photoetched in a photoetching step 42 to define the shape of the lower polycrystalline silicon layer after laying and etching the polycrystalline silicon layer the silicon nitride layer 26 is laid on the exposed oxidized surface more specifically the silicon nitride is deposited in a low pressure chemical vapor deposition step 44 on the exposed oxidized surface at the lower polycrystalline electrode 20 preferably an 8 nm thick film of silicon nitride is deposited however 3 6 18 6 nm thick silicon nitride film have also been found to be acceptable the oxide film 24 starts forming in the photoetching step during the water rinse further oxidation occurs as the polycrystalline layer is placed in the furnace for the low pressure chemical vapor depostion step these steps are controlled to minimize the thickness of the natural oxide layer preferably holding it to 1 nm or less the silicon nitride film 26 is oxidized in an oxidizing step 46 to form the silicon oxide film of the second dielectric layer 28 preferably the silicon nitride film is oxidized in a water vapor atmosphere at 900 u00b0 c to form a silicon dioxide layer that is at least 1 nm thick the oxidizing is limited such that about 3 nm or more of the silicon nitride film remain adjacent the lower electrode 20 to protect it from further oxidation the silicon nitride depositing and oxydizing steps are conducted such that the oxide film the unoxidized silicon nitride layer and the silicon oxide layer have an effective thickness that.