AFM and TEM Examination of Surface Grains in LPCVD Silicon Films
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AFM and TEM Examination of Surface Grains in LPCVD Silicon Films
Dave (Boris) Kharas, R. J. Gambino, and N. Golubovic-Liakopoulos* Dept. Materials Science, State University of New York in Stony Brook, Stony Brook, NY 11794-2275, *Standard MEMS Inc. Hauppauge, NY 11788. ABSTRACT Polycrystalline silicon is an important technological material in microelectronics, and more recently in microelectromechanical systems (MEMS). For MEMS applications polysilicon films with residual tensile stress are often used, requiring film growth in the transition zone from 550-600°C. In this study polysilicon films were grown in a hot walled LPCVD reactor at 590°C to a thickness of 450 nm. The tensile as-deposited stress in the wafers was found to decrease with distance from the reactor front and served as a marker for the changing microstructure and surface roughness of the film as measured by Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM). It was found that the surface contained a high density (≈30 /µm2) of grains that protruded 30 nm above the mean film surface. The size and volume of these grains increased linearly with decreasing stress until the surface saturated and became uniformly rough. The nature of these surface grains, their spatial characteristics, and their annealing behavior is discussed. INTRODUCTION Polycrystalline silicon finds widespread applications in all types of microelectronic devices and more recently in surface micromachined microelectromechanical systems (MEMS). Despite polysilicon’s widespread and longtime use, interest in its properties continues to be high. This is due not only to its technological importance but also due to its sensitivity to processing conditions resulting in a highly variable microstructure. There have been numerous studies tying polysilicon’s microstructure (grain size, crystalline texture, surface topography, etc.) to processing conditions [1-5]. The general trends that have been identified show that silicon deposited below 550°C is amorphous, has a smooth surface, and is in compression. Films deposited in the range of 605°C -630°C are found to be polycrystalline with strong {110} texture. Increasing {100} texture is seen at yet higher temperatures. These films have compressional intrinsic stresses in the -400 to -50 MPa range. Polysilicon deposited in the amorphous to crystalline phase transition zone of 550°C to 605°C has a highly variable microstructure. This variability is due to the competing growth kinetics of amorphous and crystalline phases in the film [6-8]. Initially small polycrystalline grains form at the substrate surface, and subsequent deposition leads to the formation of an amorphous phase that, depending on processing conditions, can crystallize during the deposition. The volume change from the solid phase crystallization of the amorphous silicon results in a tensile residual stress in the films. In all of polysilicon’s applications a controllable film surface topography is of great importance. Although in some cases such as in hemispherical
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