The plasma density radial profiles in capacitive discharges driven over a wide frequency range (60-220 MHz) are measured by a floating double probe, and the results measured at 60 MHz are compared with those obtained from the electrostatic models, i.e. particle in cell/Monte Carlo collision (PIC/MCC) and the fluid models. It was found that at low pressure the plasma density peaks at the center of the reactor, while it peaks at the electrode edge at high pressure, indicating that the power deposition transitions from 'non-local' to 'local' with increasing pressure. The plasma radial profiles obtained from the PIC/MCC simulation and fluid model show a qualitative agreement with the experiment at low pressure and high pressure, respectively. This is primarily due to the fact that at low pressure the fluid model substantially under-predicts sheath heating, which, however, is the main electron heating mechanism at low pressure. So the total power into electrons and therefore the plasma density is also under-predicted. In contrast, the PIC/MCC model takes into account these electron collisionless heating effects at low pressure, and thus the plasma density is enhanced in the central region of electrodes. At high pressure, due to local power deposition, both the experiment and fluid simulation show that as rf power increases, a density peak at the electrode edge appears, indicating an enhancement in edge field. Compared with the electrostatic case, at a higher frequency, the plasma density profile is determined by electromagnetic (EM) effects, especially the standing wave effect. To be specific, we found that the standing wave effect exhibits multi-node structure within the electrode at 130 MHz or above, and the wavelength becomes smaller as the excitation frequency increases. At high excitation frequency and high pressure, the rf power is mainly deposited at the electrode periphery due to the fact that the EM waves are strongly damped when they propagate from the discharge edge to the center. In addition, our experimental results show that the standing wave wavelength increases with rf power.

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A two-dimensional self-consistent electrostatic fluid model and the experimental diagnostic method are employed to investigate the plasma radial uniformity in capacitively coupled nitrogen discharges driven at 60 MHz. The effects of the rf power and electrode gap on the spatial profiles of the N2+ ion density and the radial profiles of the ion flux to the lower electrode are demonstrated. It is found in the simulation that with the increase of rf power or the decrease of electrode gap, the electrostatic edge effect becomes remarkable, which gives rise to an increase in the positive ion density at the electrode edge and thus the radial uniformity of plasma becomes worse. Moreover, the radial profiles of the N2+ ion flux to the lower electrode show a similar behavior to that of the ion density. These results are further understood by the calculated axial and radial components of the power deposition, which exhibit pronounced peaks at the electrode edge at high rf power or small electrode gap. In order to validate the simulation results, the radial profiles of the N2+ ion density were measured by a floating double probe. A general qualitative agreement between the experimental and calculated results is achieved.

In this paper, a self-consistent fluid model is developed focusing on the plasma parameters in capacitively coupled 20% N2–80% Ar discharges. Measurements of ion density are performed with the help of a floating double probe, and the emission intensities from Ar(4p) and N2(B) transitions are detected by an optical emission spectroscopy to estimate their relative densities. The consistency between the numerical and experimental results confirms the reliability of the simulation. Then the plasma characteristics, specifically the reaction mechanisms of active species, are analyzed under various voltages. The increasing voltage leads to a monotonous increase in species density, whereas a less homogeneous radial distribution is observed at a higher voltage. Due to the high concentration of Ar gas, Ar+ becomes the main ion, followed by the N2+ ion. Besides the electron impact ionization of neutrals, the charge transfer processes of Ar+/N2 and N2+/Ar are found to have an impact on the ionic species. The results indicate that adopting the lower charge transfer reaction rate coefficients weakens the Ar+ ion density and yields a higher N2+ ion density. However, the effect on the species spatial distributions and other species densities is limited. As for the excited-state species, the electron impact excitation of background gases remains overwhelming in the formation of Ar(4p), N2(B), and N2(a′), whereas the N2(A) molecules are mainly formed by the decay of N2(B). In addition, the dissociation of N2 collided by excited-state Ar atoms dominates the N generation, which are mostly depleted to produce N+ ions.