The Stress and Microstructure analysis of Polycrystalline Silicon Films Deposited by LPCVD
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The Stress and Microstructure analysis of Polycrystalline Silicon Films Deposited by LPCVD 1
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Y.T. Cherng , M.C. Boysel and B.D. Gates1, 2 1. 4DLABS, Simon Fraser University 8888 University Drive, Burnaby, BC, Canada 2. Chemistry Department, Simon Fraser University 8888 University Drive, Burnaby, BC, Canada ABSTRACT Amorphous (a-Si) and polycrystalline silicon(c-Si) films have been obtained by low pressure chemical vapor deposition (LPCVD) in fixed silane flow at low pressure (200 mtorr) with variable growth temperature. Measurement of residual stress of polysilicon films growth between 570oC to 620oC was reported. Residual stress of polycrystalline silicon depends on film microstructure. The poly-silicon microstructure is a strong function of LPCVD growth temperature and pressure. In this work, Raman Scattering and X-ray diffraction (XRD) were used to study the film structure and composition. The multilayer optical model of Spectroscopic ellipsometry (SE) was used to crosscheck the crystallinity fraction. The surface roughness was identified by Atomic force microscopy (AFM) and SE. The residual stress changed from compressive to tensile and back to compressive for deposition temperature between 570oC and 620oC. Film’s c-Si fraction increased as a function of deposition temperature. The roughness surface was found at deposition temperature of 580oC. For deposition temperature larger then 580oC, all films shown (200) texture. The (220) grain size increased from 18.6 nm to 25 nm when deposition temperature increased from 580oC to 620oC. The film residual stress change can be explained by grain structure, surface stress and volume stress. At deposition temperature from 580oC to 587oC, grain is equi-axes type and volume stress dominate which cause the tensile stress. For temperature higher then 587oC inverse conical grain formed from oxide interface to surface and surface stress dominate cause the stress back to compressive. The columnar structure formed when deposition temperature > 600oC, grain growth push the compressive stress decrease again. INTRODUCTION Thin films of polycrystalline silicon (c-Si) and amorphous silicon (a-Si) deposited by Low Pressure Chemical Vapor Deposition (LPCVD) have been intensively studied because of their broad range of applications [1-3]. Polysilicon is being extensive used as a structure and active material in surface micromachining technology (e.g., MEMS) for variety applications such as pressure transducers, micro-switches and other MEMS devices. For MEMS applications, film residual stress is important to understand and control. Compressive stresses may initiate film bulking while high tensor stresses can result in cracking. Stresses also influence device electrical and optical properties. For most applications, polysilicon film with low tensile stress or zero stress is desired. Residual stress of polysilicon film is a function of film microstructure, a-Si/c-Si phase, grain size and morphology. LPCVD deposition parameters, such as deposition temperature, pressure and siliane flow r
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