Method of making a nonvolatile semiconductor memory device

A method for manufacturing a nonvolatile semiconductor memory device comprising the steps of:

• forming a field oxide film to define an active region on a semiconductor substrate of a first conductive type;
• thermally growing a gate oxide film on the surface of said semiconductor substrate of said active region;
• depositing a first polycrystalline silicon layer doped with an impurity and forming a pattern of said first deposited polycrystalline silicon layer;
• covering said first patterned polycrystalline silicon layer with an interlayer insulating film;
• depositing a second polycrystalline silicon layer on said interlayer insulating film;
• forming a pattern of a first polycrystalline silicon layer provided as a floating electrode for accumulating charges and a pattern of said second polycrystalline silicon layer provided as a control electrode overlapping said floating electrode, by using a photolithography process to sequentially etch the stacked layers of the second polycrystalline silicone layer, the interlayer insulating film, and the first polycrystalline silicon layer;
• growing a thermal oxide film at least two times as thick as that of said gate oxide film on the whole surface of the above resultant structure, after the step of forming said patterns;
• thinning the portion of said thermal oxide film formed on the surface of said active region by an anisotropically etching process, after the step of growing said thermal oxide film;
• and
• forming a source region and a drain region by ion-implanting impurities of the opposite conductive type to the substrate near the surface of the semiconductor substrate of the active region through said thermal oxide film and then activating the implanted impurities;
• whereby the edge portion of said gate oxide film, which is thicker than the other portions of said gate oxide film, is formed by a bird's beak extending toward said gate oxide film from said thermal oxide film.