TEMPERATURE-DEPENDENCE OF THE DIELECTRIC FUNCTION OF SILICON USING IN-SITU SPECTROSCOPIC ELLIPSOMETRY

Silicon substrates are widely used for the optical study of transparent materials such as oxides, fluorides, etc., because of the large difference between the indices of refraction of the film and the substrate. Optimal conditions for growth of dielectric films require high substrate temperatures, and hence a good knowledge of the temperature dependence of the dielectric function of silicon. In the present study, the complex dielectric function of Si [111] was determined in the 1.5-4.7 eV spectral range from room temperature up to 450-degrees-C, from measurements of the ellipsometric parameters tan psi and cos A performed in ultra-high vacuum on silicon wafers covered with their native oxide, with a rotating polariser spectroscopic ellipsometer. A two-boundary model was used to account for the oxide layer. In the fundamental absorption spectral region, the major effect of temperature on the dielectric function is a shift and a broadening of the structures associated with critical points, in agreement with the literature. In the transparency region, the real part of the index of refraction is found to vary linearly with temperature for energies lower than approximately 3 eV. Using this set of data as a reference, the temperature of a silicon substrate can be deduced from ellipsometric measurements in the 20-450-degrees-C range with good accuracy (+/- 3-degrees-C).