Ion energy distributions driven by the ultra-low frequency electrical asymmetry effect in biased inductively coupled plasmas

Inductively coupled plasmas (ICPs) are commonly applied in plasma-assisted material processing, where precise and convenient control of ion energy distribution (IED) is important for high selectivity in plasma etching. In this work, using a validated global model coupled with a fluid ion sheath model, we investigated the IEDs bombarding on the biased electrode driven by an electrically asymmetric voltage waveform composed of an ultra-low fundamental frequency and its second harmonic with a tuning phase shift θ in ICPs. For the base case with a fundamental bias frequency of f = 400 kHz, and an amplitude of Vrf=200 V in argon discharges at 1 Pa, it is found that for θ = 0, a four-peak IED is generated corresponding to the three extremes of the bias waveform. Increasing θ to 0.25π, the peak positions of the IED shift to lower energies and the shape of IED are nearly kept constant. For θ=0.5π, the lowest energy peak almost disappears, resulting in a combined low-energy distribution from the anodic portion of the waveform. Varying Vrf in the range of 100–300 V, the energy separation width can be customized accordingly. When we increase f to 13.56 MHz, the ions respond to the time-averaged sheath field, leading to a quasi-bimodal energy distribution at θ=0.25π and triple-peak distribution at θ=0 and 0.5π. The customization of the IEDs via the ultra-low frequency-driven electrical asymmetry effect of the bias waveform shows strong potential for the optimization of plasma etching.