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

CONSTITUTION: A polycrystal silicon film 102 is formed on a silicon substrate 101 in thickness of 4500, heat diffusion 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 is subject to oxidation and grow in a thickness of 500 on the silicon film 102 at 1000°C under dry oxygen atmosphere. Then a silicon oxide nitride film 104 is formed at 400°C by the plasma CVD method. In this case, the gas pressure ratio of N2O, NH3,SiH4 is controlled to form a silicon oxide nitride film 4 where ratio of elements Si, O, N is nearly 2:1:2 in a thickness of 750. Then the 2nd polycrystal silicon film 105 is formed in a thickness of nearly 4000, an N-channel impurity is diffused to obtain nearly 260 of sheet resistance. Then the sheet is annealed under argon gas atmosphere including hydrogen at 400-800°C. Thus, the element with high capacity and high dielectric strength is manufactured.