High-temperature oxidation (HTO), which is used to create an internal thermal oxide (ITOX) layer at the superficial silicon/buried oxide (BOX) interface in the silicon-on-insulator structure, is an important process in the production of low-dose separation-by-implanted-oxygen (SIMOX) wafers. We studied the influence of oxygen partial pressure in the HTO atmosphere on ITOX growth. Low-dose SIMOX wafers were subjected to HTO processes in which the oxygen partial pressure was varied. ITOX growth at high oxygen partial pressures agreed well with the HTO model proposed by Schroer et al. At low oxygen partial pressures, however, ITOX growth was demonstrated for the first time to change to BOX shrinkage, which cannot be explained by previous HTO models. BOX shrinkage occurred simultaneously with top oxide growth, suggesting that the thermal oxidation at the top oxide/superficial silicon interface can proceed even on silicon crystals with an oxygen concentration below the solubility limit. We proposed a new HTO model that allows top oxide growth on silicon crystals below the solubility limit. Furthermore, a diffusion-limited BOX thickness change was assumed. The new HTO model showed excellent agreement with the experimental data and effectively explains the changing behavior from ITOX growth to BOX shrinkage. (c) 2006 The Electrochemical Society.