Semiconductor device and manufacturing method thereof

A semiconductor device, comprising:

• a first conductivity type well layer and second conductivity type well layer disposed on a first conductivity type semiconductor substrate;
• a first conductivity type second well layer selectively disposed in a surface layer of the second conductivity type well layer;
• a second conductivity type first offset layer and second conductivity type second offset layer spaced apart in a surface layer of the first conductivity type second well layer;
• a second conductivity type source layer disposed in a surface layer of the first offset layer;
• a second conductivity type drain layer disposed in a surface layer of the second offset layer;
• a first conductivity type semiconductor layer disposed in contact with the source layer in a surface layer of the second well layer on the side opposite to that of the second offset layer;
• a gate electrode disposed across a gate dielectric on the second well layer sandwiched by the first offset layer and second offset layer;
• and
• a LOCOS oxide film, disposed so as to enclose the drain layer, disposed in contact with the gate dielectric, wherein
• the second well layer is such that a bottom portion below the high concentration drain layer includes a region shallower than a bottom portion below the LOCOS oxide film.

A method of manufacturing a semiconductor device including a first conductivity type well layer and second conductivity type well layer disposed on a first conductivity type semiconductor substrate and a first conductivity type second well layer selectively disposed in a surface layer of the second conductivity type well layer, the manufacturing method comprising:

• a step of forming the first conductivity type well layer and second conductivity type well layer on or in a surface layer of the first conductivity type semiconductor substrate;
• a step of ion implanting first conductivity type impurities using a photoresist mask including a first opening portion and a second opening portion enclosed by the first opening portion or formed so as to divide the first opening portion, and subsequently carrying out a drive-in process, thereby forming the second well layer including a region wherein the depth of a region corresponding to the second opening portion is less than the depth of a region corresponding to the first opening portion;
• a step of forming a second conductivity type first offset layer in a surface layer of the second well layer;
• a step of forming a second conductivity type second offset layer distanced from the first offset layer on a region of the second well layer with a small depth;
• a step of forming a LOCOS oxide film including a third opening portion on the second offset layer;
• a step of forming a gate dielectric so as to be in contact with the LOCOS oxide film on the second well layer sandwiched by the first offset layer and second offset layer and on the second offset layer;
• a step of forming a gate electrode on the gate dielectric;
• a step of forming a second conductivity type source layer in a surface layer of the first offset layer;
• a step of ion implanting second conductivity type impurities into a surface layer of the second offset layer with the LOCOS oxide film as a mask, and carrying out a drive-in process, thereby forming a second conductivity type drain layer;
• a step of forming a first conductivity type semiconductor layer in contact with the second well layer from the first offset layer surface;
• and
• a step of forming a source electrode electrically connected to the source layer and semiconductor layer and a drain electrode electrically connected to the drain layer.