Abstract-A study has been made of high-cycle fatigue in 2-µm thick structural films of n + -type, polycrystalline silicon for MEMS applications. Using an "on-chip" test structure resonating at ~40 kHz, such thin-film polysilicon is shown to display "metal-like" stress-life fatigue behavior in room air environments, with failures occurring after lives in excess of 10 11 cycles at stresses as low as half the fracture strength. Through in-situ monitoring of the natural frequency to evaluate the damage evolution by notch-root oxidation and cracking, and using transmission electron microscopy to image such damage, it is concluded that the mechanism of thin-film silicon fatigue involves sequential oxidation and environmentallyassisted cracking in the native SiO 2 layer. This moisture-induced "reaction-layer fatigue" mechanism can also occur in bulk silicon but it 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. The susceptibility of thin-film silicon to fatigue failure is shown to be suppressed by the use of alkene-based self-assembled monolayer coatings that prevent the formation of the native oxide.

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When subjected to alternating stresses, most materials degrade, e.g., suffer premature failure, due to a phenomenon known as fatigue. It is generally accepted that in brittle materials, such as ceramics, fatigue can only take place in toughened solids, i.e., premature fatigue failure would not be expected in materials such as single crystal silicon. The results of this study, however, appear to be at odds with the current understanding of brittle material fatigue. Twelve thin-film (20 m thick) single crystal silicon specimens were tested to failure in a controlled air environment (30 0.1 C, 50 2% relative humidity). Damage accumulation and failure of the notched cantilever beams were monitored electrically during the "fatigue life" test. Specimen lives ranged from about 10 s to 48 days, or 1 10 6 to 1 10 11 cycles before failure over stress amplitudes ranging from approximately 4 to 10 GPa. A variety of mechanisms are discussed in light of the fatigue life data and fracture surface evaluation. [642] Index Terms-Fatigue failure, MEMS devices, single-crystal silicon, thin films. I. INTRODUCTION T HE application of common MEMS, such as pressure transducers, inertial sensors, and ink jet cartridge nozzles, has already resulted in significant improvements in performance, and reductions in cost, for many industries ranging from medical device manufacturing to computing. Bolstered by this success, manufacturers are developing micromechanical components made of silicon-based structural films in actuator, power generation, and other "safety-critical" 1 and "high-performance" applications. However, these safety-critical structures are often subjected to aggressive mechanical and chemical environments without sufficient understanding of the behavior of the material under such conditions; this is especially pertinent as the dimensions of the material components may be far smaller, i.e., micrometer-scale and below, than has been conventionally tested in mechanical property evaluations. Consequently, in order to ensure performance and reliability, design approaches must be employed that account for the time-and cycle-dependent degradation of the material at the size-scales of interest.