High-cycle Fatigue in Micron-scale Structural Films of Polycrystalline Silicon: a Reaction-layer Failure Mechanism

A study has been made of high-cycle fatigue in 2-μm thick structural films of ntype, polycrystalline silicon for MEMS applications. Using an “on-chip” test structure resonating at ~40 kHz, such thin-film polysilicon is shown to display “metal-like” stress-life fatigue behavior in room air environments, with failures occurring after lives in excess of 10 cycles at stresses as low as half the fracture strength. Through in-situ monitoring of the natural frequency to evaluate the damage evolution by notch-root oxidation and cracking, and using transmission electron microscopy to image such damage, it is concluded that the mechanism of thin-film silicon fatigue involves sequential oxidation and environmentallyassisted cracking in the native SiO2 layer. This moisture-induced “reaction-layer fatigue” mechanism can also occur in bulk silicon but it 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. The susceptibility of thin-film silicon to fatigue failure is shown to be suppressed by the use of alkene-based self-assembled monolayer coatings that prevent the formation of the native oxide.