Measurement and modeling of neutral, radical, and ion densities in H2-N2-Ar plasmas

Abstract

A comprehensive experimental investigation of absolute ion and neutral species densities in an inductively coupled H2-N2-Ar plasma was carried out. Additionally, the radical and ion densities were calculated using a zero-dimensional rate equation model. The H2-N2-Ar plasma was studied at a pressure of 1.5 Pa and an rf power of 200 W. The N2 partial pressure fraction was varied between fN2=0% and 56% by a simultaneous reduction of the H2 partial pressure fraction. The Ar partial pressure fraction was held constant at about 1%. NH3 was found to be produced almost exclusively on the surfaces of the chamber wall. NH3 contributes up to 12% to the background gas. To calculate the radical densities with the rate equation model, it is necessary to know the corresponding wall loss times twrad of the radicals. twrad was determined by the temporal decay of radical densities in the afterglow with ionization threshold mass spectrometry during pulsed operation and based on these experimental data the absolute densities of the radical species were calculated and compared to measurement results. Ion densities were determined using a plasma monitor (mass and energy resolved mass spectrometer). H3+ is the dominant ion in the range of 0.0≤fN2<3.4%. For 3.4<fN2<40%, NH3+ and NH4+ are the most abundant ions and agree with each other within the experimental uncertainty. For fN2=56%, N2H+ is the dominant ion, while NH3+ and NH4+ have only a slightly lower density. Ion species with densities in the range between 0.5% and 10% of ni,tot are H2+, ArH+, and NH2+. Ion species with densities less than 0.5% of ni,tot are H+, Ar+, N+, and NH+. Our model describes the measured ion densities of the H2-N2-Ar plasma reasonably well. The ion chemistry, i.e., the production and loss processes of the ions and radicals, is discussed in detail. The main features, i.e., the qualitative abundance of the ion species and the ion density dependence on the N2 partial pressure fraction, are well reproduced by the model.