Plasma profile control using external circuit in capacitively coupled plasma reactors

Multi-frequency capacitively coupled plasma (CCP) sources in the range of 2-180 MHz are widely used for materials processing in the semiconductor industry. Very high frequency (VHF) sources bring various benefits including low plasma potential, high electron density, and controllable dissociation. Sources in the low frequency (LF) and high frequency (HF) regimes provide high ion energy with different ion energy distributions. Multi-frequency CCP reactors have been developed to combine the benefits of both frequency regimes. However, plasma structure can be difficult to control over a wide range of operating conditions (a few mTorr to several hundred mTorr, hundred to several thousand Watts, electropositive and electronegative chemistries). This paper discusses how the plasma structure can be controlled using external circuit impedance that modifies the plasma boundary conditions. A 2-dimensional plasma model with external circuit has been developed and used for this study. The plasma model considers conservation of charge species densities, momentum and energy along with the full set of Maxwell equations. The external circuit is a network of passive circuit elements connected to different electrodes. Plasma simulations have been performed for various external circuit configurations using combinations of capacitors and inductors. We found that the plasma structure and profile can be controlled by changing the current return path impedance. In general, the plasma is pulled towards the electrode with an inductive impedance and pushed away from the electrode with a capacitive impedance. When only capacitors are used, the plasma moves towards the electrode with lower impedance. As a result, the ion flux profile to the electrode can be controlled using the external circuit impedance. Electron density measurements using the resonance cavity method are used to verify that the electron density profile can indeed be modified using the external circuit impedance.