Semiconductor device and manufacturing method thereof

A method for manufacturing a semiconductor device, comprising:

• forming a first oxide and a second oxide over the first oxide;
• forming a first insulating film covering the first oxide and the second oxide;
• forming a first dummy gate over the first insulating film to overlap with the second oxide;
• adding a first dopant to the second oxide using the first dummy gate as a mask;
• forming a second dummy gate by removing part of the first dummy gate;
• exposing part of the second oxide from the second dummy gate;
• adding a second dopant to the second oxide using the second dummy gate as a mask;
• forming a second insulating film covering the first insulating film and the second dummy gate;
• forming a third insulating film over the second insulating film;
• removing part of the second insulating film and part of the third insulating film until a top portion of the second dummy gate is exposed;
• forming an opening by removing the second dummy gate, part of the second insulating film, and part of the first insulating film;
• forming a third oxide, a fourth insulating film, and a conductive film in this order to be embedded in the opening;
• and
• removing part of the third oxide, part of the fourth insulating film, and part of the conductive film until a top portion of the third insulating film is exposed.