Method and apparatus for reducing contamination of a substrate in a substrate processing system. The apparatus has a substrate support, a gas directing shield circumscribing the substrate support and a shadow ring disposed vertically above the substrate support and gas directing shield for retaining the substrate. The gas directing shield and substrate support define an annular channel that is provided with an edge purge gas. The edge purge gas imparts a force at the edge of a substrate resting on the substrate support the lifts it off the substrate supports and against the shadow ring. The shadow ring further has a plurality of conduits extending from its upper surface to its sidewall to provide a path for the edge purge gas to vent and to impede the flow of process gases under the backside and around the edge of the substrate. The method includes the steps of providing a substrate upon the substrate support, applying a first flow of gas to a first set of ports to lift the substrate off of the substrate support, centering the substrate upon the substrate support and applying a second flow of gas to a second set of ports to establish and maintain thermal control of the substrate.