A reactor is provided for removing polymer from a backside of a workpiece. The reactor includes a vacuum chamber having a ceiling, a floor and a cylindrical side wall. A workpiece support apparatus within the chamber is configured to support a workpiece thereon, so that the workpiece has its front side facing the ceiling. The support apparatus leaves at least an annular periphery of the backside of the workpiece exposed. A confinement member defines a narrow gap with the outer edge of the workpiece, the narrow gap being on the order of about 1% of workpiece diameter, the narrow gap corresponding to a boundary dividing the chamber between an upper process zone and a lower process zone. A vacuum pump is coupled to the lower process zone. A lower external plasma-generating chamber introduces a plasma by-product into the lower process zone and a supply of a polymer etch precursor gas coupled to the lower external plasma-generating chamber. An upper external plasma-generating chamber is coupled to introduce a plasma by-product into the upper process zone and a supply of a scavenger species precursor gas coupled to the upper external plasma-generating chamber.