| AVS 58th Annual International Symposium and Exhibition | |
| Plasma Science and Technology Division | Thursday Sessions |
| Session PS-ThA |
| Session: | Plasma Diagnostics, Sensors and Control II |
| Presenter: | John Boffard, University of Wisconsin-Madison |
| Authors: | J. Boffard, University of Wisconsin-Madison L.E. Aneskavich, University of Wisconsin-Madison R.O. Jung, University of Wisconsin-Madison C.C. Lin, University of Wisconsin-Madison A.E. Wendt, University of Wisconsin-Madison |
| Correspondent: | Click to Email |
Passive, non-invasive optical emission measurements provide a means of probing important plasma parameters without introducing contaminants into plasma systems.* Due to the dominant role of electron-impact collisions in gas-phase reactions, our investigation focuses on characterization of the electron energy distribution function (EEDF). In particular, we highlight the ability to observe EEDFs under non-equilibrium conditions in which the EEDF deviates from the Maxwell-Boltzmann form. The energy dependence of the EEDF, which varies with plasma generation method and operating conditions, has significant implications for gas phase reaction rates and is thus critical to the predictive control of plasma process outcomes. EEDFs are determined using measurements of argon emission intensities in the 650-1150 nm wavelength range and measured metastable and resonance level concentrations, in conjunction with a radiation model that includes contributions from often neglected but critical processes such as radiation trapping and electron-impact excitation from metastable and resonance levels. Results using argon emission spectra will be presented for an inductively-coupled plasma (ICP) over a wide range of operating conditions (pressure, RF power, Ar/Ne/N2 gas mixtures), which show a depletion of the EEDF relative to the Maxwell-Boltzmann form at higher electron energies, in good agreement with measurements made with Langmuir probes and predictions of a global discharge model. These results are consistent with predictions of electron kinetics and can be explained in terms of reduced life times for energetic electrons due to wall losses and inelastic collisions. For Ne/Ar plasmas, analysis of neon emission spectra in addition to the argon analysis provides enhanced sensitivity to the presence of high-energy electrons. This example highlights the potential utility of this method as a tool for probing kinetics of many types of low-temperature plasma systems, which are typically characterized by non-Maxwellian EEDFs.
*Plasma Sources Sci. Technol. 19, 065001 (2010).
This work was supported by the Wisconsin Alumni Research Foundation (WARF) and by NSF Grant CBET 0714600.