Non-equilibrium electron energy distribution in oxygen plasma: observation with optical emission spectroscopy*

Partially ionized inductively-coupled RF oxygen plasmas are in widespread use for materials processing, and non-invasive diagnostics are of interest for the optimization and control of the degrees of ionization and dissociation. Our initial study involves a 2-5\% admixture of argon for optical emission spectroscopy (OES) of the oxygen plasma glow. The Ar 420.1/419.8 nm line intensity ratio, previously used in other mixtures to compute electron temperature, when $<1$, is also an indicator of a significant population of high energy ($>35$ eV) electrons;\footnote{J. B. Boffard {\it et al.}, PSST {\bf 24} (2015))} the latter is observed under conditions of low power and high pressure in the oxygen plasma. We tentatively attribute the increase in energetic electrons to a transition to capacitive coupling, leading to electron acceleration to high energy in the sheaths adjacent to the powered electrode, which in this system is a spiral flat antenna separated from the plasma by a dielectric window. Investigations of OES methods involving additional species, including other trace rare gases,\footnote{V. M. Donnelly, J. Phys. D {\bf 37} (2004).} O, and O$_2^+$, to determine oxygen plasma properties such as non-Maxwellian electron energy distributions will also be described.