An apparatus and method are described for processing workpieces in a treatment process. A multi-wafer chamber defines a chamber interior including at least two processing stations within the chamber interior such that the processing stations share the chamber interior. Each processing station includes a plasma source and a workpiece pedestal for exposing one of the workpieces to the treatment process using a respective plasma source. The chamber includes an arrangement of one or more electrically conductive surfaces that are asymmetrically disposed about the workpiece at each processing station in a way which produces a given level of uniformity of the treatment process on a major surface of each workpiece. A shield arrangement provides an enhanced uniformity of exposure of the workpiece to the respective one of the plasma sources that is greater than the given level of uniformity that would be provided in an absence of the shield arrangement.
