Nonlinear behavior of SOI free-free micromechanical beam resonator

Measured nonlinear behavior of a capacitively driven free-free micromechanical beam resonator at different driving conditions is presented. The resonator, fabricated in SOIMUMPs process, has a measured resonant frequency of 654 kHz with an average quality factor, Q value of 12,000 operating at a pressure of 37.5 μTorr. The overall nonlinearity (including mechanical and electrical) in the resonator was found to be triggered after critical ac drive voltage amplitude of about 120 mVpp was exceeded. The observed nonlinearity was relatively independent of the proof-mass dc voltage, VP, as long as the critical ac drive voltage is not exceeded. Furthermore, partial compensation of spring hardening effect (arising from mechanical nonlinearity) with spring softening effect (from capacitive force used to actuate the resonator) is observed in this work. This feature is particularly useful for MEMS oscillator applications where frequency tuning can be done by varying VP without inducing nonlinearity in the resonator.