A reaction-layer mechanism for the delayed failure of micron-scale polycrystalline silicon structural films subjected to high-cycle fatigue loading

A study has been made to discern the mechanisms for the delayed failure of 2-μm thick structural films of n +-type, polycrystalline silicon under high-cycle fatigue loading conditions. Such polycrystalline silicon films are used in small-scale structural applications including microelectromechanical systems (MEMS) and are known to display ‘metal-like’ stress-life (S/N) fatigue behavior in room temperature air environments. Previously, 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 a surface micromachined, resonant-loaded, fatigue characterization structure. In this work the accumulation of fatigue-induced oxidation and cracking of the native SiO2 of the polycrystalline silicon was established using transmission electron and infrared microscopy and correlated with experimentally observed changes in specimen compliance using numerical models. These results were 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 native SiO2 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. It is shown that the susceptibility of thin-film silicon to such failures can be suppressed by the use of alkene-based monolayer coatings that prevent the formation of the native oxide.