Nonvolatile semiconductor memory device with offset transistor and method for manufacturing the same

A method for manufacturing semiconductor memory devices comprising the steps of:

• forming a first insulation film on a semiconductor substrate of a first conductivity type;
• forming a first conductive layer on said first insulation film;
• patterning said first conductive layer and said first insulation film into a stripe form;
• forming source and drain regions of a second conductivity type in a stripe form by doping impurity of the second conductivity type into the main surface area of said semiconductor substrate with the remaining portions of said first conductive layer and said first insulation film used as a mask;
• forming a second insulation film on the entire surface of the semiconductor structure;
• coating first photoresist on said second insulation film;
• etching said first photoresist and said second insulation film to have the same surface level as the upper surface of said first conductive layer so as to leave portions of said second insulation film between said first conductive layer portions formed in the stripe form;
• coating second photoresist on the entire surface of the semiconductor structure;
• patterning said second photoresist;
• selectively removing both end portions of said first conductive layer and first insulation film with the remaining portion of said second photoresist used as a mask so as to expose those portions of the surface area of said semiconductor substrate which lie on both sides of said first conductive layer;
• forming a third insulation film on the entire surface of the semiconductor structure;
• forming a second conductive layer on said third insulation film;
• patterning said second conductive layer, third insulation film, first conductive layer and first and second insulation films into a stripe form extending a direction which intersects said source and drain regions formed in the stripe form;
• and
• forming an impurity region of the first conductivity type for element isolation by doping impurity of the first conductivity type into the main surface area of said semiconductor substrate with those portions of said second conductive layer, third insulation film, first conductive layer and first and second insulation films which are left behind in the patterning step used as a mask.