An apparatus and method of forming an oxynitride insulating layer on a substrate performed by putting the substrate at a first temperature within the main chamber of a furnace, exposing the substrate to a nitrogen containing gas at a second temperature which is higher than the first temperature, and growing the oxynitride layer on the substrate within the main chamber in the presence of post-combusted gases. The higher temperature nitrogen containing gases are combusted in a chamber outside the main chamber. The higher temperature is in the range of 800 to 1200° C., and preferably 950° C. In a second embodiment, distributed N2O gas injectors within the main chamber deliver the nitrogen containing gas. The nitrogen containing gas is pre-heated outside the chamber. The nitrogen containing gas is then delivered to a gas manifold that splits the gas flow and directs the gas to a number of gas injectors, preferably two to four injectors within the main process tube. Gas injection orifices on the order of several millimeters then distribute the pre-decomposed gas to the wafers, producing a more uniformly N-doped wafer load in a batch furnace.