The effect of oxygen on polycrystalline-Si (poly-Si) and SiO2 etching in hydrogen bromide (HBr) reactive ion etching plasmas has been studied by measuring etch rates and using x-ray photoelectron spectroscopy (XPS) to study compositional changes in the surface layer. The etch rate of the poly-Si increases dramatically from 30 to 235 nm/min as the O-2 concentration increases from 0% to 25%, whereas the SiO2 etch rate gradually decreases from 3 to 1 nm/min. Above 30% O-2 in HBr, the poly-Si etch rate abruptly decreases by a factor of 16 compared with that at 25%. From XPS analysis, it is found that the abrupt decrease of the poly-Si etch rate at O-2 concentrations of more than 30% is closely related with the composition and thickness of an SiBrxOy layer formed during the HBr/O-2 plasma exposure. The SiBrxOy layer has a composition of nearly SiO2. Br ions cannot permeate the SiBrxOy, layer formed in plasmas containing 30% O-2 in HBr (or greater), and therefore, the poly-Si etch terminates.