Fatigue of polycrystalline silicon films with thin surface oxides

The performance of structural films under cyclic loading conditions is a critical consideration when designing microelectromechanical systems (MEMS) based on silicon structural films. Empirical and theoretical studies have shown that silicon films are susceptible to fatigue at room temperature, but the underlying mechanistic origin is still an active topic of debate. This study characterized the fatigue behavior of electrostatically-actuated, n+-type, 2 μm thick polycrystalline silicon films with a thin native oxide. Electrostatically actuated resonators (natural frequency, f0 ~ 40 kHz) were used to evaluate the stress-life fatigue behavior of the films in 30°C, 50% relative humidity (R.H.) air. These tests revealed delayed failure with increasing fatigue lives (up to 1011 cycles) for decreasing stress amplitudes (down to 2.5 GPa). Long fatigue lives were associated with larger decreases in f0 and very smooth failure origins that encompassed several grains. These findings are consistent with cyclic degradation of silicon films occurring within a surface reaction layer that forms upon exposure to the service environment and that evolves during fatigue loading.