On the Role of the Underlying Microstructure on the Mechanical Properties of Microelectromechanical Systems (MEMS) Mater
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On the Role of the Underlying Microstructure on the Mechanical Properties of Microelectromechanical Systems (MEMS) Materials. G. F. Dirras 1,2, G. Coles1, A. J. Wagner3, S. Carlo3, C. Newman3, K. J. Hemker1 and W. N. Sharpe Jr1. 1 Mechanical Engineering and 3Chemistry Departments, The Johns Hopkins University 3400 N. Charles Street, Baltimore, MD 21218, USA 2 Visiting from LPMTM-CNRS, Institut Galilée, Université Paris XIII 93430 Villetaneuse, FRANCE ABSTRACT The microstructure of Low Pressure Chemical Vapor Deposition (LPCVD) Polycrystalline silicon (Polysilicon) thin films was investigated by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), atomic force microscopy (AFM) and Auger electron spectroscopy (AES). SEM characterization of tensile tested samples showed a brittle like-rupture, along with grooves located at the surface sides of the sample. TEM investigations of as-deposited samples showed equiaxed or fully columnar grains bridging from the bottom to the top of the films. A microstructural coarsening was observed with annealing. In the as-deposited state, the films exhibited a {110} texture as showed by the XRD analysis. The films' top and bottom surfaces were observed to be smooth with a roughness (standard deviation) of about 11nm and 20 nm respectively. A chemical analysis of the thin films showed the presence of carbon and oxygen impurities on the surface and oxygen through the sample as observed in the depth profile. The hypothetical influence of these findings is subsequently discussed in relation to the measured mechanical properties.
INTRODUCTION The flourishing new technology of microelectromechanical systems (MEMS) shows great promise in various areas, comprising for example critical applications such as weapons. Polysilicon thin films of a few microns in thickness are the primary structural support materials for MEMS applications. The microstructure of these thin films depends strongly on the processing parameters such as total and partial pressures, the deposition rate, the doping, and annealing [1,2]. This microstructure which comprises the grain size and morphology, the crystallographic texture, the surface roughness and chemistry influences the electrical properties [3] as well the mechanical properties of the films. A better understanding of the correlation between mechanical properties and thin films microstructure is needed for reliability of MEMS devices. Processing of Polysilicon thin films is usually conducted via chemical vapor deposition (CVD) methods. The resulting thin films usually exhibit compressive or tensile residual stresses that can be relieved by a subsequent heat treatment [4,5]. Depending on the deposition temperature Polysilicon thin films can be deposited as amorphous or crystalline [11] with or without crystallographic texture [1,3]. During a subsequent heat treatment the grain size and morphology may change [4]. Both in-situ doping and post-deposition doping play a role in the built-in stress [6] while, the n
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