High-cycle fatigue and durability of polycrystalline silicon thin films in ambient air

To evaluate the long-term durability properties of materials for microelectromechanical systems (MEMS), the stress-life (S/N) cyclic fatigue behavior of a 2-μm thick polycrystalline silicon film was evaluated in laboratory air using an electrostatically actuated notched cantilever beam resonator. A total of 28 specimens were tested for failure under high frequency (∼40kHz) cyclic loads with lives ranging from about 10s to 34 days (3×105 to 1.2×1011 cycles) over fully reversed, sinusoidal stress amplitudes varying from ∼2.0 to 4.0GPa. The thin-film polycrystalline silicon cantilever beams exhibited a time-delayed failure that was accompanied by a continuous increase in the compliance of the specimen. This apparent cyclic fatigue effect resulted in an endurance strength, at greater than 109 cycles, of ∼2GPa, i.e. roughly one-half of the (single cycle) fracture strength. Based on experimental and numerical results, the fatigue process is attributed to a novel mechanism involving the environmentally-assisted cracking of the surface oxide film (termed reaction-layer fatigue). These results provide the most comprehensive, high-cycle, endurance data for designers of polysilicon micromechanical components available to date.