Environmental contributions to fatigue failure of micron-scale silicon films used in microelectromechanical system (MEMS) devices

Over the past 40 years, the silicon semiconductor industry has explored critical properties of the silicon dioxide (SiO2)/silicon (Si) materials system. The recent advent of Si-based microelectromechanical system (MEMS) devices has, however, required new focus on the structural reliability of micron-scale Si films. The discovery over a decade ago of delayed failure for silicon films in room-temperature air caused active research to capture the underlying mechanism(s) for the fatigue process. This investigation established that the evolution of nanometer-scale surface oxide layers on micron-scale silicon films is a critical part in the fatigue damage accumulation process. The importance of service environment to the fatigue resistance of n +-type, 10 mum thick, single-crystal silicon structural films was characterized by evaluating the response of electrostatically-actuated resonators (natural frequency, f0, ∼40kHz) in controlled atmospheres. A testing methodology was developed to accurately measure stress amplitude and lifetime in humid air and vacuum environments. A short-life testing technique was introduced to measure with precision fatigue lives as low as a few hundred cycles from kHz-frequency resonators. Stress-life ( S-N) fatigue tests conducted in 30°C, 50% relative humidity (R.H.) air demonstrated the fatigue susceptibility of these films. Further characterization of the films in medium vacuum and 25% R.H. air at various stress amplitudes revealed that the rates of fatigue damage accumulation (measured via resonant frequency changes) were strongly sensitive to both stress amplitude and, more importantly, humidity. Scanning electron microcopy of high-cycle fatigue fracture surfaces (cycles to failure, Nf > 1 x 10 9) revealed clear failure origins that were not observed in short-life (Nf < 1 x 104) specimens.; Similar results were observed for the fatigue testing of 2 mum thick, n+-type polycrystalline silicon films consisting of ∼100 nm equiaxed grains. Longer fatigue lives (up to 1011 cycles) were associated with more damage accumulation (i.e., larger decreases in f0). Smooth failure origins were observed for high-cycle fatigue fracture surfaces that were not influenced by the grain morphology of polycrystalline silicon films. Finally, Auger electron spectroscopy revealed local oxide thickening within the failure origin of a high-cycle fatigue specimen.; The findings of this investigation supported the reaction-layer mechanism which asserts that fatigue of silicon films is a reaction-layer process dominated by degradation of the surface of the material. Specifically, the process involves mechanically-induced thickening and environmentally-assisted cracking of the oxide reaction layer which forms on the surface upon exposure to air. (Abstract shortened by UMI.)...