On the possibility of making a geometrically symmetric RF-CCP discharge electrically asymmetric

Abstract

A fundamental problem in technological plasmas has been how to control the ion energy and the ion flux (plasma density) independently of one another. A simple, but previously overlooked asymmetry effect is reported that should allow a high degree of control of the ion energy. The idea is that when a temporally symmetric, multi-frequency voltage waveform containing one or more even harmonics is applied to a discharge, even a geometrically symmetric one, the two sheaths are necessarily asymmetric. To balance the charged particle fluxes, a dc self-bias develops. Optimally, this is achieved with a dual frequency discharge that uses the phase locked fundamental and its second harmonic. The resulting dc self-bias and hence the ion energy are a nearly linear function of the phase angle between the two applied RF voltages. This works even for geometrically symmetric discharges, and the roles of the two electrodes can be reversed using the phase. This means that the technique can be used to increase or decrease the ion energy striking a substrate while leaving the applied RF voltage and frequency and thereby the discharge parameters effectively unchanged.

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 ;…”

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

“…Accordingly, qðuÞ ¼ QðuÞ=Q 0 with Q 0 ¼ e Ð s max 0 n i ðzÞdz, where n i ðzÞ is the ion density profile and s max is the maximum powered sheath length. The symmetry parameter e is defined as 22,23 …”

“…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).…”

A model for tailored-waveform radiofrequency sheaths

“…Robiche et al [12,13] extended the Lieberman model for this case, with the approximation of the small amplitude of the high-frequency component. However, more complex asymmetric waveforms were later introduced [14] (see also [15] and references therein for an extensive review) and the Lieberman sheath model could not easily be extended to these situations.…”