Abstract: A method of employing an ambient diffusion barrier to isolate a first dielectric on a silicon substrate against further growth during formation of a second dielectric on the silicon substrate. Subsequent to formation of the diffusion barrier, an exposed surface area of the silicon substrate is subjected to a dielectric forming ambient. The dielectric forming ambient reacts with the exposed silicon surface to form the second dielectric. However, since the first dielectric is isolated, the dielectric forming ambient cannot diffuse therethrough and react with the silicon underlying the first dielectric thereby precluding any further growth in the first dielectric. Using this method, the first and second dielectrics can be formed with differing cross-sectional thicknesses or differing materials.