Plasma process uniformity across a wafer by apportioning ground return path impedances among plural vhf sources

Abstract

In a plasma reactor chamber a ceiling electrode and a workpiece support electrode, respective RF power sources of respective VHF frequencies fand fare coupled to either respective ones of the electrodes or to a common one of the electrodes, where fis sufficiently high to produce a center-high non-uniform plasma ion distribution and fis sufficiently low to produce a center-low non-uniform plasma ion distribution. Respective center ground return paths are provided for RF current passing directly between the ceiling electrode and the workpiece support electrode for the frequencies fand f, and an edge ground return path is provided for each of the frequencies fand f. The impedance of at least one of the ground return paths is adjusted so as to control the uniformity of the plasma ion density distribution.