Abstract: A semiconductor device provides improved performance at high integration levels by utilizing a gate insulation layer formed from silicon-oxynitride which prevents impurities in the doped gate electrode from diffusing into the semiconductor substrate during the fabrication processes. A method for forming the silicon-oxynitride layer utilizes a vertical diffusion furnace to increase productivity and achieve a uniform nitrogen density in the silicon-oxynitride layer. The method includes forming an initial oxide layer on a semiconductor substrate, changing the initial oxide layer into a pure oxide layer, and then changing the pure oxide layer into a silicon-oxynitride layer. The initial oxide layer is formed by loading a semiconductor substrate into a diffusion furnace at a temperature between 550˜750° C., raising the temperature of the substrate to between 700˜950° C., and injecting a mixture of oxygen and nitrogen has into the diffusion furnace.