In the current work, the radial plasma density has been measured by utilizing a floating double probe in single and dual frequency capacitively coupled argon discharges operated in a cylindrical reactor, aiming at a better understanding of electromagnetic effects and exploring a method of improving the radial uniformity. The experimental results indicate that for single-frequency plasma sustained at low pressure, the plasma density radial profile exhibits a parabolic distribution at 90 MHz, whereas at 180 MHz, the profile evolves into a bimodal distribution, and both cases indicate poor uniformities. With increasing the pressure, the plasma radial uniformity becomes better for both driving frequency cases. By contrast, when discharges are excited by two frequencies (i.e., 90 + 180 MHz), the plasma radial profile is simultaneously influenced by both sources. It is found that by adjusting the low-frequency to high-frequency voltage amplitude ratio β, the radial profile of plasma density could be controlled and optimized for a wide pressure range. To gain a better plasma uniformity, it is necessary to consider the balance between the standing wave effect, which leads to a maximum plasma density at the reactor center, and the edge field effect, which is responsible for a maximum density near the radial electrode edge. This balance can be controlled either by selecting a proper gas pressure or by adjusting the ratio β.

The dependence of the electron density and the emission intensity on external parameters during the transitions of the electron power absorption mode is experimentally studied in asymmetric electropositive (neon) and electronegative (CF4) capacitively coupled radio-frequency plasmas. The spatio-temporal distribution of the emission intensity is measured with phase resolved optical emission spectroscopy and the electron density at the discharge center is measured by utilizing a floating hairpin probe. In neon discharge, the emission intensity increases almost linearly with the rf voltage at all driving frequencies covered here, while the variation of the electron density with the rf voltage behaves differently at different driving frequencies. In particular, the electron density increases linearly with the rf voltage at high driving frequencies, while at low driving frequencies the electron density increases slowly at the low-voltage side and, however, grows rapidly, when the rf voltage is higher than a certain value, indicating a transition from α to γ mode. The rf voltage, at which the mode transition occurs, increases with the decrease of the driving frequency/the working pressure. By contrast, in CF4 discharge, three different electron power absorption modes can be observed and the electron density and emission intensity do not exhibit a simple dependence on the rf voltage. In particular, the electron density exhibits a minimum at a certain rf voltage when the electron power absorption mode is switching from drift-ambipolar to the α/γ mode. A minimum can also be found in the emission intensity at a higher rf voltage when a discharge is switching into the γ mode.

An enhanced electron heating mechanism based on a resonance between the cyclotron motion of electrons and radio frequency electric field in the plasma bulk is reported in weakly magnetized capacitively coupled Argon plasmas at low pressure. When the electron cyclotron frequency coincides with the applied power source frequency, the bulk electrons can continuously acquire energy from the background electric field within certain rf periods during the cyclotron motion, inducing overall distinct increase of excitation rate and electron temperature in the plasma bulk. This enhanced electron heating effect has been examined by a combination of kinetic particle simulations, experimental measurements, and an analytical model, and the dynamics of electrons are revealed at resonant conditions.

In electronegative radio frequency plasmas, striations can appear if the bulk plasma is dominated by positive and negative ions, that can react to the driving frequency. Here, we investigate such self-organized structures in dual-frequency (DF, 2/10 MHz) capacitively coupled CF4 plasmas by PROES and PIC/MCC simulations. This choice of the frequencies is made to ensure that the ions can react to both the lower (2 MHz, "low frequency", LF) and the higher (10 MHz, "high frequency", HF) components of the excitation waveform. A strong interplay of the two excitation components is revealed. As the striations appear in the plasma bulk, their number depends on the length of this region. By increasing the LF voltage, ΦLF, the sheath widths at both electrodes increase, the bulk is compressed, and the number of striations decreases. The maximum ion density decreases slightly as a function of ΦLF, too, due to the compressed plasma bulk, while the minimum of the ion density remains almost constant. The spatio-temporal distribution of the excitation and ionization rates are modulated both by the LF and HF with maxima that occur at the first HF period that follows the complete sheath collapse at a given electrode. These maxima are caused by a high local ambipolar electric field. At a given phase within a HF period, the current density is different at different phases within the LF period because of frequency coupling. The LF components of the F- ion velocity and of the electric field are much lower than the respective HF components due to the lower LF component of the displacement current in the sheaths. The LF component of the total current is dominated by the ion current at low values, but by the electron current at high values of ΦLF.