A method of forming a silicon nitride-silicon dioxide composite insulator layer for use as a gate insulator stack for an MOSFET device, has been developed. The method features formation of the silicon dioxide component of the gate insulator stack, after formation of the overlying silicon nitride component, allowing the gate insulator stack to be comprised with a nitrogen profile presenting enhanced barrier characteristic and less interface charge than counterpart silicon nitride-silicon dioxide composites formed wherein the silicon nitride component was deposited on an already grown underlying silicon dioxide layer. Oxygen ions, or oxygen radicals obtained via ultra-violet procedures, penetrate the silicon nitride component and locate in a top portion of the semiconductor substrate. Subsequent annealing allows reaction of the oxygen ions or radicals with a top portion of the semiconductor substrate resulting in the desired silicon dioxide component underlying silicon nitride.