Study of CaF 2 growth on Si, a-Si O 2 by in-situ spectroscopic ellipsometry

Abstract

Ellipsometric measurements have been performed in-situ on very thin CaF 2 films at fixed wavelength λ during deposition or vs. wavelength at different deposition steps. In modelling tan ψ and cos Δ spectra as a function of λ 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 CaF 2 films. For films deposited at 400°C on Si wafers covered with their native oxide, cross-sectional high resolution electron microscopy shows that, in spite of the presence of a SiO 2 layer (2 nm thick), CaF 2 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 CaF 2 films deposited on SiO 2, no evidence for chemical interaction has been found.