Methods for forming a patterned SOI region in a Si-containing substrate is provided which has geometries of about 0.25 .mu.m or less. Specifically, one method includes the steps of: forming a patterned dielectric mask on a surface of a Si-containing substrate, wherein the patterned dielectric mask includes vertical edges that define boundaries for at least one opening which exposes a portion of the Si-containing substrate; implanting oxygen ions through the at least one opening removing the mask and forming a Si layer on at least the exposed surfaces of the Si-containing substrate; and annealing at a temperature of about 1250.degree. C. or above and in an oxidizing ambient so as to form at least one discrete buried oxide region in the Si-containing substrate. In one embodiment, the mask is not removed until after the annealing step; and in another embodiment, the Si-containing layer is formed after annealing and mask removal.