Langmuir probe and optical diagnostics of active screen N2–H2 plasma nitriding processes with admixture of CH4

Abstract

Low pressure pulsed DC N2–H2 plasmas with admixtures of CH4 used for active screen plasma nitriding (ASPN) have been studied by time resolved Langmuir probe and optical emission spectroscopy (OES) techniques (f=1kHz, 60% duty cycle, p=1mbar). The temporal behavior of parameters of the electrons inside the process volume of the ASPN reactor was at the center of interest. Based on probe diagnostics the electron density and temperature as well as the plasma potential could be determined in dependence (i) on time in the discharge pulses and (ii) on parameters of the gas mixture including the N2–H2 ratio. Electron temperatures between 0.13 and 0.31V and electron densities in the order of 109cm−3 could be measured. In addition OES has been used to receive complementary information about neutral and charged nitrogen molecules from the glow discharge between the active screen and the outer reactor wall. It was found, that the ratio of hydrogen to nitrogen content in the feed gas mixture influences the emission characteristics of the plasma at the active screen considerably, i.e. the intensity ratio of the N2+-(0–0)- band to the N2-(0–2)-band. Another focus was the analysis of differences of the electron parameters in pure N2–H2 discharges compared to feed gas mixtures containing methane and the influence on outer electrical parameter of the ASPN reactor. Finally the results of nitriding experiments showed a correlation to the findings of the plasma diagnostic studies.