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Raman scattering is used to probe the reactions at the interface of Ti and Si. Different titanium-silicide and titanium-oxide spectral signatures which can be used to characterize the thin-film are observed. In addition, the Raman spectrum of metallic Ti deposited on Si is detected and is used to monitor the Si interdiffusion. The 40-nm-thick Ti films were evaporated in UHV or high-vacuum environments, and in situ Raman measurements of the Ti film and the Si substrate are tracked as a function of vacuum annealing at temperatures up to 500°C. The results show that extensive Si interdiffusion into the Ti occurs between 300 and 400°C and that the interdiffusion precedes silicide compound formation. The TiSi2 formation is discussed in terms of a nucleation model. The silicide formation temperature is related to the nucleation barrier, and it is proposed that impurities increase the nucleation barrier, which results in the observed higher silicide formation temperature.