Plasma-Profile Control Using External Circuit in a Capacitively Coupled Plasma Reactor

Very high frequency (VHF) capacitively coupled plasma (CCP) sources offer several benefits, including low plasma potential, high electron density, and controllable dissociation. However, standing electromagnetic waves can make the spatial structure of VHF plasmas a sensitive function of operating conditions and reactor geometry. This paper discusses how the profile of VHF CCPs can be controlled using an external circuit that modifies the electrical boundary conditions. A 2-D plasma model with external circuit has been used for this study, where networks of passive circuit elements can be connected to different electrodes in the reactor. Plasma simulations have been performed for several combinations of capacitors and inductors. It is found that the external circuit can be used to change the radio-frequency current return path, thereby modifying the plasma profile. In general, the plasma is pulled toward the electrode with an inductive impedance and pushed away from the electrode with a capacitive impedance. These changes in plasma profile are related to the relative voltage and their phase on different electrodes.