How to address the issue of uniformity in large area capacitively coupled plasmas? A modeling investigation

Recently, the electrical asymmetry effect (EAE) has been proven effective not only in separate control of the ion energy and ion flux, but also in modulation of the plasma radial uniformity. Since no major change in reactor constructions is required, this method is promising and of significant importance in the plasma industry. In this work, a two-dimensional electromagnetic fluid model is employed to investigate the influence of the EAE on the plasma density distribution under various voltages (i.e., the peak-to-peak voltage V0 equals to 33 V, 56 V and 100 V) and pressures (i.e., 50 mTorr, 200 mTorr and 750 mTorr), in H2 capacitively coupled plasma (CCP) discharges sustained by multiple consecutive harmonics. The results indicate that at V0=33 V or 750 mTorr, the plasma density is characterized by a broad maximum at the radial center, and the effectiveness of the EAE on the modulation of the plasma radial uniformity is limited. When V0 becomes higher, the plasma distribution varies obviously by adjusting the phase shift of the fundamental frequency θ1. For instance, when θ1 increases from 0 to 2π at V0=56 V, the plasma density shifts from uniform over center-high to uniform. Moreover, at V0=100 V, the best uniformity is observed at θ1=π. When the pressure decreases to 50 mTorr, although the uniformity varies significantly with θ1, the plasma density stays low, which may be not welcomed in the plasma processing. Different influences of the EAE on the plasma radial uniformity can be understood by examining the electron heating induced by the sheath expansion and the electric field reversal under various discharge conditions. The results obtained in this work could help us to have a better understanding of the EAE on the plasma radial uniformity, which would be useful to improve the plasma processing in large area CCP reactors.