Optical properties of a system consisting of thin Au film and Si substrate have been measured by a spectroscopic ellipsometry over the range from ultraviolet to visible light. The ideal abrupt interface model is no longer reasonable for explaining the experimental results. The influences of voids

surface roughness and transition interface layer on ellipsometric spectra and the formation of an Au-Si alloy interface layer

in which the optical properties are described by a classical vibrator model

have been analysed. Our calculated results show that the formation of an Au-Si alloy interface lager is the major factor influening the ellipsometric spectra of Au film-Si substrate systems. The ellipsometric spectra calculated from our analysis fit in with the measured values.After low temperature annealing (100℃

10min.) Si atoms migrate through the Au film to the surface where they react with oxygen and form an oxide layer.