PURPOSE: To obtain a capacitor with large capacity density and small leak current, by forming a superthin Si nitride film at the interface between a Ta oxide film and an Si semiconductor substrate.

CONSTITUTION: On the Si nitride film 2, the Ta film 3,300 thick, is adhered by a sputtering method. The Si substrate having this double layer film is thereafter heat-treated in a dry oxygen atmosphere at 525°C for 30min, and according the Ta film 300 thick is changed into a Ta oxide film 4. Thereat, in the plyesence of the superthin Si nitride film 2 with denseness between the Ta oxide film and the Si semiconductor substrate, the direct interaction between the Ta oxide film and the Si semiconductor substrate 1 is blocked, thus the formation of Ta silicide having conductivity can be prevented, and accordingly the leak current can be decreased. On said insulation films 2 and 4, Al 1μm thick is adhered and patterned resulting in the formation of an electrode 5. Next, it is applied to a heat treatment in an N2 atmosphere at 400°C for 10min resulting in the formation of a capacitor.