Microstructures of Polysilicon
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MICaT,1JUTU
OF POMXSILICIN
R. Sinclair, A. H. Carim, J. Morgiel and J. C. Bravman Department of Materials Science and Engineering Stanford University Stanford, CA 94305-2205
ABSTRACT Sane typical microstructural studies of polycrystalline silicon using transmission electron microscopy (TEM) are described, including the application of this material for assisting TEM investigations themselves. Examples include oxidation and realignment of polysilicon thin films, the structure of polysilicon in EEPRfO4 devices, polysilicon in trench capacitors and measurement of SiO layer thicknesses with polysilicon overlayers . It is also shown tha{ grain growth in heavily phosphorus doped polysilicon films can be followed by in situ heating in the TEM. UNhCUICIGN There is a remarkable similarity between the microstructure of polycrystalline silicon thin films as seen by transmission electron microscopy (TEM) and that of etched metals as revealed by optical microscopy. The different diffraction conditions posed by the Si crystallites with respect to the imaging electron beam gives rise to shades of light and dark contrast equivalent to the varying reflectivity of light by etched metals. Also, equiaxed, twinned or columnar grain structures are produced in polysilicon as they are in cast or annealed alloys. However, one major difference is the typical dimensions associated with the structures: 5rm - 5pm for polysilicon compared to 5pm - 5ram for most metals. Consequently the electron microscope, with its high magnifying power and diffraction contrast capabilities, is virtually essential for studying polysilicon microstructures. In this article we describe some applications of TEM analysis. Both the investigation of polysilicon itself and the use of polysilicon to assist electron microscopy procedures and fundamental materials research are discussed. The majority of images in this paper are conventional bright field transmission electron micrographs in which the contrast is optimized by specimen tilting. Samples are shown in cross-section and are prepared by a well-established procedure [1]. In addition some multi-beam high-resolution pictures are shown, with careful focussing to achieve maximum contrast of the lattice image points. Microscopy was performed on either a Philips EM 400 (120kV) or a Philips EH 430 (300kv) TEM. RESULTS A. Study of Polysilicon Figure 1 oimpares the microstructure of a cast aluminum alloy ingot with that of a polysilicon thin film deposited in an etched silicon trench. The columnar growth of the solid from the sidewalls of the "container" is established fran the image appearance. Notable is the difference in scale of the micrographs, since the silicon trench is only 1-2 pm across. A more revealing micrograph of a typical polysilicon thin film deposition is shown in Figure 2. Here the extremely small grain size of Mat. Res. Soc. Symp. Proc. Vol. 106. '1988 Materials Research Society
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the film where it contacts the substrate is seen, with a clear columnar Annealing such a growth and a more equiaxed, la
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