Nonvolatile semiconductor memory device having reduced resistance value for the common source wiring region

A semiconductor device comprising:

• a semiconductor substrate of a first conductivity type;
• a plurality of striped field oxide films formed on a main surface of said semiconductor substrate, said field oxide films being spaced from each other along a first direction;
• a plurality of gate insulating films, each of said gate insulating film formed on the main surface of a semiconductor substrate between said field oxide films;
• a plurality of striped conductive layers formed on said field oxide films and said gate insulating films, said conductive layers being spaced from each other along a second direction crossing said first direction;
• a groove formed in said gate insulating films and said field oxide films between said conductive layers, said groove having a lower surface formed by said main surface of said semiconductor substrate and a plurality of pairs of first widewalls, each pair of said first sidewalls is formed by each of said field oxide films, a plurality of pairs of second sidewalls, each pair of said second sidewalls is formed by each of said gate insulating films, and wherein a width between one of said pairs of first sidewalls adjacent to said main surface of said semiconductor substrate is greater than a width between said one of said pairs of first sidewalls adjacent to said conductive layers;
• and
• a common source region of a second conductivity type formed in said semiconductor substrate under said groove.