OXIDIZING METHOD OF POLYCRYSTALLINE SILICON

PURPOSE:To manufacture an EPROM and an EEPROM wherein the dielectric breakdown voltage of an intermediate oxide film is high, and the irregularity in threshold voltage is small, by a method wherein, when polycrystalline silicon doped with impurity is subjected to wet-oxidation, mixed gas of steam and inert gas is used, partial pressure of which steam is equal to or less than a prescribed value. CONSTITUTION:When polycrystalline silicon 3 doped with impurity is subjected to wet-oxidation, mixed gas of steam and inert gas is used, partial pressure of which steam is equal to or less than 200Torr. For example, in the case of a transistor wherein the thickness of a gate oxide film 4 is 100Angstrom , and the design value of threshold voltage is 1V, a lower layer gate electrode 3 doped with P is oxidized at 1000 deg.C, and an intermediate oxide film 2 of 300Angstrom thick is formed. In this case, bubbling method using N2 as carrier gas is applied to the generation of steam, and its partial pressure is adjusted to 100Torr, by changing the water temperature. Thereby, the diffusion of P through the gate oxide film 4 is restrained, so that the irregularity of threshold voltage caused by the diffusion of P can be reduced.