Strong Ionization Asymmetry in a Geometrically Symmetric Radio Frequency Capacitively Coupled Plasma Induced by Sawtooth Voltage Waveforms

Abstract

The ionization dynamics in geometrically symmetric parallel plate capacitively coupled plasmas driven by radio frequency tailored voltage waveforms is investigated using phase resolved optical emission spectroscopy (PROES) and particle-in-cell (PIC) simulations. Temporally asymmetric waveforms induce spatial asymmetries and offer control of the spatiotemporal dynamics of electron heating and associated ionization structures. Sawtooth waveforms with different rise and fall rates are employed using truncated Fourier series approximations of an ideal sawtooth. Experimental PROES results obtained in argon plasmas are compared with PIC simulations, showing excellent agreement. With waveforms comprising a fast voltage drop followed by a slower rise, the faster sheath expansion in front of the powered electrode causes strongly enhanced ionization in this region. The complementary waveform causes an analogous effect in front of the grounded electrode.

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

“…23. A reactor with an inter-electrode gap of 2.5 cm was made geometrically symmetric by adding a thick Teflon ring (inner diameter 10 cm), fitted with a 2.5 Â 2.5 Â 10 cm piece of borosilicate glass to allow optical access.…”

“…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.…”

“…These waveforms were shown to induce a fast sheath expansion and a slow sheath contraction in front of one electrode, and a slow sheath expansion and a fast sheath contraction on the other side. 23 As a consequence, a strong asymmetry of the electron power absorption (and therefore the ionization rate) can be produced in geometrically symmetric argon discharges, with larger ionization closer to the electrode where the sheath expands rapidly. A DC self-bias was also shown to appear in argon discharges in this case, despite the absence of an amplitude asymmetry (i.e., V m1 ¼Ṽ m2 ), because of different mean ion densities in the sheaths (e 6 ¼ 1 in Eq.…”

“…Such waveforms, with N ¼ 4, are shown in Fig. 1(a) for different values of H. Alternatively, it was recently shown [21][22][23] that some control over the asymmetry of argon discharges can be obtained using sawtooth waveforms (i.e., with differing rise and fall rates), defined by the following expression:…”

Atmospheric-pressure diffuse dielectric barrier discharges in Ar/O2gas mixture using 200 kHz/13.56 MHz dual frequency excitation

“…This dual frequency (DF) excitation was previously investigated in low-pressure inductively coupled plasmas (ICPs) [18,19], capacitively coupled plasmas (CCPs) [20][21][22] and dielectric barrier discharges (DBDs) [23,24]. It was found that in low-pressure plasmas the LF and HF component can facilitate separate control of the ion energy and ion flux to the surface [19,20,25,26].…”