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

TL;DR

This study uses simulations to investigate how surface quenching of a specific metastable molecule affects electron heating mechanisms in capacitively coupled oxygen discharges across different pressures.

Contribution

It provides new insights into how surface quenching influences electron heating modes and the electron energy distribution in oxygen plasmas at various pressures.

Findings

At 10 mTorr, quenching has no effect on electron heating, which is dominated by drift-ambipolar heating.

At 25 mTorr, electron heating combines drift-ambipolar and alpha modes, with the former decreasing as quenching decreases.

At 50 mTorr, sheath electron heating dominates, with minimal contribution from drift-ambipolar mode at low quenching coefficients.

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 O$_2$(a$^1\Delta_{\rm g}$) 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 $\alpha-$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 $\alpha-$mode electron heating is observed for a surface quenching coefficient of 0.001 or smaller.