The influence of atomic-layer-deposited alumina surface coatings on the fatigue of polycrystalline silicon thin films was investigated. Tests were performed on 2-μm-thick notched cantilever-beam structures actuated at resonance (∼40 kHz) that were coated with ∼20 nm of alumina deposited at 100 °C. The coated devices show a drastically different frequency evolution behavior as compared to uncoated specimens, while no surface oxide thickening during cycling is observed in bright-field transmission electron microscopy. Both results are consistent with a surface degradation mechanism for polycrystalline silicon fatigue such as the reaction-layer mechanism. The improved frequency evolution has the potential of increasing the performance of resonator-based microelectromechanical system sensors.
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