PURPOSE: To reduce an occupied area of a cell section without decreasing capacitance, and to increase the capacity of information storage by thermally oxidizing thinly the surface section of a silicon nitride film of the high degree of density and using an insulating film, converting one part of the surface thereof into a silicon oxide film.

CONSTITUTION: The thin silicon nitride film layer 103 is formed by depositing a silicon nitride film through a thermal nitriding method or a CVD method of the surface of a silicon substrate 101. The surface is thermally oxidized, and one part is converted into the thin silicon oxide film 104. A capacitor section electrode 105 is coated with an insulating film 106 such as a silicon oxide film, and the transfer gate transistor of the cell section is constituted by forming a gate oxide film 107 to the surface of the silicon substrate exposed and forming a gate electrode 108. An N- diffusion region 109 is formed, the whole regions are coated with a passivation film 110, and a MOSDRAM cell having high capacitance value per unit area is formed. The film obtained by thinly oxidizing the surface of the thin silicon nitride film is used as a capacitance insulating film because of the effect of the lowering of leakage currents.