A micro-hollow cathode discharge (MHCD) operated in Ar/N2 gas mixture, working in the normal regime, was studied both experimentally and with a 0D (volume-averaged) model in this work. This source provides high electron densities (up to 1015 cm−3) at low injected power (1 W). To understand the mechanisms leading to the production of N atoms, the densities of electrons, N atoms and argon metastable atoms (Ar*) were monitored over a wide range of experimental conditions. Electrons, N atoms and Ar* densities were probed by means of optical emission spectroscopy, vacuum ultra violet Fourier transform spectroscopy and tunable diode laser absorption spectroscopy, respectively. Measurements showed that using a smaller hole diameter enables to work with less injected power, while increasing the power density inside the hole and, subsequently, increasing the densities of excited species. Varying the percentage of N2 in the gas mixture highlighted that, up to 80%, the density of N atoms increases although the dissociation rate drops. Looking at the processes involved in the production of N atoms with the help of the 0D model, we found that at very low N2 fraction, N atoms are mostly produced through dissociative electron-ion recombination. However, adding more N2 decreases drastically the electron density. The density of N atoms does not drop thanks to the contribution of Ar* atoms, which are the main species dissociating N2 between 5% and 55% of N2 in the gas mixture. A reasonable agreement is found between the experiments and the model results. This study shows that, with this MHCD, it is possible to significantly modify the production of N atoms when modifying the physical parameters, making it particularly relevant for applications requiring a N atoms source, such as nitride deposition.
In this work, nanosecond two-photon absorption laser-induced fluorescence (TALIF) is used to probe the absolute density of nitrogen atoms in a plasma generated using a micro-hollow cathode discharge (MHCD). The MHCD is operated in the normal regime, and the plasma is ignited in an Ar/N2 gas mixture. First, we study a MHCD configuration having the same pressure (50 mbar) on both sides of the electrodes. A good agreement is found between the density of N atoms measured using TALIF in this work and previous measurements using vacuum ultraviolet Fourier transform absorption spectroscopy. Then, we introduce a pressure differential between the two electrodes of the MHCD, creating a plasma jet. The influence of the discharge current, the percentage of N2 in the gas mixture, and pressures on both sides of the MHCD is studied. The current has a small impact on the N-atom density. Furthermore, an optimal N-atom density is found at around 95% of N2 in the discharge. Finally, we demonstrate that the pressure has a different impact depending on the side of the MHCD: the density of N atoms is much more sensitive to the change of the pressure in the low-pressure side when compared to the pressure change in the high-pressure side. This could be due to several competing phenomena: gas residence time in the cathodic region, recirculation, or recombination of the N atoms at the wall. This study contributes to the optimization of MHCD as an efficient N-atom source for material deposition applications.
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