On The Mechanism of Fatigue in Micron-Scale Structural Films of Polycrystalline Silicon

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On The Mechanism of Fatigue in Micron-Scale Structural Films of Polycrystalline Silicon C. L. Muhlstein1, E. A. Stach2, 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 National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, CA 94720

ABSTRACT 2-µm thick structural films of polycrystalline silicon are shown to display “metal-like” stresslife fatigue behavior in room air, with failures occurring after >1011 cycles at stresses as low as half the fracture strength. Using in situ measurements of the specimen compliance and transmission electron microscopy to characterize such damage, the mechanism of thin-film silicon fatigue is deduced to be sequential oxidation and moisture-assisted cracking in the native SiO2 layer. This mechanism can also occur in bulk silicon but it is only relevant in thin films where the critical crack size for catastrophic failure can be exceeded within the oxide layer. The fatigue susceptibility of thin-film silicon is shown to be suppressed by alkene-based selfassembled monolayer coatings that prevent the formation of the native oxide. INTRODUCTION Silicon-based structural films are currently the dominant material for microelectromechanical systems (MEMS) because the micromachining technologies are readily adaptable from the microelectronics industry and are compatible with fabrication strategies for actuation and control integrated circuits. However, the long-term durability of MEMS may be compromised by a susceptibility of thin-film silicon to premature failure by fatigue [1-12]. Cyclic fatigue is the most commonly encountered mode of failure in structural materials [13]. In ductile (metallic) materials, fatigue is attributed to cyclic plasticity involving dislocation motion that causes alternating blunting and resharpening of a pre-existing crack tip as it advances [14]. In brittle (ceramic) materials where dislocation mobility is restricted, fatigue conversely occurs by cycle-dependent degradation of the (extrinsic) toughness of the material in the wake of the crack tip [13]. As silicon is a brittle material (no dislocation activity is generally observed below ~500°C) that displays little evidence of extrinsic toughening [15] or susceptibility to environmental cracking [16], it should not fatigue at room temperature. Indeed, there is no evidence that bulk silicon is prone to fatigue failure. However, cyclically-stressed micron-scale silicon films are known to fail in room air at stresses well below their fracture strength [1-12]. Although first reported a decade ago [1], the mechanistic origins of such thin-film silicon fatigue have remained elusive. Speculations have included static fatigue of the native oxide [1,8], dislocation activity, impurity effects, and stress-induced phase transformations, although there has never been any conclusive evidence to support these mechanisms, nor any direct observations of fatigue