Off-center ground return for RF-powered showerhead

A method for coupling RF power to a plasma chamber, comprising the steps of:

• providing an electrically grounded chamber wall that surrounds a plasma chamber interior, wherein the electrically grounded chamber wall includes a chamber cover and a chamber side wall;
• providing a workpiece passageway in the chamber side wall;
• providing within the plasma chamber a showerhead that is electrically conductive and includes a plurality of gas passageways;
• and
• providing an RF impedance matching network comprising:
• (i) an input adapted to receive RF power, (ii) an output electrically connected to the showerhead, and (iii) an electrical ground that is electrically connected to a plurality of distinct, non-contiguous connection areas on the chamber cover;
• wherein said connection areas on the chamber cover collectively are characterized by a geometric centroid having a position that is offset away from the center of the chamber cover toward the workpiece passageway;
• and
• wherein said connection areas on the chamber cover collectively constitute the entire area on the chamber cover that is electrically connected to the electrical ground of the RF impedance matching circuit.