The authors present a TEM (transmission electron microscopy) analysis of the structural evolution of SIMOX (separation by implanted oxygen) through three subsequent implantation/anneal cycles. Optimizing the process parameters on the basis of the TEM analysis resulted in a better understanding of the mechanisms of defect formation, and the reduction of defect density. The structural evolution of defects through the processing steps is described. A set of silicon (100) wafers was prepared in six steps with implantation and anneal alternating three times. The oxygen doses of the three implantation steps were 5 × 1017, 5 × 1017, and 8 × 1017 cm-2. All implantations were performed at 200 keV, 620°C, and at a current density of 1 mA/cm2. The anneals after each implant were carried out at 1300°C for 6 h in an argon atmosphere. After each implantation and anneal step the wafers were examined by TEM with a Philips 420 at 100 keV.