US Patent 6841943 Plasma processor with electrode simultaneously responsive to plural frequencies

A plasma in a vacuum chamber where a workpiece is processed is bounded by a plasma confinement volume including a region between a first electrode simultaneously responsive to power at first and second RF frequencies and a DC grounded second electrode. A DC grounded extension is substantially aligned with the first electrode. A substantial percentage of power at the first frequency is coupled to a path including the first and second electrodes but not the extension while a substantial percentage of power at the second frequency is coupled to a path including the first electrodes and extension, but not the second electrode. Changing the relative powers at the first and second frequencies, as applied to the first electrode, controls DC bias voltage of the first electrode.