Thin SiO2 layers were exposed to an HBr/O2 plasma for a variety of short periods, reproducing the over-etching process after polycrystalline Si gate electrodes have been etched and the gate oxide layer is exposed. Samples were transferred under vacuum to an x-ray photoelectron spectrometer for analysis. After relatively thick (>60 Å) films were exposed to a 10% O2/HBr plasma at an average ion energy of ∼ 150 eV, the near-surface region becomes brominated, and the thickness of the film decreases, indicating an etching rate of ∼ 1–2 Å/s. When the starting film thickness is between 10 and 20 Å, however, exposure to the plasma results in an increases in the thickness of the film, and is enhanced with the increasing addition of oxygen to the feed gas. At mean ion energies of 40 or 150 eV, the transition from etching to deposition occurs at oxygen additions of ∼ 1% or ∼ 8%, respectively. The increase in SiO2 thickness is ascribed mainly to oxidation of the Si at the oxide-substrate interface, and not to deposition resulting from sputtering of reactor materials. Consumption of crystalline Si beneath a 12 Å thick SiO2 gate oxide, adjacent to a 600 Å linewidth, polycrystalline gate electrode was also observed after etching of this transistor structure, as confirmed by transmission electron microscopy. © 1999 American Institute of Physics.