The electrical asymmetry effect (EAE) enables separate control of the ion flux and the mean ion energy in capacitively coupled plasmas (CCP). While a variety of plasma processing applications benefit from this, large-area, very-high-frequency CCPs still suffer from lateral nonuniformities caused by electromagnetic standing wave effects (SWE). Many of such plasma sources are geometrically asymmetric and are operated at low pressure so that high frequency nonlinear plasma series resonance (PSR) oscillations of the RF current are self-excited. These PSR oscillations lead to the presence of short wavelength electromagnetic waves and a more pronounced SWE. In this work, we investigate the influence of the EAE on the nonlinear standing wave excitation in a geometrically asymmetric, low pressure capacitively coupled argon plasma driven by two consecutive harmonics (30 MHz and 60 MHz) with an adjustable phase shift, θ. We use a hairpin probe to determine the radial distribution of the electron density in combination with a high-frequency B-dot probe to measure the radial distribution of the harmonic magnetic field, which in turn is used to calculate the harmonic current density based on Ampere’s law. Our experimental results show that the asymmetry of the discharge can be reduced electrically via the EAE. In this way the self-excitation of high frequency PSR oscillations can be attenuated. By tuning θ, it is, therefore, possible to switch on and off the nonlinear standing wave excitation caused by the PSR and, accordingly, the plasma uniformity can be optimized.

Temporal evolution of electrical and plasma parameters over 300 mm-diameter electrodes during the pre-ignition, ignition, and post-ignition phases of a pulsed capacitively coupled radio-frequency (RF) argon discharge is investigated by multi-fold experimental diagnostics. The electron density, n e, and the optical emission intensity (OEI) at different radial positions are measured time-resolved by using a hairpin probe and an optical probe, respectively. A B-dot probe is employed to determine the waveforms of the azimuthal magnetic field at different radii, from which the waveforms of the axial current density at corresponding radial positions are derived based on Ampere’s law. Then, the time evolution of the power density at various radii can be calculated, provided that the voltage drop across the electrodes is independent of radius. Meanwhile, the time-dependent total power deposited into the reactor is calculated with the voltage and the current waveforms measured by a voltage and a current probe at the power feeding point. It was found that during pre-ignition phase, the OEI and n e cannot be measurable due to extremely low power deposition when the system exhibits pure capacitive impedance. During the ignition phase, the OEI, the power density, and the current density exhibit the most significant increase at the electrode center, while time evolution of n e seems to exhibit a relatively weak radial dependence. In particular, at small radii, i.e. r ≤ 8 cm, the OEI was observed to change with time in the same manner as the power density during the ignition phase, because the RF power is absorbed primarily by electrons, which dissipate their energy via inelastic collisions. The more drastic ignition at the center is possibly associated with a center-high profile of Ar metastable density at the beginning of each pulse. Shortly, the profile of n e becomes edge-high during the post-ignition phase and remains thereafter until the end of the pulse-on periods. Methodologically, the synergistic diagnostics lay the foundation for extensive studies on spatiotemporal evolution of plasma ignition process under broader conditions, e.g. low gas pressure and very high frequency, widely used by practical etching process.

This letter reports the detailed observations and investigations of striations in the positive column region of helium glow discharges with a pin-to-plate copper electrode geometry in the pressure range 9–101 kPa. The striations are characterized by several plasma layers with alternate brightness and darkness. In this discharge, features such as negative glow, Faraday dark space, and striated positive column regions can be clearly observed. The evolution of a striation structure in the positive column region was found to be sensitive to gas pressure and electrode spacing. The striated discharge propagates in the form of an ionization wave with a velocity of 20.78 m/s and a frequency of 5.2 kHz.

We performed an experimental investigation on the electromagnetic effect and the plasma radial uniformity in a larger-area, cylindrical capacitively coupled plasma reactor. By utilizing a floating hairpin probe, dependences of the plasma radial density on the driving frequency and the radio-frequency power over a wide pressure range of 5–40 Pa were presented. At a relatively low frequency (LF, e.g. 27 MHz), an evident peak generally appears near the electrode edge for all pressures investigated here due to the edge field effect, while at a very high frequency (VHF, e.g. 60 or 100 MHz), the plasma density shows a sharp peak at the discharge center at lower pressures, indicating a strong standing wave effect. As the RF power increases, the center-peak structure of plasma density becomes more evident. With increasing the pressure, the standing wave effect is gradually overwhelmed by the ‘stop band’ effect, resulting in a transition in the plasma density profile from a central peak to an edge peak. To improve the plasma radial uniformity, a LF source is introduced into the VHF plasma by balancing the standing wave effect with the edge effect. A much better plasma uniformity can be obtained if one chooses appropriate LF powers, pressures and other corresponding discharge parameters.

In this work, a fluid/Monte Carlo Collision (fluid/MCC) hybrid model is newly developed based on the framework of Multi-Physics Analysis of Plasma Sources (MAPS). This hybrid model could enjoy great accuracy in predicting the nonequilibrium phenomena in capacitively coupled plasmas (CCPs) and meanwhile avoid the limitation caused by the computational cost. Benchmarking against the well-established particle-in-cell/Monte Carlo collision (PIC/MCC) method and comparison with experimental data have been presented both in electropositive N2 discharges and electronegative O2 discharges. The results indicate that in N2 discharges, the ion density evolves from a uniform distribution to an edge-high profile as power increases. Besides, the electron energy distribution function (EEDF) at the bulk center exhibits a “hole” at about 3 eV, and the “hole” becomes less obvious at the radial edge, because more low energy electrons are generated there. In O2 discharges, the EEDF exhibits a Druyvesteyn-like distribution in the bulk region, and it evolves to a Maxwellian distribution in the sheath, indicating the dominant influence of the electric field heating there. The results obtained by the hybrid model agree well with those calculated by the PIC/MCC method, as well as those measured by double probe, except for slight discrepancy in absolute values. The qualitative agreement achieved in this work validates the potential of this hybrid model as an effective tool in the deep understanding of the plasma properties, as well as in the improvement of the plasma processing.