A substrate processing chamber, particularly a chemical vapor deposition (CVD) chamber used both for thermal deposition of a conductive material and a subsequently performed plasma process. The invention reduces thermal deposition of the conductive material on peripheral portions of the pedestal supporting a wafer and in a pumping channel exhausting the chamber. A peripheral ring placed on the pedestal, preferably also used to center the wafer, is thermally isolated from the pedestal so that its temperature is kept substantially lower than that of the wafer. Despite its thermal isolation, the peripheral ring is electrically connected to the pedestal to prevent arcing. The pumping channel is lined with various elements, some of which are electrically floating and which are designed so that conductive material deposited on these elements do not deleteriously affect a plasma generated for processing the wafer.