Wafer edge deposition elimination

Abstract

This invention provides a method and apparatus for substantially eliminating deposition on the edge of a wafer supported on a pedestal in a processing chamber. Process gas flow onto the wafer surface is inhibited from reaching the wafer edge and backside, by means of a shadow ring placed over the wafer without touching it. Deposition on the edge and backside of the wafer are therefore substantially eliminated. The shadow ring defines a cavity which circumscribes the wafer edge, into which purge gas is flowed. This purge gas flows out from the cavity through the gap between the shadow ring and the upper surface of the wafer. Alignment pins are placed on the wafer supporting surface of the pedestal. These pins have sloping surfaces and are arranged to guide the wafer to a centered position on the pedestal when the wafer is placed on the pedestal. These pins also serve to align the shadow ring to the pedestal and thence to the wafer. The shadow ring has a plurality of keyed formations which mate to the pins, and as the shadow ring and pedestal are brought together, the pins serve to align the shadow ring. This precise rotational alignment and centering of the shadow ring results in substantial elimination of edge deposition. The keyed formations have elliptical cross-sections to provide for radial movement of the pins with respect to the keyed formations due to thermal expansion.