under the same bi-state bias condition through independent direct optical measurements conducted with a confocal microscope-based setup. The measurement uncertainty for the vertical displacement reported in these measurements is less than 50 nm. These measurements show for the first time the entire profile evolution of the deflected beams and membranes for up to 168 h. Both the capacitance and optical measurements reveal the same trends for the creep behavior of the devices. A physics-based creep model for the ultrafine crystalline nickel devices is also included in this paper. The required model parameters are obtained by fitting the simulation results to the measured creep curve under a bias voltage of 20V. Using the same material parameters and geometry, this model correctly captures the rate of deformation during the steady-state creep stage at 40 V. The model reveals that the observed steady-state creep deformation at both 20 and 40 V is dominated by Coble creep.