Semiconductor device and method of manufacturing semiconductor device

A method of manufacturing a semiconductor device, comprising the steps of:

• forming, over a first surface of a semiconductor substrate located in a region where a first transistor is to be formed, a first gate insulating film having a first oxide film, a first nitride film placed over the first oxide film, and a second oxide film placed over the first nitride film;
• and
• forming a second gate insulating film over the first surface located in a region where a second transistor is to be formed,
• wherein the second oxide film includes a first layer and a second layer placed over the first layer,
• wherein the first gate insulating film formation step includes a step of forming a second nitride film placed over the first layer, and
• wherein the second layer is formed by oxidation of at least a portion of the second nitride film,
• wherein the second gate insulating film is formed by thermal oxidation,
• wherein the method further comprises forming, over the first surface located in a region where a third transistor is to be formed, a third gate insulating film thinner than the second gate insulating film,
• wherein the portion of the second nitride film remains at the time of formation of the second gate insulating film,
• wherein the second nitride film oxidized at the time of formation of the second gate insulating film is removed by etching prior to the third gate insulating film formation step, and
• wherein the portion of the second nitride film which has remained at the time of formation of the second gate insulating film is oxidized into the second layer at the time of formation of the third gate insulating film.