At high temperatures and low oxygen partial pressures, decomposition of SiO2 can occur to form volatile SiO by the reaction Si+SiO2→2SiO. Since the Si atoms are supplied from the substrate, vacancies are generated at the SiO2/Si interface. This has been confirmed by observations of enhanced diffusion of Sb, retarded diffusion of P, and an accelerated shrinkage of extrinsic stacking faults in Si under thin SiO2 annealed in Ar at 1100 °C. As the oxide thickness increases, the vacancy supersaturation rapidly decreases, which is probably a result of a reduced SiO generation flux with increasing oxide thickness as the out‐diffusion of SiO tends to control the rate of the overall decomposition process. When a polysilicon layer is on top of SiO2/Si, the SiO formation process occurs nonuniformly and circular features appear where the poly‐Si layers have been completely removed. Vacancies were injected only through these circular features while the vacancy concentration in the rest of the region was not perturbed.