Environment-induced failure modes of thin film resonators

Resonant mode micromechanical devices have great potentials due to their high sensitivity and easy signal processing. As they are also sensitive to environmental effects, vacuum packaging is often required, which largely increases the costs. The current study focuses on such environment induced reliability problems and degradation processes. Stiffening effect was observed on thin silicon nitride and silicon carbide cantilever beams in air. The resonance frequency gradually increases in time. When the cantilever is subjected to mechanical shock or large deflection, the resonance frequency suddenly drops, and then increases again. Air, increased humidity, argon rich and nitrogen rich atmosphere influence the stiffening and the shock response behavior. The effects are explained with the surface oxidation model. The oxide layer introduces stress in the structure increasing the overall stiffness, while mechanical shocks crack the layer. Silicon resonators gather airborne particles from the atmosphere due to electrostatic charging. The extra mass results in decrease of the resonant frequency. All these processes lead to unstable resonance frequency and thus to failure of the resonant mode device. Tests in inert environment suggest cheap atmospheric packaging solution to obtain reliable operation and yet good performance.