Heat treatment method and heat treatment apparatus for heating substrate by irradiating substrate with light

A method of heating a substrate by irradiating the substrate with light, the method comprising the steps of:

• (a) heating a substrate to a predetermined preheating temperature;
• (b) irradiating a first surface of said substrate with a flash of light from a flash lamp to keep increasing the temperature of said first surface of said substrate from said preheating temperature to a maximum target temperature over a time period longer than the time required for heat conduction from said first surface to a second surface of said substrate which is a surface opposite from said first surface, wherein an emission output of said flash lamp keeps increasing until a temperature of said first surface reaches said maximum target temperature in said step (b);
• and
• (c) irradiating said first surface of said substrate with a flash of light from said flash lamp to maintain the temperature of said first surface of said substrate within a ±25° C. range around said maximum target temperature for not less than 5 milliseconds, said step (c) being performed after said step (b), wherein said maximum target temperature is the highest temperature reached by said substrate throughout said heating method, except that it may exceed same during said irradiating step (c) but not more than by 25° C.