Utilizing On-Chip Testing and Electron Microscopy to Study Fatigue and Wear in Polysilicon Structural Films
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UTILIZING ON-CHIP TESTING AND ELECTRON MICROSCOPY TO STUDY FATIGUE AND WEAR IN POLYSILICON STRUCTURAL FILMS D.H. Alsem1,2,3, E.A. Stach3, C.L. Muhlstein4, M.T. Dugger5, and R.O. Ritchie1,2 1 Department of Materials Science and Engineering, University of California, Berkeley 2 Materials Sciences Division, Lawrence Berkeley National Laboratory 3 National Center for Electron Microscopy, Lawrence Berkeley National Laboratory 4 Department of Materials Science and Engineering, Pennsylvania State University 5 Materials and Process Sciences Center, Sandia National Laboratory, Albuquerque Abstract Wear and fatigue are important factors in determining the reliability of microelectromechanical systems (MEMS). While the reliability of MEMS has received extensive attention, the physical mechanisms responsible for these failure modes have yet to be conclusively determined. In our work, we use a combination of on-chip testing methodologies and electron microscopy observations to investigate these mechanisms. Our previous studies have shown that fatigue in polysilicon structural thin films is a result of a ‘reaction-layer’ process, whereby high stresses induce a room-temperature mechanical thickening of the native oxide at the root of a notched cantilever beam, which subsequently undergoes moisture-assisted cracking. Devices from a more recent fabrication run are fatigued in ambient air to show that the post-release oxide layer thicknesses that were observed in our earlier experiments were not an artifact of that particular batch of polysilicon. New in vacuo data show that these silicon films do not display fatigue behavior when the post release oxide is prevented from growing, because of the absence of oxygen. Additionally, we are using polysilicon MEMS side-wall friction test specimens to study active mechanisms in sliding wear at the microscale. In particular, we have developed in vacuo and in situ experiments in the scanning electron microscope, with the objective of eventually determining the mechanisms causing both wear development and debris generation. Introduction Recent advances in micromachining have increased the demand for more reliable microscale structures. Although silicon is an excellent construction material at the microscale and is most widely used in these structural thin film applications, it is very brittle. Consequently, reliability is often the limiting factor as far as commercial applications are concerned. Since the surface to volume ratio of these structural films is very large, classical models for failure modes cannot always be applied. At these size scales, surface effects may become dominant in determining the mechanical properties of the material. The main reliability issues for microelectromechanical systems (MEMS) are stiction, fatigue and wear. Stiction occurs when freestanding parts stick to the substrate or to each other when releasing the device (release stiction), or while operating, due to particular surface contact conditions (in-use stiction) [1]. Fatigue is important in cases where
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