A method of forming a field oxide isolation region includes: forming a first pad oxide layer over a semiconductor substrate; forming a silicon nitride layer over the first pad oxide layer; patterning and etching the silicon nitride layer and the first pad oxide layer to expose a portion of the substrate, and simultaneously forming an undercut cavity; forming a second pad oxide layer over the exposed portion of the substrate; depositing a layer of polysilicon over the second pad oxide layer, the polysilicon layer filling the undercut cavity to form a polysilicon plug; removing portions of the polysilicon layer to form a polysilicon spacer; thermally oxidizing the substrate to substantially consume the polysilicon spacer but leave a polysilicon residual of the polysilicon plug, the thermal oxidation forming a thick oxide above the exposed portion of the substrate; substantially removing the silicon nitride layer; applying a first etching solution to the first pad oxide layer and the polysilicon residual, the first etching solution providing selective etching of the first pad oxide layer and the polysilicon residual so that the polysilicon residual is substantially removed and the first pad oxide layer is partially removed leaving a first pad oxide layer residual; and applying a second etching solution to remove the first pad oxide layer residual, thereby leaving the thick oxide to form the isolation region.