Apparatus and method are provided for obtaining improved measurement and control of the temperature of a semiconductor wafer (W) during processing. The apparatus includes a chuck for holding a wafer during processing, a coolant gas supply (16), and a temperature sensing arrangement for measuring and controlling the temperature of the wafer during processing. A top face of the chuck (22) over which the wafer is positioned, is configured with a plurality of holes (34) into which the coolant gas, such as helium, is admitted at controlled rate and pressure. The coolant gas passes through a narrow space (36) between the top face of the chuck and the underside of the wafer and is evacuated via an exhaust line (30) after being heated to (or nearly to) the temperature of the wafer. Temperature of the now-heated coolant gas is continuously measured by a temperature sensor arrangement which generates a signal controlling the pressure and flow of coolant gas to the wafer. Close control of the temperature of the wafer is thereby maintained continuously at a desired value during processing.