A substrate processing apparatus for processing a substrate having a peripheral edge, an upper surface for processing and a lower surface lying on a support. The apparatus includes a processing chamber which houses the substrate support, in the form of a heater pedestal including a substrate receiving surface for receiving the lower surface of the substrate. A circumscribing shadow ring is located around the pedestal to cover peripheral edge portion of the substrate. The shadow ring also defines a cavity, between itself and the pedestal, at the peripheral edge of the substrate. In operation, the chamber receives processing gas at a first pressure and purge gas is introduced into the cavity, between the ring and the pedestal, at a second pressure which is greater than the first pressure. Fluid conduits are provided to enhance the flow of the purge gas away from the peripheral edge of the substrate.