Adhesion improvement of plasma-deposited silica thin films on stainless steel substrate studied by x-ray photoemission spectroscopy and in situ infrared ellipsometry

Abstract

Adhesion of plasma-deposited SiO2 on stainless steel is shown to be enhanced after Ar, N2, and NH3 plasma pretreatments of the substrate. Adhesion is related to the chemical bonding at the interface. Therefore, it is studied by x-ray photoemission spectroscopy (XPS) and in situ IR ellipsometry performed on very thin films (≈30 Å thick). IR ellipsometry reveals the removal of adsorbed hydrocarbons on the metallic surface by all plasma treatments. XPS measurements show the removal of NOx species, related to the sample electropolishing, using N2 and NH3 plasmas; in contrast, Ar has practically no effect in this case. Plasma-induced modifications are not limited to surface cleaning. Some nitrogen is incorporated in the substrate after N2 and NH3 plasmas. In particular, nitrogen is found bonded to Si and to Cr after NH3 and N2 treatments, respectively, we interpret the adhesion enhancement by hardening of the substrate surface region in the case of N2 plasma, while, in the case of NH3, it may be related to the formation of silicon nitride. Cr–N–Si linkages between the substrate and the film may also contribute to adhesion improvement. We also observed changes in the early stages of the film growth. A higher sticking coefficient of dissociated species on the surface is revealed after plasma pretreatment. Narrowing of the Si 2p peak in XPS spectra observed in pretreated samples is attributed to a better ordered SiO2 structure at Si sites in terms of bond angles and/or lengths. In contrast, both IR ellipsometry and XPS show no change at O sites.