The present invention is directed to a semiconductor thermal processing apparatus and a method for thermally cooling a semiconductor substrate. According to one aspect of the present invention, a gas-cooled clamp and associated method is disclosed which provides cooling of a substrate by thermal conduction generally in the free molecular regime. The gas-cooled clamp comprises a clamping plate having a plurality of protrusions that define gaps therebetween, wherein a distance or depth of the gaps are associated with a mean free path of the cooling gas therein. The gas-cooled clamp further comprises a pressure control system operable to control a backside pressure of the cooling gas within the plurality of gaps to thus control a heat transfer coefficient of the cooling gas, wherein the heat transfer coefficient of the cooling gas is primarily a function of the pressure and substantially independent of the gap distance.