Plasma immersion ion implantation (PIII) is used to fabricate buried oxide layers in silicon. This "separation by plasma implantation of oxygen" (SPIMOX) technique can achieve a nominal oxygen atom dose of 2×10 17cm-2 in implantation time of about 3 min. SPIMOX is thus presented as a practical high-throughput process for manufacturing silicon-on-insulator. In the SPIMOX samples prepared, three distinct modes of buried oxide microstructure formation are identified and related to the as-implanted oxygen profiles. A first-order model based on oxygen transport and oxide precipitation explains the formation mechanisms of these three types of SPIMOX layers.