PURPOSE: To machine the parts of the side walls and the bottom part of a trench groove selectively, by coating the upper part of a semiconductor substrate having the trench groove with a film to be etched, coating the film with an organic film having a pattern so as to distinguish the side walls and the bottom part, and performing etching with the film as a mask.

CONSTITUTION: The upper part of a silicon semiconductor substrate 1, in which a trench groove is formed, is coated with a nitride film, 2, which is a film to be etched. The upper part of the film 2 is coated with an organic film 3 having a pattern, which distinguishes the side walls and the bottom part of the trench groove. Then, with the pattern 3 as a mask, the nitride film is etched, and the organic film 3 undergoes ashing and is removed by using oxygen plasma. By such an operation, only distinguished machining of the side parts and selective machining of only a part of the bottom part can be implemented. Since the nitride film of the silicon substrate is conductive, the device, in which the side walls and the bottom part are separately machined, can be used as a conductive semiconductor as it is.