Electron heating and control of ion properties in capacitive discharges driven by customized voltage waveforms

Derzsi, Aranka and Korolov, Ihor and Schüngel, Edmund and Donkó, Zoltán and Schulze, Julian (2013) Electron heating and control of ion properties in capacitive discharges driven by customized voltage waveforms. Plasma Sources Science and Technology, 22. pp. 1-13. ISSN 0963-0252 ; ESSN: 1361-6595

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Abstract

We investigate the electron heating dynamics in capacitively coupled radio frequency plasmas driven by customized voltage waveforms and study the effects of modifying this waveform and the secondary electron emission coefficient of the electrodes on the spatio-temporal ionization dynamics by particle-in-cell simulations. We demonstrate that changes in the electron heating dynamics induced by voltage waveform tailoring strongly affect the dc self-bias, the ion flux, i, and the mean ion energy, ⟨Ei⟩, at the electrodes. The driving voltage waveform is customized by adding N consecutive harmonics (N 4) of 13.56 MHz with specific harmonics’ amplitudes and phases. The total voltage amplitude is kept constant, while modifying the number of harmonics and their phases. In an argon plasma, we find a dc self-bias, η, to be generated via the electrical asymmetry effect for N 2. η can be controlled by adjusting the harmonics’ phases and is enhanced by adding more consecutive harmonics. At a low pressure of 3 Pa, the discharge is operated in the α-mode and ⟨Ei⟩ can be controlled by adjusting the phases at constant i. The ion flux can be increased by adding more harmonics due to the enhanced electron-sheath heating. ⟨Ei⟩ does not remain constant as a function of N at both electrodes due to a change in η. These findings verify previous results of Lafleur et al. At a high pressure of 100 Pa and using a high secondary electron emission coefficient of γ = 0.4, the discharge is operated in the γ -mode and mode transitions are induced by changing the driving voltage waveform. Due to these mode transitions and the specific ionization dynamics in the γ -mode, i is no longer constant as a function of the harmonics’ phases and decreases with increasing N.

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