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U7.2.1

Fatigue Degradation of Nanometer-Scale Silicon Dioxide Reaction Layers on Silicon Structural Films C.L. Muhlstein1, E.A. Stach2, and R.O. Ritchie3 1 Department of Materials Science and Engineering and the Materials Research Institute, The Pennsylvania State University, University Park, PA 16802 2 National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 3 Materials Sciences Division, Lawrence Berkeley National Laboratory, and Department of Materials Science and Engineering, University of California, Berkeley, CA 94720

ABSTRACT Although bulk silicon is ostensibly immune to cyclic fatigue and environmentally-assisted cracking, the thin film form of the material exhibits significantly different behavior. Such silicon thin films are used in small-scale structural applications, including microelectromechanical systems (MEMS), and display ‘metal-like’ stress-life (S/N) fatigue behavior in room temperature air environments. Fatigue lives in excess of 1011 cycles have been observed at high frequency (~40 kHz), fully-reversed stress amplitudes as low as half the fracture strength using surface micromachined, resonant-loaded, fatigue characterization structures. Recent experiments have clarified the origin of the susceptibility of thin film silicon to fatigue failure. Stress-life fatigue, transmission electron microscopy, infrared microscopy, and numerical models have been used to establish that the mechanism of the apparent fatigue failure of thin-film silicon involves sequential oxidation and environmentally-assisted crack growth solely within the nanometerscale silica layer on the surface of the silicon, via a process that we term ‘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 surface 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. Finally, the successful mitigation of reaction-layer fatigue with monolayer coatings is shown. INTRODUCTION Conventional theories would suggest that silicon should not fatigue at room temperature; several studies have now confirmed that micron-scale mono and polycrystalline silicon structural films, as used in microelectromechanical systems (MEMS), are susceptible to premature failure in cyclic fatigue in ambient atmospheres [1-11]. This research, which has often involved electrostatically-actuated, notched, cantilever-beam fatigue test structures (Fig. 1) to measure the stress-life (S/N) fatigue curves (e.g. [8,9]), has revealed that in room-temperature air, thin-film silicon (~2 µm thick) can exhibit “metal-like” fatigue behavior with failures occurring at fullyreversed stress amplitudes of roughly one-half of the (single-cycle)