The adhesion and growth of plasma deposited a-SiO2 thin films on stainless steel substrate were studied. The adherence, measured by the microscratch test, is shown to be enhanced by N-2, NH3 and O-2 plasma treatments prior to deposition of SiO2. In situ IR ellipsometry suggests that these plasmas remove hydrocarbon contaminants from the surface, resulting in better adhesion. During the growth of a-SiO2 thin films a vibrational mode near 1180 cm(-1) is observed, in addition to the usual mode near 1070 cm(-1). This effect is discussed and correlated with the variations of the position of the longitudinal optical mode as a function of the film thickness. An interpretation related to the presence of interface stress or disorder in the layer is proposed. More generally, IR ellipsometry is found to be a good tool to analyse plasma surface pretreatment and growth of thin films.