Electrostatic probe measurements in an expanding microwave discharge sustained in an Ar–N2–H2 gas mixture: Investigation on plasma parameters

Abstract

This work is devoted to the study of plasma parameters in the expansion of a microwave discharge sustained within an Ar–N2–H2 gas mixture. The plasma expansion in the reactor depends on the wave propagation conditions.We show that in a pure argon discharge the electron energy distribution function (EEDF) is identical to a Maxwell energy distribution function whatever the distance between the discharge exit and the substrate holder is and whatever the incident power is, ranging between 100 and 400 W. However, results show that the electron density is larger close to the discharge exit than to the substrate holder.When N2 or H2 is mixed with Ar, the discharge shrinks. The EEDF is displaced towards a low electron energy, and this behaviour increases with increasing N2 or H2 percentage. The EEDF cannot be compared with a Maxwell energy distribution function for a gas mixture containing more than 10% N2. Beyond this value, the plasma is unstable 3 cm below the discharge exit, and for larger N2 concentrations, the wave propagation is not observed in these regions.When the three gases H2, N2 and Ar are all mixed together in the gas mixture, the plasma is more stable and the expansion is detected even for gas mixtures containing a large concentration of (N2, H2) mixed with argon, in a 80% Ar–20% (N2, H2) or 48% Ar–52% (N2, H2) gas mixture. However, when the concentration of H2 is too low (49% Ar–50% N2–1% H2), the plasma shrinks and wave propagation is not observed. This behaviour could be explained assuming that these gas mixtures containing Ar–H2–N2 are producing NHx species in plasma that are low ionization energy species. However, numerical models are necessary in order to study such a complex plasma and to confirm the accuracy of this last assumption.