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

Using tailored voltage waveforms (TVWs) to excite a low pressure, low-temperature plasma discharge, we compare the behavior of three gas mixtures, namely Ar, O₂ and SF₆/O₂ 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 O₂ mixtures always display behavior associated with electropositive plasmas (a solely α heating mode). However, with the addition of SF₆ to an O₂ 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. (paper)