Theoretical insight into Ar–O₂surface-wave microwave discharges

Abstract

Abstract.A zero-dimensional kinetic model has been developed to investigate the coupled electron and heavy-particle kinetics in Ar-O 2 surface-wave microwave discharges generated in long cylindrical tubes, such as those launched with a surfatron or a surfaguide. The model has been validated by comparing the calculated electron temperature and species densities with experimental data available in the literature for different discharge conditions. Systematic studies have been carried out for a surfacewave discharge generated with 2.45 GHz field frequency in a 1 cm diameter quartz tube in Ar-O 2 mixture at 0.5-3 Torr pressures, which are typical conditions found in different applications. The calculations have been performed for the critical electron density n e =3.74×10 11 cm −3 . It has been found that the sustaining electric field decreases with Ar percentage in the mixture, while the electron kinetic temperature exhibits a minimum at about 80%Ar. The charged and neutral species densities have been calculated for different mixture compositions, from pure O 2 to pure Ar, and their creation and destruction processes have been identified. The O 2 dissociation degree increases with Ar addition into O 2 and dissociation degrees as high as 60% can be achieved. Furthermore, it has been demonstrated that the dissociation degree increases with the discharge tube radius, while decreases with the atomic surface recombination of O-atoms. The density of O − negative ions is very high in the plasma, the electronegativity of the discharge can be higher than 1, depending on the discharge conditions. §

O2 dissociation in Ar–O2 surface-wave microwave discharges

“…This effect is well represented by the EEDFs, which reveal that the energy range interesting for the O 2 dissociation is more populated at elevated Ar percentages in the mixture than in pure O 2 . Furthermore, in high Ar content mixtures the collisions of O 2 molecules with Ar(4s) atoms also contribute to the oxygen dissociation [15]. Three-body recombination of O-atoms starts to play role at pressures higher than 10 mbar and mixtures with significant amounts of O 2 .…”

“…For more details the reader should refer to [9,[13][14][15], where the gas phase and surface reactions taken into account in the model, as well as all other parameters used, are described. A surface wave microwave discharge generated in a small diameter tube is a plasma column with a decreasing electron density profile as discussed in [16,17].…”

“…For this purpose the continuity and transport equations for the electrons, different positive ions and O − ions are solved, in order to describe the charged particle motion to the wall under the presence of the radial space-charge field. The reduced electric field is determined using the requirement that under steady-state conditions the total rate of ionization must compensate exactly for the rate of electron loss [15].…”

A Powerful Remote Source of O Atoms for the Removal of Hydrogenated Carbon Deposits

“…Most of these arise from high voltages involved when running discharges at high power. It is well known that highfrequency electrode-less discharges are almost perfectly optimized for low powers [19][20][21][22][23][24][25]. The voltage on the RF coil is reasonable low, at a comfortably level of about 500 V. Such systems are suitable for treatment of small samples.…”

“…Average neutral oxygen atom densities measured in these discharges are: *1910 21 m -3 in a DC discharge [16], *5910 20 m -3 in a microwave discharge [17], *1910 20 m -3 in a capacitively coupled RF plasma (CCP) [18], *2910 21 m -3 in an oxygen plasma created by electrode-less high-frequency discharges (ICP) [19][20][21][22][23][24][25]. In inductively coupled plasmas, because of relatively-low dissociation energies for oxygen molecules as compared with their ionization energy, and due to a relatively low probability for recombination on chamber walls, the dissociation fraction in plasma created in such discharges easily exceed 10 %.…”