Negative ions and the role of metastable molecules in a capacitively coupled radiofrequency excited discharge in oxygen

Abstract

Axial distributions of negative oxygen ions in an asymmetric, capacitively coupled oxygen radiofrequency discharge (of the gaseous electronics conference reference cell type) are investigated. The O − ion densities are measured by detecting the electrons released after photo-detachment with a Langmuir probe.At small power and a pressure of approximately 10 Pa the axial distributions of both the O − ions and the electrons are flat. At higher power (characterized by voltage amplitudes in excess of 200 V) both quantities exhibit maxima in the central region between the electrodes. These maxima are displaced with increasing pressure (>50 Pa) towards the driven electrode. The absolute concentration of O − is insensitive to admixtures of argon up to 50%. This can be understood, if one assumes that the dominant loss channel for the negative ions involves metastable O 2 (a 1 g ) molecules. This assumption is confirmed by measurements taken with the discharge operating in a pulsed mode.

Particle-in-cell Monte Carlo and fluid simulations of argon-oxygen plasma: Comparisons with experiments and validations

“…6͑b͒, the plasma potential increases with increasing voltage and the sheath length also increases because of the confinement of O − by the increased plasma potential. The radial distribution of electron and O − , and the O − /O 2 + density ratio calculated by the 1D PIC-MCC simulation are consistent with the experimental data of Katsch et al 14 Figure 7 shows the charged species density, potential, and electron temperature with the ratio of Ar to O 2 at 115 mTorr and 160 V. As the ratio increases, the concentration of electrons and Ar + increases due to the increase of background argon atoms T n is the neutral gas temperature in K.…”

“…Figure 1͑b͒ corresponds to a 1D PIC-MCC simulation in an asymmetric cylindrical voltage-driven capacitively coupled discharge. Simulations for this reactor are compared with the experimental data obtained by Katsch et al 14 Here, the effects of gas composition and driving voltage are shown. Figure 1͑c͒ corresponds to the twodimensional model used for the fluid simulations of the reactor used by Kiehlbauch et al in Ref.…”

Radio-frequency discharges in oxygen: I. Particle-based modelling

“…[27,30]. Ignoring the electric asymmetry between the powered and grounded electrode, this part of the discharge can be simulated by the planar, one-dimensional model shown in figure 1.…”

“…It can be, for instance, accompanied by a double layer (internal sheath [20]). Electro-negative gas discharges are thus rather complex and the investigation of the spatio-temporal structure of the discharge as a function of external control parameters (current, voltage, frequency, pressure, and geometry) is a great theoretical [21,22,23,24] and experimental [25,26,27,28,29,30] challenge. In addition, electro-negative processing gases are reactive molecular gases, with internal degrees of freedom, which lead to a quite involved plasma chemistry.…”

“…1: Schematic geometry of the rf discharges used in Refs. [27,30]. We use a planar, one-dimensional model to simulate the central axial part of the discharge (thick solid line).…”