A comparison of the plasma etching characteristics of SiO2 and Si3N4 in high-density fluorocarbon discharges with the goal of identifying an etching chemistry with a very high SiO2-to-Si3N4 etch selectivity has been initiated. High-density plasmas were excited in an electron cyclotron resonance apparatus equipped with a cooled rf powered electrostatic chuck. Gas mixtures of either CF4/H-2, CHF3/H-2 (low carbon/fluorine ratio fluorocarbon gases), or C2F4/H-2, C2F6/H-2, and C3F6/H-2 (high C/F ratio) were used in this work. We will show that a carbon-rich fluorocarbon gas like C-2/F-4 and a modest amount of H-2 are useful in achieving high SiO2/Si3N4 etch selectivity (>28). On the other hand, hydrogen-rich fluorocarbon gas mixtures which contain less carbon, e.g., CHF3/H-2, are not useful for achieving SiO2 over Si3N4 etch selectivity although they will enable SiO2/Si etch selectivity. The gas phase and surface chemical aspects of the different gas mixtures were studied by optical emission spectrometry and line of sight mass spectrometry and by post plasma x-ray photoelectron spectroscopy. The results of these measurements can explain the etch rate data by selective deposition of fluorocarbon films on Si3N4 and Si surfaces.