Light irradiation type heat treatment method

Heating treatment is performed on a semiconductor wafer in an ammonia atmosphere formed in a chamber by light irradiation from halogen lamps and flash lamps. For the formation of the ammonia atmosphere in the chamber, pressure in the chamber is once reduced. The pressure in the chamber is also reduced after the heating treatment of the semiconductor wafer. Light irradiation from the halogen lamps is performed to heat the atmosphere in the chamber before the pressure in the chamber is reduced by exhausting the atmosphere from the chamber. The heating of the atmosphere in the chamber before the pressure reduction activates the thermal motion of gas molecules in the atmosphere and decreases a gas density. As a result, the gas molecules in the chamber are discharged rapidly during the pressure reduction, so that the pressure in the chamber is reduced to a predetermined pressure in a short time.