Etching mechanisms of silicon and silicon oxide in a fluorocarbon environment are studied in an ICP reactor. Optimization of the process for deep etching Of SiO2 with a Si mask has been discussed in a previous article. In this article, adequate plasma conditions are chosen both (a) to allow separation of parametric variables and (b) to get appreciable variation of the different plasma and surface experimental results versus parameters. Hence, pressure, source power, ion energy, and subsequently ion flux are kept constant. The influences of the gas composition and the residence time (varying gas flow rate) are studied. We show that silicon etching depends both on the atomic fluorine concentration in the plasma and of the fluorocarbon blocking layer at the surface. We pay particular attention to the formation of the fluorocarbon overlayer. We establish that the thickness of this layer is linked to the plasma species through the (C+H)/F ratio, calculated from the radical densities in the gas, taking into account the etching-deposition competition. Concerning oxide etching in the so-called sputtering regime, we show that the overlayer detected by x-ray photoelectron spectroscopy does not control the etching mechanism and seems to be an interaction layer between plasma and oxide rather than a blocking layer. We also establish a linear relation between oxide etch yield and the average F/C ratio calculated from the ion densities indicating that the radical species are not a limiting factor in the oxide etching mechanism. Finally, from these results for oxide and silicon etching, a correlation between plasma species and the evolution of the etch rates versus ion energy is done by varying gas composition and residence time. © 2005 American Vacuum Society.