An oxygen-impervious pad structure which reduces the bird's beak profiles in semi-recessed oxide isolation regions. The sidewalls of a conventional silicon oxide - silicon nitride pad are coated with a thick layer of oxynitride. A thin layer of oxynitride is grown on the substrate surface prior to deposition of the thick oxynitride layer. The thick oxynitride layer prevents lateral oxidizing specie diffusion through the oxide layer of the conventional pad, and the thin oxynitride layer prevents lateral oxidizing specie diffusion through the pad-substrate interface into the substrate region beneath the pad.