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To evaluate the reliability of thin thermally grown oxide films, we investigated both step stress breakdown and time-dependent dielectric breakdown (TDDB) which exhibited two distinguished slopes in Weibull plots. It is demonstrated that the intermediate breakdown mode (B mode) in the breakdown histogram corresponded to the steep slope in the short time range of the TDDB plot. The steep slope is observed in the shorter time range with stress field and temperature. The electric field acceleration factor decreases with decreasing tbe oxide thickness. The TDDB data give us minimum voltage in the step stress breakdown histogram necessary to guarantee the device operation for 10 years. Comparison between the breakdown histogram and the minimum voltage indicates that the B mode defect should be decreased. Major origins of the B mode defect are oxygen microprecipitates and metallic contamination in the Si substrates. We found that both high temperature preoxidation annealing and phosphorus diffusion into the back side of wafers greatly increase time to failure of thin thermally grown SiO2 films because of decreasing both the number of oxygen microprecipitates and metallic contamination level.