PIC simulations of the separate control of ion flux and energy in CCRF discharges via the electrical asymmetry effect

Abstract

Recently a novel approach for achieving separate control of ion flux and energy in capacitively coupled radio frequency (CCRF) discharges based on the electrical asymmetry effect (EAE) was proposed (Heil et al 2008 J. Phys. D: Appl. Phys. 41 165202). If the applied, temporally symmetric voltage waveform contains an even harmonic of the fundamental frequency, the sheaths in front of the two electrodes are necessarily asymmetric. A dc self-bias develops and is a function of the phase angle between the driving voltages. By tuning the phase, precise and convenient control of the ion energy can be achieved while the ion flux stays constant. This effect works even in geometrically symmetric discharges and the role of the two electrodes can be reversed electrically. In this work the EAE is verified using a particle in cell simulation of a geometrically symmetric dual-frequency CCRF discharge operated at 13.56 and 27.12 MHz. The self-bias is a nearly linear function of the phase angle. It is shown explicitly that the ion flux stays constant within ±5%, while the self-bias reaches values of up to 80% of the applied voltage amplitude and the maximum ion energy is changed by a factor of 3 for a set of low pressure discharge conditions investigated. The EAE is investigated at different pressures and electrode gaps. As geometrically symmetric discharges can be made electrically asymmetric via the EAE, the plasma series resonance effect is observed for the first time in simulations of a geometrically symmetric discharge.

Slope and amplitude asymmetry effects on low frequency capacitively coupled carbon tetrafluoride plasmas

“…4 It is possible, however, to control the ion energy using well-separated multiple frequencies or through frequency coupling of harmonics, for example, via the electrical asymmetry effect (EAE), independently of other plasma parameters and, in particular, independently of the ion flux. [5][6][7][8][9][10][11][12][13][14][15][16] In this paper, we will investigate the effect of both amplitude asymmetric and slope asymmetric waveforms on ion properties in CF 4 plasmas.…”

“…According to an analytical model developed by Czarnetzki et al, the appearance of a self-bias, g, [5][6][7] due to the EAE, can be expressed as g ¼ÀṼ m1 þ eṼ m2 1 þ e ;…”

“…Although the impact of the amplitude and slope asymmetry of a waveform has been studied in detail in argon discharges, [6][7][8][9][10][11][12][21][22][23] only one article by Schulze et al 25 has looked at the effect of TVWs on CF 4 discharges, and it is briefly studied in an article by Bruneau et al 26 The former study contained only simulations with no experimental measurements and was limited to two harmonics. Furthermore, there is an interest in studying the behavior of CF 4 discharges when excited with low fundamental frequencies, as used in etching applications.…”

On the self-excitation mechanisms of plasma series resonance oscillations in single- and multi-frequency capacitive discharges

“…1,15,16,18,20 The Fourier spectrum of the electron current density of the 54. 24 MHz discharge again reveals the alternating amplitudes for odd and even …”

“…Another way of inducing an asymmetry is via the Electrical Asymmetry Effect (EAE), [17][18][19][20][21][22][23][24][25][26] i.e., by using a driving voltage waveform with unequal absolute values of the global maximum and minimum. Donko et al 17 have shown that the PSR is self-excited in geometrically symmetric, electrically asymmetric discharges operated at a fundamental frequency (13.56 MHz) and its second harmonic (27.12 MHz).…”