Optical diagnostics for characterization of electron energy distributions: argon inductively coupled plasmas

The assumption of thermodynamic equilibrium in low-temperature plasmas can lead to errors in determination of electron energy distribution functions (EEDFs) using optical diagnostics. Significant improvements in the accuracy of EEDFs determined using optical diagnostics on argon-containing inductively coupled plasmas (ICPs) are possible by fitting an analytic representation of the EEDF with two adjustable parameters (x and Tx) to recorded optical emission spectra. This so-called x-form of the EEDF captures the well-known suppression of the high-energy portion of the EEDF (compared with the Maxwellian form) observed in ICPs. A review of electron kinetics summarizes the physical mechanisms that lead to this form, and explains the weak dependence of x on operating pressure. For the 1–25 mTorr pressure range of argon-containing mixtures in the system examined, the fixed value x = 1.2 is found to represent the EEDF very well, followed by a near linear increase in x with pressure from x = 1.2 to 1.6 in the 25–50 mTorr range. Significantly improved agreement between predictions of effective electron temperature with a global model and both probe and optical measurements when a Maxwellian EEDF is replaced by the x-form further highlights the benefit of the x-form in improved accuracy in determining the EEDF with optical diagnostics.