Patent ID: 7884025

Claim:
A method of processing a workpiece in a plasma reactor chamber having electrodes including at least a ceiling electrode and a workpiece support electrode, said method comprising: coupling respective RF power sources of respective VHF frequencies f 1 and f 2 through respective RF impedance matches to one of: (a) respective ones of the electrodes, (b) a common one of the electrodes, where f 1 is sufficiently high to produce a center-high non-uniform plasma ion distribution and f 2 is sufficiently low to produce a center-low non-uniform plasma ion distribution; providing respective center ground return paths for RF current passing directly between the ceiling electrode and the workpiece support electrode for the frequencies f 1 and f 2 ; providing an edge ground return path for each of the frequencies f 1 and f 2 ; providing in at least one of said center and edge ground return paths for each of said frequencies f 1 and f 2 a tunable reactance element separate from said respective RF impedance matches; and adjusting the impedance of at least one of the ground return paths by adjusting the corresponding one of said tunable reactance elements until a desired change in plasma ion distribution is achieved so as to control the uniformity of the plasma ion density distribution.