It has recently been shown that SIMOX (separation by implantation of oxygen) technology can be extended to provide both vertical and lateral isolation of device islands by a single implantation stage. This technology (TDI) entails implantation of O⁺ ions through a deposited masking layer of SiO₂ in which windows are opened to define the silicon device islands. However, the structures had detrimental characteristics (e.g. nonplanar surfaces and entrapped silicon islands in the synthesized SiO₂) that detracted from the utility of the technique. A process is reported that produces improved structures, which are suitable for application to circuits.