Methods and heat treatment apparatus for heating a substrate and any layer carried on the substrate during a bake process. A heat exchange gap between the substrate and a heated support is at least partially filled by a gas having a high thermal conductivity. The high thermal conductivity gas is introduced into the heat exchange gap by displacing a lower thermal conductivity originally present in the heat exchange gap when the substrate is loaded. Heat transfer across the heat exchange gap is mediated by the high thermal conductivity gas.