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Surface Engineering of Polycrystalline Silicon Microelectromechanical Systems for Fatigue Resistance C.L. Muhlstein1, W.R. Ashurst2, E.A. Stach3, R. Maboudian2, and R.O. Ritchie1 1 Materials Sciences Division, Lawrence Berkeley National Laboratory, and Department of Materials Science and Engineering, University of California, Berkeley, CA 94720 2 Department of Chemical Engineering, University of California, Berkeley, CA 94720 3 National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, CA 94720

ABSTRACT Recent research has established that for silicon structural films used in microelectromechanical systems (MEMS), the susceptibility to premature failure under cyclic fatigue loading originates from a degradation process that is confined to the surface oxide. In ambient air environments, a sequential, stress-assisted oxidation and stress-corrosion cracking process can occur within the native oxide on polycrystalline silicon (referred to as reaction-layer fatigue); for the structural films of micron-scale dimensions, such incipient cracking in the oxide can lead to catastrophic failure of the entire silicon component. Since the degradation process is intimately linked to the thin reaction layer on the silicon, modification of this surface and the access of the environment to it can dramatically alter the fatigue resistance of the material. The purpose of this paper is to evaluate the efficacy of modifying the fatigue behavior of polycrystalline silicon with alkene-based monolayers. Specifically, 2-µm thick polysilicon fatigue structures were coated with a monolayer film based on 1-octadecene and cyclically tested to failure in laboratory air. By applying the coating, the formation of the native oxide was prevented. Compared to the fatigue behavior of untreated polysilicon, the lives of the coated samples ranged from 105 to >1010 cycles at stress amplitudes greater than ~90% of the ultimate strength of the film. The dramatic improvement in fatigue resistance was attributed to the monolayer inhibiting the formation of the native oxide and stress corrosion of the surface. It is concluded that the surprising susceptibility of thin structural silicon films to premature fatigue failure can be inhibited by such monolayer coatings. INTRODUCTION Refinement of bulk and surface micromachining technologies over the past decade has led to the development of a variety of integrated sensor technologies including advanced inertial sensors and gyros. During this timeframe, silicon has remained the dominant structural material for microelectromechanical systems (MEMS) because of ease of manufacturing and electronics integration. In biomedical applications, biocompatibility and other considerations have motivated exploration of other materials systems, yet silicon is still a mainstay of research programs. Consequently, the durability of silicon under cyclic loading conditions is of general interest to many researchers and designers. Fatigue failure of silicon thin films, first reported by Connally and Brow