Highly localized surface oxide thickening on polycrystalline silicon thin films during cyclic loading in humid environments

Previous studies on very high-cycle fatigue behavior of thin silicon (Si) films suggest a strong environmental dependence of the degradation mechanism, the precise nature of which is still the subject of debate. This is partly due to contradictory evidence on the presence of thick post-cycling surface oxides. In the present study, 2 μm thick polycrystalline Si structures subjected to fully reversed stresses at 40 kHz are used to investigate fatigue degradation in a harsh environment (80 °C, 90% relative humidity). Transmission electron microscopy (TEM) on vertical through-thickness slices reveals highly localized thick oxides (∼50 nm) in the area of large cyclic stress, but not in control specimens. Such localized oxides are likely to be missed with horizontal TEM slices, as done in previous studies. This study highlights the challenges in characterizing nanometer-scale phenomena with micron-scale specimens, and confirms the viability of the reaction-layer fatigue mechanism for the high-cycle/very high-cycle fatigue behavior of micron-scale silicon.