Optical and electrical characterization of an atmospheric pressure microplasma jet for Ar∕CH4 and Ar∕C2H2 mixtures

Abstract

A rf microplasma jet working at atmospheric pressure has been characterized for Ar, He, and $\mathrm{Ar}∕\mathrm{C}{\mathrm{H}}_4$ and $\mathrm{Ar}∕{\mathrm{C}}_2{\mathrm{H}}_2$ mixtures. The microdischarge has a coaxial configuration, with a gap between the inner and outer electrodes of $250\mu \mathrm{m}$ . The main flow runs through the gap of the coaxial structure, while the reactive gases are inserted through a capillary as inner electrode. The discharge is excited using a rf of $13.56\mathrm{MHz}$ , and rms voltages around $200–250\mathrm{V}$ and rms currents of $0.4–0.6\mathrm{A}$ are obtained. Electron densities around $8\times {10}^{20}{\mathrm{m}}^{-3}$ and gas temperatures lower than $400\mathrm{K}$ have been measured using optical emission spectroscopy for main flows of $3\mathrm{slm}$ and inner capillary flows of $160\mathrm{SCCM}$ . By adjusting the flows, the flow pattern prevents the mixing of the reactive species with the ambient air in the discharge region, so that no traces of air are found even when the microplasma is operated in an open atmosphere. This is shown in $\mathrm{Ar}∕\mathrm{C}{\mathrm{H}}_4$ and $\mathrm{Ar}∕{\mathrm{C}}_2{\mathrm{H}}_2$ plasmas, where no CO and CN species are present and the optical emission spectroscopy spectra are mainly dominated by CH and ${\mathrm{C}}_2$ bands. The ratio of these two species follows different trends with the amount of precursor for $\mathrm{Ar}∕\mathrm{C}{\mathrm{H}}_4$ and $\mathrm{Ar}∕{\mathrm{C}}_2{\mathrm{H}}_2$ mixtures, showing the presence of distinct chemistries in each of them. In $\mathrm{Ar}∕{\mathrm{C}}_2{\mathrm{H}}_2$ plasmas, $\mathrm{C}{\mathrm{H}}_x$ species are produced mainly by electron impact dissociation of ${\mathrm{C}}_2{\mathrm{H}}_2$ molecules, and the $\mathrm{C}{\mathrm{H}}_x∕{\mathrm{C}}_2{\mathrm{H}}_x$ ratio is independent of the precursor amount. In $\mathrm{Ar}∕\mathrm{C}{\mathrm{H}}_4$ mixtures, ${\mathrm{C}}_2{\mathrm{H}}_x$ species are formed mainly by recombination of $\mathrm{C}{\mathrm{H}}_x$ species through three-body reactions, so that the $\mathrm{C}{\mathrm{H}}_x∕{\mathrm{C}}_2{\mathrm{H}}_x$ ratio depends on the amount of $\mathrm{C}{\mathrm{H}}_4$ present in the mixture. All these properties make our microplasma design of great interest for applications such as thin film growth or surface treatment.