Optical emission measurements of electron energy distributions in low-pressure argon inductively coupled plasmas

Optical modeling of emissions from low-temperature plasmas provides a non-invasive technique to measure the electron energy distribution function (EEDF) of the plasma. While many models assume the EEDF has a Maxwell-Boltzmann distribution, the EEDFs of numerous plasma systems deviate significantly from the Maxwellian form. In this paper, we present an optical emission model for the Ar(3p54p → 3p54s) emission array which is capable of capturing details of non-Maxwellian distributions. Our model combines previously measured electron-impact excitation cross sections with Ar(3p54s) number density measurements and emission spectra. The model also includes corrections for radiation trapping of the Ar(3p54p → 3p54s) emission lines. Results obtained with this optical technique are compared with corresponding Langmuir probe measurements of the EEDF for Ar and Ar/N2 inductively coupled plasma systems operating under a wide variety of source conditions (1-25 mTorr, 20-1000 W, %N2 admixture). Both the optical emission method and probe measurements indicate the EEDF shapes are Maxwellian for low electron energies, but with depleted high energy tails.