A plasma processing apparatus has a chamber with an open top and a cover plate extending across the open top of the chamber. The cover plate has an opening therethrough. An annular shield of an electrical insulating material is secured to the cover plate around the opening and extends partially across the opening. An aluminum showerhead is within the shield and has holes therethrough through which a gas can pass into the chamber. The showerhead is connected to a source of RF voltage to provide a flow of RF power between the showerhead and an electrode within the chamber. The shield has a plurality of openings therethrough which allows the RF power to flow through the openings from the showerhead to the electrode in the event that the showerhead becomes coated with particles of an insulating material.