MANUFACTURE OF MOS FET

PURPOSE:To realize a highly reliable MOS FET Of LDD structure only by an ordinary wafer process, by stacking a polysilicon gate on a source drain diffusion layer of low concentration, and covering a source drain diffusion layer of high concentration with a thick oxide film. CONSTITUTION:After a polysilicon gate pattern 3 is formed on a gate oxide film 2, a nitride film 4 is formed on the whole surface, and the flat part is etched and eliminated so as to leave the part covering the side wall of the polysilicon gate 3, thereby forming an oxide film 5 thicker than the gate oxide film, on a source.drain region. The nitride film 4 covering the side wall of the polysilicon gate 3 is eliminated; a source.drain diffusion layer 6 of low concentration is formed by ion implantation, and the flat part is again etched and eliminated so as to leave the part covering the side wall of the silicon gate 3; a source.drain diffusion layer 8 of high concentration is formed by ion implantation. Thereby a highly reliable MOS FET of fine structure can be realized by using an ordinary wafer process.