Thicker corner of a gate dielectric structure around a recessed gate electrode for an mv device

A semiconductor device comprising:

• a well region within a substrate;
• a source region and a drain region arranged within the substrate on opposite sides of the well region;
• a gate electrode arranged within the substrate and directly between the source region and the drain region;
• a trench isolation structure continuously surrounding the source region, the drain region, and the gate electrode;
• and
• a gate dielectric structure arranged on outer sidewalls and bottom surfaces of the gate electrode,
• wherein the gate dielectric structure comprises a central portion having a first thickness, an inner corner portion having a second thickness that is greater than the first thickness, and an outer corner portion that has a third thickness greater than the second thickness, and
• wherein the first, second, and third thicknesses are measured in a same direction from a bottommost surface of the gate dielectric structure.

A method comprising:

• forming a trench isolation structure around a well region of a substrate;
• removing a portion of the substrate to form a recess in the substrate, wherein outer sidewalls of the recess are defined by the trench isolation structure;
• forming a first gate dielectric layer over surfaces of the recess in the substrate;
• forming a second gate dielectric layer over the first gate dielectric layer;
• performing a removal process to remove central portions of the first and second gate dielectric layers, wherein after the removal process, portions of the first and second gate dielectric layers remain on the trench isolation structure;
• forming a third gate dielectric layer over surfaces of the recess in the substrate and over the second gate dielectric layer;
• and
• forming a gate electrode over the third gate dielectric layer.