A chamber includes a lower electrode and an upper electrode. The lower electrode is defined to transmit a radiofrequency current through the chamber and to support a semiconductor wafer in exposure to a plasma within the chamber. The upper electrode is disposed above and in a spaced apart relationship with the lower electrode. The upper electrode is electrically isolated from the chamber and is defined by a central section and one or more annular sections disposed concentrically outside the central section. Adjacent sections of the upper electrode are electrically separated from each other by a dielectric material. Multiple voltage sources are respectively connected to the upper electrode sections. Each voltage source is defined to control an electric potential of the upper electrode section to which it is connected, relative to the chamber. The electric potential of each upper electrode section influences an electric potential of the plasma within the chamber.