Experimental Investigation of Nonlinear Standing Waves in DC/VHF Hybrid Capacitive Discharges

It is recognized that in very-high-frequency capacitively coupled plasma (VHF CCP) reactors, the standing wave effects can be enhanced by the presence of nonlinear higher harmonics self-excited by the plasma series resonance (PSR). In this work, we investigated the influence of the dc bias on the excitation of the nonlinear standing waves in a parallel-plate capacitively coupled dc/VHF (60 MHz) hybrid discharge sustained in argon, with both the dc source and the VHF source applied to the top electrode. A hairpin probe was employed to determine the axial/radial distribution of the electron density, while a high-frequency B-dot probe was used to measure the radial distributions of the harmonic magnetic field and the harmonic current density. With no dc bias applied, the nonlinear standing wave excitation was observed to be predominant; the high-order harmonic current density exhibited a main peak at the electrode center and one or more minor peaks between the electrode center and the edge, leading to a center-high electron density profile. With the addition of the dc source, the harmonic excitations and the corresponding nonlinear standing wave effect tend to be suppressed with the increase of dc bias, resulting in improved plasma uniformities. Underlying mechanisms of the suppression effect of the dc bias on the nonlinear standing wave excitation were analyzed.