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The importance of service environment to the fatigue resistance of n+-type, 10 μm thick, deep-reactive ion-etched (DRIE) silicon structural films used in microelectromechanical systems (MEMS) was characterized by testing of electrostatically actuated resonators (natural frequency, f0, ∼40 kHz) in controlled atmospheres. Stress-life (S-N) fatigue tests conducted in 30°C, 50% relative humidity (R.H.) air demonstrated the fatigue susceptibility of silicon films. Further characterization of the films in medium vacuum and 25% R.H. air at various stress amplitudes revealed that the rates of fatigue damage accumulation (measured via resonant frequency changes) are strongly sensitive to both stress amplitude and, more importantly, humidity. Scanning electron microscopy of high-cycle fatigue fracture surfaces (cycles to failure, Nf>1×109) revealed clear failure origins that were not observed in short-life (Nf<1×104) specimens. Reaction-layer and microcracking mechanisms for fatigue of silicon films are discussed in light of this empirical evidence for the critical role of service environment during damage accumulation under cyclic loading conditions.