is retreated in advance so that the oxide film that has grown on the side surface of the second film may not be hung over the oxide film that has grown on the side surface of the first film after the thermal treatment step of forming the oxide film by thermal treatment is performed.
According to this production method, the side surface of the second film is retreated in advance for a predetermined length so that the oxide film that has grown on the side surface of the second film may not be hung over the oxide film that has grown on the side surface of the first film. Therefore, voids can be prevented from being formed when the buried oxide film is buried in the trench. This canthe trench by retreating the side surface of the second film before forming the trench, thereby forming an isolation oxide film including an oxide film and a buried oxide film having a desired shape.
Furthermore, it is preferable that the first film, the second film, and the layer serving as a mask material specifically include a silicon oxide film, a polysilicon film, and a silicon nitride film, respectively.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.