The role of surface quenching of the singlet delta molecule in a capacitively coupled oxygen discharge

Abstract

We use the one-dimensional object-oriented particle-in-cell Monte Carlo collision code oopd1 to explore the influence of the surface quenching of the singlet delta metastable molecule O2(a 1 ∆g) on the electron heating mechanism, and the electron energy probability function (EEPF), in a single frequency capacitively coupled oxygen discharge. When operating at low pressure (10 mTorr) varying the surface quenching coefficient in the range 0.00001 -0.1 has no influence on the electron heating mechanism and electron heating is dominated by drift-ambipolar (DA) heating in the plasma bulk and electron cooling is observed in the sheath regions. As the pressure is increased to 25 mTorr the electron heating becomes a combination of DA-mode and α−mode heating, and the role of the DA-mode decreases with decreasing surface quenching coefficient. At 50 mTorr electron heating in the sheath region dominates. However, for the highest quenching coefficient there is some contribution from the DA-mode in the plasma bulk, but this contribution decreases to almost zero and pure α−mode electron heating is observed for a surface quenching coefficient of 0.001 or smaller.

Experimental and computational investigations of the effect of the electrode gap on capacitively coupled radio frequency oxygen discharges

“…(We note that data for this coefficient show large scattering in the literature. 58,67,69,70 For a recent study of the effect of metastable molecules in oxygen plasmas, see Ref. 71.)…”

“…56 As mentioned above, in order to gain access to various discharge characteristics such experimental studies should also be accompanied by modeling/simulations, which is now possible with accurate kinetic PIC/MCC codes developed recently. 44, [57][58][59][60][61] Therefore, in this work, we investigate the effects of changing the electrode gap on important plasma characteristics such as the central electron density, electronegativity, mean electron energy, and ion flux-energy distribution functions in low pressure single-frequency CCPs operated at 13.56 MHz in O 2 . We also study the time averaged optical emission from atomic oxygen lines and the spatio-temporal ionization dynamics.…”

The Influence of Secondary Electron Emission and Electron Reflection on a Capacitively Coupled Oxygen Discharge

“…For the surface quenching coefficients of the singlet metastables molecules on the electrode surfaces, we assume for the singlet metastable O 2 (a 1 ∆ g ) a value of γ wqa = 0.0001, and for the singlet metastable O 2 (b 1 Σ + g ), we assume a value of γ wqb = 0.1, based on the suggestion by O'Brien and Myers [39] that the surface quenching coefficient for the b 1 Σ + g state is significantly larger than for the a 1 ∆ g state. We explored the influence of the surface quenching coefficients of the singlet metastable molecule O 2 (a 1 ∆ g ) on the discharge properties in an earlier work [37]. There, we demonstrated that the influence of γ wqa on the discharge properties and the electron heating mechanism can be significant indeed.…”

“…The energy threshold values and the particle weights used here for the various neutral species included in the simulation are listed in Table 1. The partial pressures of the background thermal neutral species were calculated using a global (volume averaged) model of the oxygen discharge, as discussed in Proto and Gudmundsson [37]. The fractional densities for the neutrals O 2 (X 3 Σ − g ), O( 3 P), O 2 (a 1 ∆ g ), and O 2 (b 1 Σ g ), estimated using the global model calculations at 10, 25, and 50 mTorr, are listed in Table 2.…”

“…There, we demonstrated that the influence of γ wqa on the discharge properties and the electron heating mechanism can be significant indeed. The partial pressures listed in Table 2 were calculated by a global model using these surface quenching and recombination parameters as discussed in our earlier study [37]. In the simulations, we either neglect electron reflection from the electrode or assume that electrons are reflected from the electrodes with a probability of 0.2, which is the number of elastically-reflected electrons per incoming electron, independent of their energy and angle of incidence.…”