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. Gas flow is confined at the edge of the workpiece within a gap at the edge of the workpiece, the gap configured to be 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. The process further includes evacuating the lower process zone, generating a plasma in an external chamber from a polymer etch precursor gas, and introducing a by-product from the plasma into the lower process zone. The process further includes pumping a purge gas into the upper process zone to remove polymer etch species from the upper process zone.