O 2 18 isotope labeling studies of stress effect on oxidation kinetics

Abstract

It is observed that in sequential two-step thermal oxidation in dry O2 at 800 °C with Ar annealing at 1100 °C following the first oxidation, the oxidation rate increases by nearly four times at the early stage in the second oxidation. Because an intrinsic stress of about 1.4×109 dyne/cm2 induced during the thermal oxidation in the as-grown SiO2 films was sufficiently relaxed following the annealing, the increase in the oxidation rate is supposed to be due to relaxation of the intrinsic oxide stress. For the purpose of investigating this stress effect in terms of the oxygen transport mechanism during thermal oxidation of silicon, O218 isotope oxidation is performed and the isotope profiles in oxides are measured by secondary ion mass spectroscopy. It is found that the bulk O18 concentration in the annealed oxide is much higher than that in the unannealed oxide and increases with the annealing temperature. This strongly suggests that the increase in oxygen concentration is due to the stress relaxation. Since the bulk O18 concentration in the oxide annealed at 1100 °C is over three times higher than that in the unannealed oxide, the increase in oxygen concentration is most likely the cause for the enhanced oxidation. The mechanism of the stress effect is discussed. Furthermore, it is found that the oxygen concentration in oxide on the (111) Si substrate is higher than that on the (100) Si substrate. This could also be explained by the stress effect.