In-line Characterization of Thin Polysilicon Films by Variable Angle Spectroscopic Ellipsometry
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In-line Characterization of Thin Polysilicon Films by Variable Angle Spectroscopic Ellipsometry S. Paprotta1, K. S. Röver1, R. Ferretti1, U. Höhne2, J. D. Kähler2, J. Haase2 1 Institut für Halbleitertechnologie und Werkstoffe der Elektrotechnik, Universität Hannover Appelstr. 11a, 30167 Hannover, Germany 2 Centrotherm Elektrische Anlagen GmbH+Co. Johannes-Schmid-Str. 3, 89143 Blaubeuren, Germany ABSTRACT Up to now an in-line method for parameter determination of deposited thin polysilicon films is not available. In this paper a method for monitoring the polysilicon deposition process in device manufacturing by variable angle spectroscopic ellipsometry (VASE) is demonstrated. Therefore several polysilicon films are deposited on thermally oxidized [100] silicon wafers. These samples are characterized by VASE in the optical range of 450 - 850 nm. Parameters are determined by simulation using a multilayer model consisting of air, interface layer (surface roughness), polysilicon, SiO2 and silicon substrate. Different optical models representing the properties of polysilicon are tested. The free parameters are the oxide thickness, the composition and the thickness of the interface layer (air, polysilicon) as well as the thickness and the complex refractive index of the polysilicon layer. Results of the spectroscopic analysis are verified by AFM and SEM measurements. It can be shown that parameters of the deposited polysilicon films, which often could only be determined by complex and destructive off-line analysis methods are also accessible by non-destructive in-line VASE measurements. INTRODUCTION Today polysilicon is still an important material for a large area of applications in semiconductor technology. For example polysilicon is widely used for MOSFET´s, nonvolatile memory devices and polysilicon thinfilm-transistors [1]. Often polysilicon is deposited on a thermally oxidized silicon substrate. In this study it is shown that variable angle spectroscopic ellipsometry (VASE) can be used very effectively as an in-line characterization tool to monitor the layer thickness and the surface roughness of the polysilicon films. This two parameters strongly influence the conductivity of the polysilicon [1] and the electrical breakdown strength and reliability of insulators deposited on top [2]. VASE exceeds conventional characterization methods like cross-section SEM, TEM, AFM and surface profiler because it is faster and nondestructive and has mapping capability.
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EXPERIMENTAL DETAILS In this study [100] silicon wafers (6 inch) were used as substrates. After a RCA cleaning process the wafers were thermally oxidized at 950° or 1000°C. The resulting thickness of the silicon oxide was between 15 and 100 nm. For the deposition of polysilicon a silane LPCVDprocess at 200 mtorr and within a temperature range of 600 to 640°C was used. The thickness of the polysilicon films was between 50 and 350 nm. The samples showed a strong variation in respect of surface roughness. For some samples CMP-polishing was used to reduce the sur
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