a method for forming a soi substrate device having multiple buried oxide regions comprising the steps of ; forming a thin buried oxide layer in a silicon - containing substrate , forming a mask with openings therein on the substrate , implanting oxygen into the substrate through the openings in the mask , forming a buried oxide region in the substrate and a thermal oxide layer at the substrate surface in the mask openings by annealing , exposing the regions of the substrate that were not thermally oxidized by removing the mask , planarizing the crystalline silicon surface of the substrate by thermally oxidizing the substrate surface , removing the oxide layer from the surface of the substrate , exposing a crystalline silicon surface that has no steps between different buried oxide regions of the substrate .