Thermally processing a substrate

A thermal processing method is described in which a temperature response of a substrate may be controlled during a heat-up phase or a cool-down phase, or during both phases. This reduces the thermal budget of the substrate and improves the quality and performance of devices formed on the substrate. In particular, by controlling the rate of heat transfer between the substrate and a thermal reservoir (e.g., a water-cooled reflector plate assembly), the temperature response of the substrate may be controlled during the thermal process. The rate of heat transfer may changed by changing the thermal conductivity between the substrate and the thermal reservoir, by changing the emissivity of a surface of the thermal reservoir, or by changing the distance between the substrate and the thermal reservoir. The thermal conductivity may be changed by changing the characteristics of a thermal transport medium (e.g., a purge gas) located between the substrate and the thermal reservoir. For example, the thermal conductivity may be changed by changing the composition of the purge gas or the pressure of the purge gas between the substrate and the thermal reservoir. In one implementation, the substrate is heated in accordance with a heating schedule and, during the heating schedule, the rate of heat transfer between the substrate and a thermal reservoir inside the thermal processing system is changed. In another implementation, a first purge gas is supplied into the thermal processing system, the substrate is heated in accordance with a heating schedule, and a second purge gas that is different from the first purge gas is supplied into the thermal processing system.