The field oxide thinning behavior in a modified local oxidation of silicon isolation process has been studied. The oxidation rates of silicon were deliberately modified by implantation or by high pressure oxidation (HiPOX). Although oxidation of heavily doped silicon with arsenic (As) or germanium (Ge) greatly enhanced the oxidation rate on the wide-field region, severe field oxide thinning was observed at the small isolation spacings. The dry HiPOX process also showed aggravated thinning compared to the atmospheric dry oxidation process. These results can be attributed to the increased oxide stresses during field oxidation due to low temperature and to fast oxidation rate, respectively These results suggest that, in order to mitigate the thinning, the stress should be relieved by the slower oxide growth at the higher oxidation temperatures. Much reduced thinning was observed, as was expected, when the stress was relieved by introducing an interanneal during field oxidation.