Silicon dioxide to silicon reactive ion etch selectivities in CF4 plasmas containing either H2 or C2F4 additives were measured as a function of additive gas concentration. X-ray photoelectron spectroscopy showed the presence of a fluorocarbon film on etched silicon surfaces that slows the silicon etch rate and is responsible for the selectivities attained in both cases (SiO2/Si etch rate ratio: >30 for CF4/35%H2, 4–6 for CF4/35%C2F4). The poor selectivity attained using C2F4-containing plasmas is due to a significantly lower SiO2 etch rate relative to the H2 case and is attributed to less energetic ion bombardment of the surface. A slightly higher etch rate of silicon in the C2F4-containing plasma was attributed to a higher fluorine concentration in the plasma and a thinner, more fluorinated residue on the surface that allowed etching to continue even at high additive concentrations (>50% additive). The thinner film, despite a higher concentration of polymerizing species in the gas phase, indicates that plasma-surface interactions play an important role in film formation. © 1991, The Electrochemical Society, Inc. All rights reserved.