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.