Method of forming a device isolation region

A method of forming a device isolation region, comprising the steps of:

• forming a first dielectric film and an oxidation-resistant deposition film successively on a semiconductor substrate;
• forming a trench groove in the semiconductor substrate by successively processing the oxidation-resistant deposition film, the first dielectric film and the semiconductor substrate by anisotropic etching;
• forming a second dielectric film to cover at least an inner surface of the trench groove;
• depositing a third dielectric film in the trench groove so that the thickness of the third dielectric film is larger than a depth of the trench groove;
• planarizing a surface of the third dielectric film and an upper surface of the trench groove;
• removing the oxidation-resistant deposition film and the first dielectric film to form the device isolation regions;
• thermally treating the entire substrate to densify the third dielectric film before the planarizing step, and further thermally treating in another thermal treatment the entire substrate to oxidize an interface between the second dielectric film and the semiconductor substrate after the planarizing step to form a fourth dielectric film on the inner surface of the trench groove.