Ion flux uniformity in large area capacitively coupled dual-frequency discharges

The lateral plasma homogeneity and uniformity of plasma surface interaction is of crucial importance for all large area processing applications of capacitively coupled radio frequency (CCRF) discharges. While the operation of the discharge at higher frequencies typically leads to an increase in the overall deposition rate, the ion properties are inhomogeneous and dominated by electromagnetic effects, e.g. standing wave effects and/or the skin effect. In this experimental study, we investigate the ion flux distribution at the grounded electrode of a CCRF discharge operated in hydrogen at various single- and dual-frequency combinations. The results show that the ion flux remains approximately uniform between the two electrodes at low frequencies. A standing wave pattern occurs in a 81.36 MHz single-frequency discharge, strongly reducing the ion flux uniformity. However, applying a dual-frequency voltage waveform consisting of 40.68 MHz + 81.36 MHz, the lateral distribution of the ion flux can be controlled via the phase angle between the two applied harmonics. According to the Electrical Asymmetry Effect (EAE), a phase angle dependent DC self-bias develops in the geometrically symmetric discharge. Tuning the phase angle to strongly positive DC self-bias values allows for the compensation of ion flux inhomogeneities due to the standing wave effect. Thus, a high and laterally uniform ion flux can be generated in electrically asymmetric high frequency plasmas. We demonstrate, how the ion flux uniformity can be improved and controlled in this type of dual-frequency discharges and provide a theoretical hypothesis to explain this effect.