Fatigue of polycrystalline silicon films with thin surface oxides

Abstract

ABSTRACT The performance of structural films unde r cyclic loading conditions is a cri tical consideration when designing microelectromechanical systems (MEMS) based on silicon stru ctural films. Empirical and th eoretical studies have shown that silicon films are susceptible to fatigue at room temperat ure, but the underlying mechanistic origin is still an active topic of debate. This study characterized the fa tigue behavior of electrostatically-actuated, n + -type, 2 P m thick polycrystalline silicon films with a thin native oxide. Elect rostatically actuated resonators (natural frequency, f 0 ~ 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 10 11 cycles) for decreasing stress amplitudes (down to 2.5 GPa). Long fatigue lives were as sociated with larger decreases in f 0 and very smooth failure origins that encompassed several grains. These findings are consis tent with cyclic degradation of silicon films occurring with in a surface reaction layer that forms upon exposure to the service environment and that evolves during fatigue loading. Keywords: silicon, fatigue, MEMS, reaction-layer fatigue