Abstract
Over the past ten years a variety of techniques for characterizing the fatigue behavior of structural films have emerged. In particular, micromachined resonant fatigue characterization structures have been used to evaluate the stress-life fatigue behavior of thin films. In this work we will first review the design, testing, and analysis of micromachined fatigue characterization resonators. Subsequent discussion will focus on how resonant-loaded fatigue characterization structures were used to evaluate the high-cycle fatigue behavior of silicon films commonly used in microelectromechanical systems (MEMS). Although bulk silicon is not known to be susceptible to cyclic fatigue, micron-scale structures made from mono and polycrystalline silicon films display "metal-like" stress-life (S/N) fatigue behavior in room temperature air environments. Fatigue lives in excess of 10^<11> cycles have been observed at high frequency (〜40 kHz), fully-reversed stress amplitudes as low as half the fracture strength. Stress-life fatigue, transmission electron microscopy, infrared microscopy, and numerical models were used to establish that the mechanism of the apparent fatigue failure of thin-film silicon involves the sequential oxidation and environmentally-assisted crack growth solely within the native silica layer, a process termed "reaction-layer fatigue". Only thin films are susceptible to such a failure mechanism because the critical crack size for catastrophic failure of the entire silicon structure can be exceeded by a crack solely within the native oxide layer. The growth of the oxide layer and the environmentally-assisted initiation of cracks under cyclic loading conditions are discussed in detail. Furthermore, the importance of interfacial fracture mechanics solutions and the synergism of the oxidation and cracking processes are described.
