The plasma assisted etching of SiO2 in a commercial RF reactor with a variety of C4F8/Ar/O-2 chemistries has been studied by XPS, SIMS and FTIR. A simple model of surface reactions is proposed. In particular, the role of oxygen in the etch process has been investigated. According to our experiments, oxygen inhibits the formation of CFH based polymeric films on the surface. As the etching process is due to an exchange reaction between the oxygen in the SiO2 and the gaseous fluorine species in the plasma, the presence of oxygen in the etch hinders this process by occupying adsorption sites on the surface. The results would confirm that argon does not participate in chemical reactions with the SiO2 substrate but provides energy for reactions in which F, C and O are involved. The results also indicate that a thick fluorocarbon layer only forms on the surface in the absence of oxygen, regardless of the oxygen source. Consequently, only when the SiO2 layer has been substantially removed does this film form. (C) 2005 Springer Science + Business Media, Inc.