NONVOLATILE SEMICONDUCTOR STORAGE DEVICE AND ITS MANUFACTURING METHOD

PROBLEM TO BE SOLVED: To provide a method of manufacturing a semiconductor storage device wherein irregularity of threshold voltage of a nonvolatile semiconductor storage device can be reduced by isolating P wells every plural memory cells which are adjacent to each other in the lengthwise direction of a bit line diffusion layer and a common source diffusion layer, without increasing a cell array area and manufacturing cost. SOLUTION: In this method of manufacturing a nonvolatile semiconductor storage device, the following processes are included. A second and a first conductivity type wells 2, 3 are formed on a first conductivity type semiconductor substrate 1. A tunnel oxide film 4 and floating gates(FG) 5 stretching in the Y direction are formed on the first conductivity type well 3, and an impurity diffusion layer 7 is formed by using the FG's 5 as masks. The impurity diffusion layer 7 is divided into two parts in the X axis direction. Trenches which penetrate the impurity diffusion layer 7 and reach the second conductivity type well 2 are formed, and insulating films 12 are buried in the trenches.