Patent ID: 8907423

Claim:
A method of manufacturing a semiconductor device in which a first transistor having a first gate oxide film is formed in a first region and a second transistor having a second gate oxide film thinner than the first gate oxide film is formed in a second region, the method comprising: forming a protective film to the surface of a silicon substrate, the protective film containing a protective oxide film in contact with the surface of the silicon substrate; providing a first opening to the protective film in the first region and providing a second opening to the protective film in the second region; etching the silicon substrate thereby forming a first trench and a second trench in communication with the first opening and the second opening respectively; burying a buried oxide film in the first trench and in the second trench, removing the protective film; forming the first gate oxide film to a portion of the first region from which the protective film has been removed; and forming the second gate oxide film to a portion of the second region from which the protective film has been removed, the second gate oxide film being thinner than the first gate oxide film, wherein the first trench has a first tapered portion extended from the surface of the silicon substrate and having a taper and a first trench main portion extended from the bottom of the first tapered portion and having a taper steeper than the first tapered portion, wherein the second trench has a second tapered portion extended from the surface of the silicon substrate and having a taper and a second trench main portion extended from the bottom of the second tapered portion and having a taper steeper the second tapered portion, and wherein the depth form an upper end of the second trench to a position where the second tapered portion and the second trench main portion are in contact to each other is shallower than the depth from an upper end of the first trench to a position where the first tapered portion and the first trench main portion are in contact to each other, wherein the step of forming the first trench and the second trench comprises forming the first trench main portion and the second trench main portion after forming the first tapered portion and the second tapered portion.