Field-oxide-ungrowth (FOU) phenomenon in recessed local oxidation of silicon (R-LOCOS) isolation process has been studied. Field oxidation temperature has been revealed to play a decisive role in the occurrence of FOU and nitrogen-containing polymers originated from the etching process are believed to be converted into oxidation barrier materials at a certain temperature and above. This transition temperature lies between 1000 and 1050 degrees C, independent of the ambient in the furnace. Although field oxidation below the transition temperature can eliminate the FOU, it causes severe thinning of the field oxide at a small isolation spacing. In order to remove the FOU and at the same time to minimize the field oxide thinning, we propose a new field oxidation method which is a combination of a breakthrough field oxidation at below the transition temperature and a high temperature field oxidation.