Optical emission spectroscopy and collisional-radiative modeling for non-equilibrium, low temperature helium plasma

An optical emission spectroscopy (OES) technique combined with a collisional-radiative (CR) model is developed for measuring the electron temperature and density of low temperature helium (He) plasma. The CR modeling self-consistently solves the steady state population balance equation, including nonlinear radiation trapping and heavy particle collisional ionization as well as linear electron-impact processes and metastable diffusion. The CR modeling considers both Maxwellian and non-Maxwellian electron energy distributions, and the gas temperature parameter needed in the CR modeling is measured by adding a small amount of nitrogen gas and analyzing the rotational spectra of N${}_2^{+}$ species. The electron temperature and density determined by the CR modeling and OES in He inductively-coupled plasma are compared with the values measured by a Langmuir probe for the RF power and the gas pressure variations. The considered plasmas are in the ranges of electron temperature 2–5 eV and density 10⁸–10¹⁰ cm⁻³, and the gas pressure 200–800 mTorr and temperature 500 K. The trend and absolute values by the two methods are in overall agreement within the uncertainty range for the various RF powers and the gas pressures. The underlying population kinetics for the OES spectra is analyzed in detail.