A method and apparatus are provided to control the radial or non-radial temperature distribution across a substrate during processing to compensate for non-uniform effects, including radial and angular non-uniformities arising from system variations, or process variations, or both. The temperature is controlled, preferably dynamically, by flowing backside gas differently across different areas on a wafer supporting chuck to vary heat conduction across the wafer. Backside gas flow, of helium, for example, is dynamically varied across the chuck to control the uniformity of processing of the wafer. Ports in the support are grouped, and gas to or from the groups is separately controlled by different valves responsive to a controller that controls gas pressure in each of the areas to spatially and preferably dynamically control wafer temperature to compensate for system and process non-uniformities.