Patent ID: 7696107

Claim:
A nitride film forming method comprising: forming an oxide film on the semiconductor substrate; loading the semiconductor substrate into a reaction furnace and decompressing the inside of the reaction furnace to remove oxygen and water from the inside of the reaction furnace and the semiconductor substrate; heating the reaction furnace to further remove the oxygen and the water from the inside of the reaction furnace and the semiconductor substrate; modifying the oxide film by feeding an inert gas, a reducing gas or a mixed gas thereof into the reaction furnace without nitriding the oxide film; and purifying nitrogen gas to have the oxygen concentration of 1 ppb or below, and performing a thermal process while feeding the purified nitrogen gas into the reaction furnace to form a nitride film by nitriding the oxide film without activating the purified nitrogen gas by plasma, said method further comprising purifying the inert gas, the reducing gas or the mixed gas thereof before said feeding of the inert gas, the reducing gas or the mixed gas thereof into the reaction furnace, wherein in the modifying the oxide film, the oxide film is modified so that the oxide film is easily nitrided in the performing the thermal process.