Thermal oxidation mechanism based on formation and diffusion of volatile SiO molecules

The thermal oxidation kinetics of Si with dry oxygen was investigated by X-ray and ultraviolet photoelectron spectroscopy. The SiO desorption was found to be maintained for a while even after the oxide growth during initial oxidation. By the annealing of thick oxides in O₂, the oxidation of SiO molecules in SiO₂ was verified more straightforwardly. Further, it was observed that the decomposition kinetics are closely concerned with the SiO₂/Si strained interface at 730°C, suggesting that the point defect produced by the strain is responsible for the SiO formation. Based on the observations, a unified thermal oxidation model is proposed, in which oxidation and decomposition always take place simultaneously by vacancies and Si self-interstitials due to point defect generation at the interface, respectively, in addition to the oxidation of SiO by O₂ permeating into SiO₂. It is pointed out that SiO molecules are so unstable that they diffuse through the repeated exchange reaction of SiO with SiO₂. Although SiO can be oxidized under O₂ in SiO₂, SiO would desorb from the SiO₂ surface when the O₂ pressure is too low and in vacuum, leading to the removal of oxide. By the use of the present unified model, one can interpret successfully the ¹⁸O profile measured by SIMS for the oxidi reoxidized with ¹⁸O₂ isotope [J.A. Costello and R.E. Tressler, J. Electrochem. Soc. 131 (1984) 1944] and the initial rapid oxidation below 300 Å.