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Phase Resolved Optical Emission Spectroscopy (PROES) measurements combined with 1d3v Particle-in-Cell/Monte Carlo Collisions simulations are performed to investigate the excitation dynamics in low-pressure capacitively coupled plasmas (CCPs) in argon-oxygen mixtures. The system used for this study is a geometrically symmetric CCP reactor operated in a 70% Ar-30% O2 mixture at 120 Pa, applying a peak-to-peak voltage of 350 V, with a wide range of driving RF frequencies (2 MHz ⩽ f ⩽ 15 MHz). The measured and calculated spatio-temporal distributions of the electron impact excitation rates from the Ar ground state to the Ar 2 p 1 level show good qualitative agreement. The distributions show significant frequency dependence, which is generally considered to be predictive of transitions in the dominant discharge operating mode. Three frequency ranges can be distinguished, showing distinctly different excitation characteristics: (i) in the low frequency range ( f ⩽ 3 MHz), excitation is strong at the sheaths and weak in the bulk region; (ii) at intermediate frequencies (3.5 MHz ⩽ f ⩽ 5 MHz), the excitation rate in the bulk region is enhanced and shows striation formation; (iii) above 6 MHz, excitation in the bulk gradually decreases with increasing frequency. Boltzmann term analysis was performed to quantify the frequency-dependent contributions of the Ohmic and ambipolar terms to the electron power absorption. [ABSTRACT FROM AUTHOR]

Plasma simulations are powerful tools for understanding fundamental plasma science phenomena and for process optimisation in applications. To ensure their quantitative accuracy, they must be validated against experiments. In this work, such an experimental validation is performed for a one dimensional in space and three dimensional in velocity space particle-in-cell simulation complemented with the Monte Carlo treatment of collision processes of a capacitively coupled radio frequency plasma driven at 13.56 MHz and operated in neon gas. In a geometrically symmetric reactor the electron density in the discharge centre and the spatio-temporal distribution of the electron impact excitation rate from the ground into the Ne 2p1 level are measured by a microwave cutoff probe and phase resolved optical emission spectroscopy, respectively. The measurements are conducted for electrode gaps between 50 mm and 90 mm, neutral gas pressures between 20 mTorr and 50 mTorr, and peak-to-peak values of the driving voltage waveform between 250 V and 650 V. Simulations are performed under identical discharge conditions. In the simulations, various combinations of surface coefficients characterising the interactions of electrons and heavy particles with the anodised aluminium electrode surfaces are adopted. We find, that the simulations using a constant effective heavy particle induced secondary electron (SE) emission coefficient of 0.3 and a realistic electron–surface interaction model (which considers energy-dependent and material specific elastic and inelastic electron reflection, as well as the emission of true SEs from the surface) yield results which are in good quantitative agreement with the experimental data. [ABSTRACT FROM AUTHOR]

Particle-in-cell/Monte Carlo collisions (PIC/MCC) simulations are performed to investigate the sputtering and the secondary electron emission (SEE) in geometrically symmetric capacitively coupled Ar discharges with Cu electrodes driven by tailored voltage waveforms (TVWs). The driving voltage waveform is composed of multiple consecutive harmonics (1 ⩽ N ⩽ 4) of the fundamental frequency (13.56 MHz) and is tailored by adjusting the identical phases (θ) of the even harmonics. The simulations are based on a discharge model in which realistic approaches are implemented for the description of the SEE induced by electrons and heavy-particles at the electrodes, as well as for the sputtering of the electrodes by heavy-particles. In case of applying a single frequency, the voltage amplitude is varied (250 V ⩽ ϕ1 ⩽ 2500 V), while in case of applying multi-frequency TVWs, N and θ are varied at a fixed total voltage amplitude (ϕtot = 1000 V) at a low pressure of 0.5 Pa. By applying more than one harmonic to drive the discharge, the mean energy of Ar+ ions and fast Ar atoms at the electrodes can be controlled by changing the phase angles. Due to the dependence of the sputtering yield on the heavy-particle energies, the flux of sputtered atoms can as well be controlled by the phase angles at both electrodes. The domain over which the sputtered atom flux can be varied is enlarged by adding more harmonics to the driving voltage waveform. For all conditions investigated, electron induced SEs (δ-electrons) induce strong ionization in the α-mode and dominate the ionization dynamics at high voltage amplitudes. [ABSTRACT FROM AUTHOR]

The spatio-temporal ionization and excitation dynamics in low-pressure radiofrequency (RF) discharges operated in neon are studied and a detailed comparison of experimental and kinetic simulation results is provided for a wide parameter regime. Phase resolved optical emission spectroscopy (PROES) measurements and 1d3v particle-in-cell/Monte Carlo collisions (PIC/MCC) simulations are performed in a geometrically symmetric capacitively coupled plasma (CCP) reactor at driving frequencies ranging from 3.39 MHz to 13.56 MHz, pressures between 60 Pa and 500 Pa, at a peak-to-peak voltage of 330 V. We examine the applicability of PROES (which provides information about the spatio-temporal distribution of the electron-impact excitation dynamics from the ground state into the Ne 2p1 state) to probe the discharge operation mode in neon (which is determined by the spatio-temporal distribution of the ionization dynamics). We find that the spatio-temporal excitation rates measured by PROES are in a good agreement with the excitation rates obtained from the PIC/MCC simulations, for all the discharge conditions studied here. However, the ionization dynamics is found to be significantly different from the excitation dynamics under most of the discharge conditions studied here, especially at higher values of the driving frequency and lower values of the pressure, when energetic heavy particle induced secondary electrons (γ-electrons) are more likely to ionize than to excite. PROES does not probe the discharge operation mode under these conditions. At a fixed frequency and peak-to-peak voltage, the spatio-temporal distribution of the ionization rate obtained from PIC/MCC simulations shows a transition of the discharge operation mode from the α-mode to the γ-mode by increasing the pressure. However, PROES fails to show this transition. While in the spatio-temporal distribution of the excitation rate obtained from the PROES measurements and the PIC/MCC simulations the α-peak (the intensity maximum at the bulk side of the expanding sheath edge) is dominant and a γ-peak (a maximum near the edge of the fully expanded sheath) becomes visible only at high values of the pressure or at the lowest frequency of 3.39 MHz, a γ-peak is visible in the ionization rate for all operation conditions, and it dominates the ionization in the vast majority of the cases investigated. [ABSTRACT FROM AUTHOR]