A novel method is proposed to determine the electron energy distribution function (EEDF) including highly-populated tail originated from by energetic beam-like electrons (> 20 eV), which often occur in the narrow gap, very-high-frequency capacitively coupled plasma (VHF-CCP). This method combines the conventional Langmuir probe analysis for representing the EEDF in low-energy regime and the line-ratio method taken from optical emission spectrum for the highly-populated tail of EEDF. Here the emission lines are chosen with considering the excitation rates which is a function of shape of cross-section of spin- and dipole-forbidden state of argon atom and EEDF at corresponding energy.  In this method, the analytical EEDF model is chosen as the composition of Maxwellian for lowenergy (bulk) electrons and J Schulze’s time-average form of shifted Maxwellian [J. Schulze et al. 2008 J. Phys. D: Appl. Phys. 41 042003] for highly-populated tail of EEDF. The variables of timeaverage form of shifted Maxwellian, the fraction and mean energy of beam-like electrons, are determined by using emission model of line-intensity ratios which describes the line-intensities as a function of EEDF. This method has an advantage for diagnostics of the time averaged EEDF having highly-populated tail, especially due to beam-like electrons originated from stochastic heating, rather than two temperature EEDF. 

Roh et al 2017 J. Phys. D: Appl. Phys. https://doi.org/10.1088/1361-6463/aa6cf8