A process is provided for removing polymer from a backside of a workpiece. The process includes supporting the workpiece on the backside in a vacuum chamber while leaving a peripheral annular portion of the backside exposed. The process further includes confining gas flow at the edge of the workpiece within a gap at the edge of the workpiece on the order of about 1% of the diameter of the chamber, the gap defining a boundary between an upper process zone containing the front side and a lower process zone containing the backside. A first plasma is generated in a lower external chamber from a polymer etch precursor gas, and an etchant by-product is introduced from the first plasma into the lower process zone. A second plasma is generated in an upper external plasma chamber from a precursor gas of a scavenger of the etchant by-product, and scavenger species are introduced from the second plasma into the upper process zone.