Formation of a buried oxide (BOX) layer during high-temperature annealing in a separation by implanted oxygen (SIMOX) process was evaluated by means of Fourier-transform infrared (FTIR) absorption spectroscopy. The evaluation results suggested that the formation process could be controlled by changing the ramping rate and the oxygen concentration in the atmosphere during high-temperature annealing. This was confirmed through TEM observation after annealing under various conditions. Reduction of the O+ implantation dose was enabled by adopting a slow ramping rate for the annealing. Novel Si-on-insulator (SOI) structures with a BOX layer at the damage-peak (Dp) depth and a double BOX layer at both Dp and Rp (projection range) depths were obtained, depending on the O+ implantation dose, by applying a combination of a slow ramping rate and a high oxygen concentration in the atmosphere. The atmospheric oxygen enhanced the growth of the oxide precipitate and smoothed the SiO2/Si interfaces of the SOI structures.