Thin SiO2 layers were subjected to short exposures (10-40 s) to HBr/O-2 and Cl-2 high-density plasmas, simulating the over-etching process encountered when polycrystalline Si gate electrodes are etched down to the gate oxide layer. Following this treatment, the samples were transferred under vacuum to an x-ray photoelectron spectrometer and spectra were recorded as a function of the take-off angle between the sample surface plane and the photoelectron collection lens. These angle-resolved measurements were inverted, using a maximum entropy approach, to obtain depth profiles. After etching in Cl-2 or HBr plasmas at an ion energy of similar to 40 eV (obtained with a grounded stage and a plasma potential of 40 V), surface layers were formed with halogen areal densities of similar to 2X10(15) cm(-2), distributed over a half-depth of 10-20 Angstrom. These results (both absolute areal densities and depth distributions) are similar to those found previously for etching of Si under the same conditions. For SiO2, buildup of Cl or Br near the surface is accompanied by a depletion of O. Addition of 10% O-2 to HBr plasmas decreases the Br content in the film by nearly a factor of 2 (with the stage grounded), and dramatically slows the etching rate from similar to 30 to <2 Angstrom/min. Increasing the mean ion energy to similar to 150 eV by applying an rf bias (resulting in a de bias of -110 V) increases the etching rate in the 10% O-2/HBr plasma to about 10 Angstrom/min, and increases the Br areal density by 50. Implications for etching of polycrystalline-Si gate electrodes and selectivity to SiO2 are discussed.