Semiconductor device with high breakdown voltage and manufacture thereof

A method of manufacturing a semiconductor device comprising:

• forming a first n-type well by ion implantation of an n-type impurity into a p-type semiconductor substrate;
• forming a first isolation film and a second first isolation film in the p-type semiconductor substrate;
• forming a second n-type well by ion-implantation of an n-type impurity in the p-type semiconductor substrate, the second n-type well being shallower than the first n-type well;
• forming a first p-type backgate region in the first n-type well and a second p-type backgate region in the second n-type well by ion-implantation of a p-type impurity;
• forming a first gate electrode that extends from above the first p-type backgate region onto the first isolation film, and forming a second gate electrode that extends from above the second p-type backgate region onto the second isolation film;
• forming a first n-type source region in the first p-type backgate region and a second n-type source region in the second p-type backgate region by ion-implantation of an n-type impurity;
• and
• forming a first n-type drain region in the first n-type well at position that is on opposite side to the first n-type source region with respect to the first p-type backgate region and the first isolation film, and forming a second n-type drain region in the second n-type well at position that is on opposite side to the second n-type source region with respect to the second p-type backgate region and the second isolation film by ion-implantation of an n-type impurity.