A machine for processing the front side of a flat media workpiece, such as a silicon wafer, seals the backside of the wafer from processing chemicals. A rotor has an inside ring and an outside ring protruding from the rotor face. The inside ring and outside ring are separated by an annular groove in the rotor. A O-ring is positioned between the inside ring and the outside ring. A membrane extends from the inside ring, over the annular groove and the O-ring, to the outside ring. The membrane and face of the rotor form a sealable wafer back face chamber between them. Vacuum is applied to the back face chamber to hold the wafer against the membrane. The back surface of the wafer is sealed from processing chemicals, which are allowed to contact only the front surface and edges of the wafer.