Ellipsometric measurements have been performed in situ on very thin CaF2 films at fixed wavelength lambda during deposition or vs. wavelength at different deposition steps. In modelling tan PSI and cos DELTA spectra as a function of lambda at different stages of growth, the Bruggeman effective medium theory is used to describe the interfaces. The value of the index of refraction n depends strongly on the nature and the temperature of the substrate, and on the thickness of CaF2 films. For films deposited at 400-degrees-C on Si wafers covered with their native oxide, cross-sectional high resolution electron microscopy shows that, in spite of the presence of a SiO2 layer (2 nm thick), CaF2 grows on Si through this layer. A model including interfacial layers, which takes into account this interaction with the Si substrate, is proposed for reproducing ellipsometric data. For CaF2 films deposited on SiO2, no evidence for chemical interaction has been found.
