dc plasma etching experiments were carried out on polycrystalline Si wafers using CF4and CF4/O2mixtures as etchants. The etchants were injected into a slow stream of Ar while a continuous discharge was maintained. The resulting transients of intermediate and product species were determined mass spectrometrically; some of these were found to depend very much on the oxygen concentration, even though hysteresis effects which complicated earlier results were not noticed in this study.The results do not support existing simplified models, but yield some detailed information concerning the etching mechanism. The etch rates increase when small amounts of oxygen are added to the CF4, probably due to an increase in the atomic fluorine concentrations, while at larger oxygen concentrations they decrease, in part because the fragmentation of the etchant gas CF4is then suppressed. CF2radicals are found to be present in the plasma in appreciable concentrations; presumably, this species is directly involved in the formation of SiF4at the surface.