Thermal oxidation of silicon, in the presence of chlorine or bromine compounds, effectively improved the dielectric breakdown characteristics (as measured with a voltage ramp) of the resulting SiO2 films. At high halogen concentrations the oxidation rate was erratic; the optimal initial breakdown properties were attained by oxidizing with about one-half of the halogen-containing additive that would cause etching of the Si. Oxidation in the presence of halides increased the resistance of SiO2 films to dielectric breakdown under accelerated bias-temperature stressing. The maximum time to failure varied semilogarithmically with HCl concentration and increased by 0.3 to 0.65 decade per per cent of HCl (0-3% HCl range) for applied fields from 2 to 7 MV/cm, respectively. Higher oxidation temperatures further improved the resistance of HCl oxides to dielectric wear-out. The longer wear-out times were attributed to hydrogen rather than the halogen and could be achieved by oxidation in small concentrations of H2O, annealing in H2, or implantation of H2 or H2O. © 1974, The Electrochemical Society, Inc. All rights reserved.