Relating mechanical testing and microstructural features of polysilicon thin films
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Relating mechanical testing and microstructural features of polysilicon thin films S. Jayaramana) Department of Materials Science and Engineering, The Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218
R. L. Edwards Applied Physics Laboratory Department of Technical Services, The Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218
K. J. Hemkerb) Department of Mechanical Engineering, The Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218 (Received 17 March 1998; accepted 31 August 1998)
Polycrystalline silicon thin films (polysilicon) have been deposited on single crystalline silicon substrates, and square and rectangular windows have been etched into these substrates using standard micromachining techniques. Pressure-displacement curves of the resulting polysilicon membranes have been obtained for these geometries, and this data has been used to determine the elastic constants E and n. The microstructural features of the films have been investigated by transmission electron microscopy (TEM) and x-ray diffraction. The grains were observed to be columnar and were found to have a k011l out-of-plane texture and a random in-plane grain orientation. A probabilistic model of the texture has been used to calculate the bounds of the elastic constants in the thin films. The results obtained from bulge testing (E 162 6 4 GPa and n 0.20 6 0.03) fall in the wide range of values previously reported for polysilicon and are in good agreement with the microsample tensile measurements conducted on films deposited in the same run as the present study (168 6 2 GPa and 0.22 6 0.01) and the calculated values of the in-plane moduli for k110l textured films (E 163.0–165.5 GPa and n 0.221–0.239). I. INTRODUCTION
Polysilicon thin films have traditionally been employed in integrated circuits to form gate electrodes, interconnect networks, emitter structures, and other circuit elements.1 More recently, polysilicon thin films have come to be used as the structural material in a wide variety of microelectromechanical systems (MEMS) applications. The reliability of these polysilicon devices are closely tied to the mechanical performance of these thin films. While equiaxed grains have been observed in films annealed at temperatures above 1040 ±C, low pressure chemical vapor deposited (LPCVD) polysilicon thin films are most commonly observed to have a dendritic grain structure. The need for an understanding of the structure-property relations in these films is highlighted by the fact that their microstructures are often highly textured and far from their equilibrium state. a)
Swaminathan Jayaraman was killed in an automobile accident on November 27, 1998. He was, at the time of his death, a graduate student in the Department of Materials Science and Engineering at Johns Hopkins University, and he will long be remembered as a gregarious and hard working individual by all of his colleagues at Johns Hopkins. b)
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