Electron power absorption in capacitively coupled neon–oxygen plasmas: a comparison of experimental and computational results

Abstract

Phase resolved optical emission spectroscopy (PROES) measurements combined with 1d3v particle-in-cell/Monte Carlo collisions (PIC/MCC) simulations are used to study the electron power absorption and excitation/ionization dynamics in capacitively coupled plasmas (CCPs) in mixtures of neon and oxygen gases. The study is performed for a geometrically symmetric CCP reactor with a gap length of 2.5 cm at a driving frequency of 10 MHz and a peak-to-peak voltage of 350 V. The pressure of the gas mixture is varied between 15 Pa and 500 Pa, while the neon/oxygen concentration is tuned between 10% and 90%. For all discharge conditions, the spatio-temporal distributions of the electron-impact excitation rate from the Ne ground state into the Ne 2p53p0 state measured by PROES and obtained from PIC/MCC simulations show good qualitative agreement. Based on the emission/excitation patterns, multiple operation regimes are identified. Localized bright emission features at the bulk boundaries, caused by local maxima in the electronegativity are found at high pressures and high O2 concentrations. The relative contributions of the ambipolar and the Ohmic electron power absorption are found to vary strongly with the discharge parameters: the Ohmic power absorption is enhanced by both the high collisionality at high pressures and the high electronegativity at low pressures. In the wide parameter regime covered in this study, the PROES measurements are found to accurately represent the ionization dynamics, i.e. the discharge operation mode. This work represents also a successful experimental validation of the discharge model developed for neon–oxygen CCPs.

Metastable argon atom kinetics in a low-pressure capacitively coupled radio frequency discharge

“…The data for x < 0.25 cm and for x > 2.75 cm are cut since the signal-tonoise ratio and the quality of the Gaussian fit are significantly reduced in these regions due to the very low absorbance signal. We find that, within the accuracy of the measurements, the temperature can be assumed to be constant along the gap for fixed discharge conditions, which is consistent with the findings of Derzsi et al [89], where the spatial distribution of the temperature profile was computed based on a heat conduction model. Figure 4 shows the measured gas temperature as a function of the gas pressure.…”

Frequency-dependent electron power absorption mode transitions in capacitively coupled argon-oxygen plasmas

“…A geometrically symmetric plasma reactor (our 'Budapest v.3' cell) is used for the measurements [54]. In the discharge cell, the plane parallel electrodes, made of stainless steel, with identical diameters of 14 cm, are situated within a quartz cylinder.…”

“…The optical emission from the Ar 2p 1 excited level with a wavelength of 750.4 nm is measured (by applying an interference filter with a central wavelength of 750 nm and a spectral full width at half maximum of ~10 nm), from which the electron impact excitation rate from the ground state into the observed level is calculated as introduced in [52]. A more detailed description of the experimental setup and diagnostics is given in [54]. The reason for selecting a gas mixture of 70% Ar-30% O 2 at a pressure of 120 Pa for this study was motivated by the presence of the large variety of spatiotemporal emission patterns which develop under these conditions at different frequencies.…”

“…PROES images are frequently used to formulate statements on the operation mode of low-pressure CCP discharges despite the fact that PROES provides information on the spatio-temporal distribution of the electron-impact excitation dynamics from the ground state into the selected excited atomic level in the discharge, while the discharge operation mode is determined by the spatiotemporal distribution of the ionization dynamics. Recently, the applicability of PROES to probe the discharge operation mode was tested in low-pressure CCPs in pure neon [53] and in neonoxygen mixtures [54], and limiations were revealed especially around the transition regime between the α-mode and the γmode.…”

“…The computational investigation of the spatio-temporal dynamics of the electron power absorption is based on the Boltzmann term analysis, a computational diagnostic tool, which is capable of providing a complete description of the electron power absorption in CCPs [55,56]. This method, first proposed in [57] and later revisited in [58], has recently been applied to CCPs under various conditions: in inert gases [8,55,56,[59][60][61][62][63], in electronegative gases [36,54,64,65], in CCPs at atmospheric pressure [66], and in magnetized CCPs as well [46,[67][68][69].…”