Fatigue Testing of Polycrystalline Silicon Thin-Film Membrane Using Out-of-Plane Bending Vibration

This paper describes a new fatigue testing method for polycrystalline-silicon (polysilicon) thin-film membrane to evaluate its mechanical reliability not affected by surface roughness of etched sidewalls. The test specimen is a thin membrane consisting of polysilicon, silicon dioxide, and silicon nitride films with a circular weight at the center, and its outer edge is supported by a square frame. Stress on polysilicon for fatigue test is applied by deformation of the membrane generated by oscillating the weight in the out-of-plane direction near the resonant frequency. The polysilicon film fractured by fatigue damage accumulated by the cyclic stress. Stress and number of cycles to fracture ($S$--$N$) plots were well formulated on the basis of Weibull statistics and Paris' law. Weibull modulus and fatigue index of the 250-nm-thick polysilicon film were 19.2 and 21.8, respectively. These parameters make it possible to predict the lifetime of polysilicon thin-film membrane under arbitrary cyclic stress.