Semiconductor device and method of manufacturing a semiconductor device

A method of manufacturing a semiconductor device that comprises an N-channel MOS transistor, the N-channel MOS transistor comprising:

• a field oxide film and a gate oxide film formed on a semiconductor substrate;
• a gate electrode formed on the gate oxide film, the gate electrode having one end extended to the field oxide film;
• an N-type high concentration source region formed at another end of the gate electrode;
• a channel region formed under the gate oxide film and sandwiched between the N-type high concentration source region and one end portion of the field oxide film;
• an N-type high concentration drain region formed at another end portion of the field oxide film which is opposite from the one end portion of the field oxide film;
• and
• a plurality of electric field relaxing areas formed under the field oxide film, around the N-type high concentration drain region,
• the method comprising:
• forming a P-type well region and a first N-type well region on a surface of the semiconductor substrate;
• forming a second N-type well region so that a depth of the second N-type well region is shallower than a depth of the first N-type well region;
• forming a field oxide film and an N-type intermediate concentration diffusion region simultaneously by oxidization and diffusion of an N-type impurity introduced by ion implantation under a region where the field oxide film is to be formed;
• forming a channel region in a region where no field oxide film is formed;
• forming a gate oxide film on the channel region;
• forming a gate electrode on the gate oxide film;
• forming an N-type high concentration source region and an N-type high concentration drain region by ion implantation of an N-type impurity at a high concentration by using the gate electrode and the field oxide film as a mask;
• forming an interlayer insulating film;
• forming a contact via-hole;
• forming wiring;
• and
• forming a protective film.