PURPOSE: To reduce the leakage currents of a tantalum oxide film by forming an ultra-thin silicon nitride film between the tantalum oxide film and a silicon substrate, forming a nitrogen-concentration gradient, and reducing stress in the interface between the tantalum oxide film and the ultra-thin silicon nitride film.

CONSTITUTION: A direct interaction between the tantalum oxide film 4 and the silicon semiconductor substrate 1 is obstructed owing to the ultra-thin silicon nitride film 2, and leakage currents are reduced. The nitrogen concentration gradient of which nitrogen concentration in the tantalum oxide film 4 increases in the interface 5 in the film thickness direction and decreases gradually toward the surface 6 of the tantalum oxide film and gradually increases again and reaches the surface 6 of the tantalum oxide film can be formed through heat treatment for two hrs. at 1,100°C in NH3 gas, and tantalum oxide film layers 7, 7' of high nitrogen concentration are each formed. The oxide film layer 7 functions as a buffer layer relaxing stress resulting from the difference of thermal expansion coefficients because it has an intermediate thermal expansion coefficient of the tantalum oxide film 4 containing no nitrogen and the ultra-thin silicon nitride film 2, and leakage currents in the tantalum oxide film can be reduced.