PURPOSE: To manufacture a product with a high capacity and a high dielectric strength by using a silicon thermo nitride film and a silicon nitride film as insulation films.

CONSTITUTION: A polycrystal silicon film 102 is formed on a silicon substrate 101 in thickness of 4500, heat treatment is applied to the film at 900°C under nitrogen and POCl3 atmosphere to decrease the sheet resistance to nearly 25Ω. Then a silicon oxide film 103 in a thickness of 500 on the silicon film 102 at 1000°C under dry oxygen atmosphere, heat treatment is applied at 1100°C under nitrogen ammonium atmosphere for nearly 2 hours, part of the silicon oxide film 103 is subject to thermo nitrogen processing to form a silicon nitride film 104. Then a silicon nitride film 105 is deposited and grown by nearly 500 by the CVD method. Then the 2nd polycrystal silicon film 106 is formed and annealed under argon gas atmosphere including hydrogen at 400-1000°C. Thus, the element with high capacity and excellent dielectric strength is manufactured.