Dual-mode device for in situ testing of MEMS packaging quality

A method is presented to rapidly characterize the hermeticity of a vacuum cavity for a high frequency (ω0/(2π) = 56 MHz) micromechanical bulk-wave resonator. The method relies on operating the device in a low-stiffness out-of-plane resonance mode. By monitoring the Q-factor and resonance frequency shift of this out-of-plane mode, the pressure change inside the cavity can be measured with a sensitivity that is 100–1000 times higher than when using the functional bulk mode. This high sensitivity pressure sensing is vital for estimating the life-time of vacuum packaged resonators within a limited test-time. The experimental results are well described by the squeeze-film damping model of Suijlen et al. [27] and the classical predictions for kinetic damping and drag forces based on gas kinetic theory, within the experimental error of 15% for Q and 3% for Δω0. For this purpose the model has been extended to include the effect of coupled squeeze-film boxes, which can be expressed in terms of an effective gap height and an effective escape time.