Interfacial cyclic fatigue of atomic-layer-deposited alumina coatings on silicon thin films.

A microresonator-based interfacial fatigue testing technique was used to investigate the subcritical delamination of atomic-layer-deposited alumina coatings along the sidewalls of deep-reactive-ion-etched monocrystalline silicon thin films. This technique ensures loading conditions relevant to microelectromechanical system devices, including kHz testing frequency and fully reversed cyclic stresses. Four different coating thicknesses (4.2, 12.6, 25, and 50 nm) were investigated in two environments (30 °C, 50% relative humidity (RH) and 80 °C, 90% RH). Fatigue damage, in the form of channel cracks and delamination of the alumina coating, was found to accumulate slowly over more than 1 × 10(8) cycles. The average delamination rates increase with increasing energy release rate amplitude for delamination, modeled with a power law relationship. In the harsher environment, the rates are roughly 1 order of magnitude higher. Additionally, a few tests under static load were conducted for which no delamination (or crack growth) occurred, demonstrating that the governing interfacial fatigue mechanism is cycle-dependent.