Measurement of resonance level densities in rare gas plasmas and modeling of their resulting VUV emissions

In the rare gases, the vacuum ultraviolet (VUV) emissions are dominated by the decays from the $1s_2$ and $1s_4$ (Paschen's notation) principal resonance levels. In isolation, atoms excited to these resonance levels have a short radiative lifetime ($<10$ns), but resonance blockade of the VUV transitions to the ground state significantly extend the effective lifetimes of these levels under typical plasma conditions with pressures greater than a mTorr. Despite this re-absorption, rare gas plasmas do produce copious VUV emissions that may play an important role in critical surface reactions under certain process conditions. We have measured the resonance level densities as a function of pressure in rare-gas discharges (Ne,Ar,Kr,Xe) in an inductively coupled plasma using both white-light absorption spectroscopy and optical emission spectroscopy by monitoring changes in the $2p_x \to 1s_y$ branching fractions [1]. The measured resonance level concentrations are subsequently used as inputs to a simple VUV transport model to determine the VUV flux to surfaces. These model VUV flux calculations are compared to measurements made with an absolutely calibrated VUV photodiode. \\[4pt] [1] Plasma Sources Sci. Technol. \textbf{18} (2009) 035017.