Excitation of Ar, O₂, and SF₆/O₂ plasma discharges using tailored voltage waveforms: control of surface ion bombardment energy and determination of the dominant electron excitation mode

Abstract

Using tailored voltage waveforms (TVWs) to excite a low pressure, low-temperature plasma discharge, we compare the behavior of three gas mixtures, namely Ar, O 2 and SF 6 /O 2 mixtures, the last of which is currently used for the plasma-texturing of silicon wafers for photovoltaics. The primary goal of using TVWs is to control the ion bombardment energy at the surface of the wafer, and this control is demonstrated through retarding field energy analyzer (RFEA) measurements. However, the complicated electrical response of the plasma to such waveforms makes the ab initio prediction of the ion energy difficult, although by using said RFEA measurements, we show that it can be done approximately by using measured electrical data. In addition, we utilize the response of the plasma to mirror-image 'sawtooth' waveforms as a predictor of the dominant electron heating mode (α or drift-ambipolar, DA). At equivalent pressures and coupled powers, the Ar and O 2 mixtures always display behavior associated with electropositive plasmas (a solely α heating mode). However, with the addition of SF 6 to an O 2 gas flow, a transition can be observed towards a behavior associated with a more electronegative plasma (i.e. a dominant DA heating mode). This crossover in the dominant heating mode is observed through the relative self-bias voltage for each type of sawtooth waveform, and is therefore a useful predictor of the dominant electron heating mode in low pressure, cold plasma discharges.

Tailored voltage waveforms applied to a capacitively coupled chlorine discharge

“…This has also been demonstrated experimentally, in a capacitively coupled SF 6 /O 2 discharge driven by tailored voltage waveforms, that higher electronegativity significantly influences the dc selfbias, as its available range with phase angle θ is decreased [22]. This is suggested to being due to an electron power absorption mode transition and SAE is found to dominate over AAE in the highly electronegative discharge [22]. Also Bruneau et al [13] demonstrate control of the ion energy, while significant changes are observed in the ion flux, as well in a CF 4 discharge operated at 80 Pa.…”

“…By decreasing the argon content in a CF 4 /Ar discharge a transition from the αmode to the DA electron power absorption mode is observed while a significant change in the plasma symmetry, the dc selfbias, and the mean electron energy occurs [21]. This has also been demonstrated experimentally, in a capacitively coupled SF 6 /O 2 discharge driven by tailored voltage waveforms, that higher electronegativity significantly influences the dc selfbias, as its available range with phase angle θ is decreased [22]. This is suggested to being due to an electron power absorption mode transition and SAE is found to dominate over AAE in the highly electronegative discharge [22].…”

Helium metastable species generation in atmospheric pressure RF plasma jets driven by tailored voltage waveforms in mixtures of He and N2

“…In order to expand the operational regime of µAPPJs and to provide a simple concept for effective and selective control of reactive as well as excited species, including metastables and RONS, Voltage waveform tailoring (VWT) was proposed recently [31,32]. The idea is adopted from low pressure discharges [33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49] and is based on driving the discharge by a voltage waveform that consists of a finite Fourier series of N consecutive harmonics of a fundamental frequency, f 0 . By varying the relative phases and amplitudes of the harmonics different types of waveforms can be generated, e.g.…”

Experimental demonstration of multifrequency impedance matching for tailored voltage waveform plasmas

“…The external circuit is depicted in Fig. 31 is installed at the input of the MFMB (SOLAYL Probe Position I) to quantify the total impedance as well as the coupled power to the total load (comprising the matching network, a coaxial coupling cable, and the plasma reactor). For certain measurements, the VI probe is installed at the output of the MFMB (but before the 6.7-m coaxial coupling cable, SOLAYL Probe Position II) to quantify the load impedance.…”