Mask etch plasma reactor with variable process gas distribution

A plasma reactor for processing a workpiece such as a mask or wafer includes a vacuum chamber having a cylindrical side wall, a ceiling overlying the side wall and a ring supported on a top edge of the side wall and supporting the ceiling, the ring comprising an external surface and an interior surface. An RF plasma source power applicator and an RF source power generator coupled to the applicator furnish plasma source power. Plural passages extend in a radial direction through the ring from the external surface to the interior surface and are spaced apart along a circumference of the ring. A process gas supply furnishes process gas. An external gas flow conduit apparatus outside of the chamber extends around a circumference of the chamber and is coupled to the process gas supply. Plural external gas flow valves outside of the chamber are coupled to the external conduit at respective locations spaced apart along the conduit, each of the valves having: (a) a controlled gas output port coupled to a respective one of the plural passages at the external surface of the ring and (b) a valve control input. A gas valve configuration controller controls the valve control input of each of the valves.