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.
Single-crystal silicon thin films were forced to resonate at high frequency (~40 kHz) in different environments to study the long-term durability of this structural material used in microelectromechanical (MEMS) devices. The fatigue characterization structure consists of a notched cantilever beam attached to a plate shaped mass and is actuated at resonance, creating fully reversed, constant amplitude, sinusoidal stresses at the notch root. The dynamic behavior of the resonating structure has been meticulously quantified to allow accurate stress measurements from the knowledge of the driving voltage amplitude and the calculation of the quality factors in air and vacuum. In addition, the change in resonant frequency is periodically monitored for long-life specimens. Fatigue failure was observed for specimens tested in humid air and medium vacuum. In air, the stress-life (S-N) curve confirms the unique fatigue behavior already attributed to silicon thin films. In vacuum, the strength of the specimens appears to increase, and fatigue failure is delayed. Fracture surface examination reveals distinct features on the fracture surfaces of long-life fatigued specimens, not found in quasistatic failure, that are clear indications of initiation regions. The decrease rate in resonant frequency during cycling is demonstrated to be related to damage accumulation rate, and is strongly sensitive to both stress amplitude and humidity. The different currently proposed mechanisms are discussed in light of this new set of experimental evidence.
Previous research has attributed the fatigue susceptibility of silicon films to the sequential oxidation of the silicon and environmentally-assisted crack growth solely within the SiO2 surface layer. This “reaction-layer fatigue” mechanism 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. Fracture mechanics analyses can provide important insight into the limitations of structural silicon films. In this paper, our current understanding of the reaction-layer fatigue mechanism will be reviewed. Current results suggest that surface oxide layer thicknesses as low as 10-20 nm may induce reaction-layer fatigue when considering failure of the specimen for a crack reaching the silica/silicon interface. In contrast, 3-fold thicker surface oxide layers are required for failure due to a crack within the oxide layer.