An apparatus for controlling the
voltage applied to a shield interposed between an
induction coil powered by a power supply via a matching network, and the
plasma it generates, comprises a shield, a first feedback circuit, and a second feedback circuit. The power supply powers the shield. The first feedback circuit is connected to the
induction coil for controlling the power supply. The second feedback circuit is connected to the shield for controlling the
voltage of the shield. Both first and second
feedback circuits operate at different frequency ranges. The first feedback circuit further comprises a first controller and a first sensor. The first sensor sends a first
signal representing the power supplied to the inductive coil to the first controller. The first controller adjusts the power supply such that the power supplied to the
inductor coil is controlled by a first
set point. The second feedback circuit further comprises a second sensor, a second controller, and a variable
impedance network. The shield is powered via a variable
impedance network. The second sensor sends a second
signal representative of the
voltage of the shield to the second controller. The second controller adjusts the variable
impedance network such that the voltage of the shield is controlled by a second
set point.