Power coupling mode transitions induced by tailored voltage waveforms in capacitive oxygen discharges

Abstract

Low-pressure capacitively coupled radio frequency discharges operated in O 2 and driven by tailored voltage waveforms are investigated experimentally and by means of kinetic simulations. Pulse-type (peaks/valleys) and sawtooth-type voltage waveforms that consist of up to four consecutive harmonics of the fundamental frequency are used to study the amplitude asymmetry effect as well as the slope asymmetry effect at different fundamental frequencies (5, 10, and 15 MHz) and at different pressures (50-700 mTorr). Values of the DC self-bias determined experimentally and spatio-temporal excitation rates derived from phase resolved optical emission spectroscopy measurements are compared with particle-in-cell/Monte Carlo collisions simulations. The spatio-temporal distributions of the excitation rate obtained from experiments are well reproduced by the simulations. Transitions of the discharge electron heating mode from the drift-ambipolar mode to the α-mode are induced by changing the number of consecutive harmonics included in the driving voltage waveform or by changing the gas pressure. Changing the number of harmonics in the waveform has a strong effect on the electronegativity of the discharge, on the generation of the DC self-bias and on the control of ion properties at the electrodes, both for pulse-type, as well as sawtooth-type driving voltage waveforms The effect of the surface quenching rate of oxygen singlet delta metastable molecules on the spatio-temporal excitation patterns is also investigated.

Electron power absorption in low pressure capacitively coupled electronegative oxygen radio frequency plasmas

“…To simplify the analysis, secondary electron emission is omitted in the model, only the reflection of the impinging electrons is taken into account with a probability of 20% [51]. The gas temperature is fixed at T g = 350 K. For the surface quenching probability of O 2 (a 1 ∆) singlet delta molecules we use the value of α = 6 · 10 −3 [25,39,48]. The computations are carried out using a spatial grid with N x = 100−1600 points and N t = 2000−85000 time steps within the RF period.…”

“…[24]. Derzsi et al investigated electron power absorption mode transitions in oxygen discharges driven by tailored voltage waveforms [25]. They concluded that changing the number of consecutive harmonics included in the driving voltage waveform has a strong effect on the electronegativity of the discharge.…”

Controlling plasma properties under differing degrees of electronegativity using odd harmonic dual frequency excitation

“…Since the original study many investigations have demonstrated this effect through both simulation and experiment and extended it to the use of more than two frequencies to increase the amplitude asymmetry of the waveform [28][29][30][31][32][33][34][35][36][37][38][39][40][41]. It has also been demonstrated that an electrical asymmetry can be generated using sawtooth-like waveforms where the rise and fall times of the voltage waveform differ [41][42][43][44][45][46][47][48]. In this case, the dc self-bias is generated as a result of a so-called slope asymmetry as opposed to the previously discussed amplitude asymmetry.…”

“…For example, it has been shown that the dc self-bias formed in electrically asymmetric plasmas can change over a wide range dependent on the gas in which the plasma is formed [44] and can thus be difficult to predict. The electronegativity of the plasma, which in part determines the electron heating mode [52], has been found to be particularly important in this regard [31,37,44,48].…”

“…A prominent example is the role played by the surface quenching probability of the molecular oxygen metastable molecule O 2 (a g 1 D ) in determining the electronegativity (i.e. the ratio of the negative ion density to the electron density) of oxygen plasmas [38,48,57,59,63]. This dependence occurs as a result of the high rate coefficient for electron detachment from O -ions in collisions with O 2 (a g 1 D ) molecules in comparison with the equivalent reaction with ground state molecular oxygen [64].…”