High-cycle fatigue of micron-scale polycrystalline silicon films : fracture mechanics analyses of the role of the silica/silicon interface

It is known that micron-scale polycrystalline silicon thin films can fail in room air under high frequency (40kHz) cyclic loading at fully-reversed stress amplitudes as low as half the fracture strength, with fatigue lives in excess of 10¹¹ cycles. This behavior has been attributed to the sequential oxidation of the silicon and environmentally-assisted crack growth solely within the SiO₂, surface layer. This 'reaction-layer fatigue' mechanism is only significant in thin films where the critical crack size for catastrophic failure can be reached by a crack growing within the oxide layer. In this study, the importance of the bimaterial (e.g., Si/SiO₂) interface to reaction-layer fatigue is investigated, and the critical geometry and stress ranges where the mechanism is a viable failure mode are established.