The characterization of Ar metastables in electron beam generated plasmas*

Electron beam generated plasmas are attractive for low-damage surface processing applications (deposition and etching) because of their ability to produce plasmas with large densities (>10¹⁰ cm^-3) at low electron temperatures (< 1 eV) and thus deliver a large fluence of very low energy (< 5 eV) ions to adjacent surfaces. These inherent features originate from the use of high energy electrons beams, rather than the heating of the plasma electrons via the use of electric fields, to drive plasma generation. The production and the diffusion of ions from the beam volume to adjacent substrates has been extensively studied, while neutrals, particularly metastables have received less attention. The interest in metastable species is facilitated by the fact that they have long lifetimes and large internal energy and can therefore distribute energy and drive reactions far from their point of origin. .  Previous modeling work has indicated the production and density of Ar metastables in electron beam generated plasmas produced in mixtures of argon and nitrogen is sensitive to the relative concentration of nitrogen. In this work, we have investigated the spatially-resolved density of metastable Ar species in electron beam generated plasmas produced in pure and diluted Ar using laser-induced fluorescence (LIF). Specifically, the density of the 1s₅ excited, metastable level was measured radially with respect to the beam axis at pressures of about 25 and 60 mTorr in Ar/N₂ gas backgrounds, where the relative N₂ concentration was varied up to approximately 20%. The measurements are compared to a zero-dimensional model of the system in an effort to understand the kinetics.