A technique is presented for forming a silicon‐on‐insulator material with an ultrathin buried oxide by utilizing the separation by implantation of oxygen or SIMOX method. It overcomes the problem of oxide continuity encountered during standard SIMOX processing when the O+‐implanted dose is scaled down to decrease the thickness of the buried oxide below ∼0.1 &mgr;m. To promote the formation of ultrathin buried oxides (during post‐implantation annealing), the implantation process was modified to produce a microstructure which promotes coalescence of the oxygen into a continuous layer. This was accomplished by slightly modifying the standard (≳500 °C) process so that the final increment of the dose is implanted near room temperature. This dose is chosen to selectively amorphize the region near the ion’s range which will yield a high‐defective layer during subsequent annealing. It is shown that this layer, which can consist of polysilicon, provides a template or guide upon which the oxide forms. Buried oxides prepared in this way are shown to be continuous and without Si inclusions while the standard process yields a broken layer with severe discontinuities. ©1996 American Institute of Physics.