Diagnostics of processing plasmas, using optical emission from trace rare gases

Abstract

Summary form only given. This talk will describe the use of Trace Rare Gases Optical Emission Spectroscopy (TRG-OES) for quantitative determination of electron temperatures (T/sub e/), electron energy distribution functions (EEDF), positive ion densities (n/sup +/), and selected species concentrations in plasmas. Small, non-perturbing amounts of the five rare gases are added to the main plasma gases. Collisions of electrons with the rare gases result in excitation of the Paschen 2p levels. These levels quickly decay by emitting a photon in the red-near-infrared region. The emission intensities are a function of several parameters, including the number of electrons within a range of energies. The electron energy dependencies are dictated by the electron impact excitation cross sections. From published cross sections, and a detailed treatment of the excitation mechanism, T/sub e/, and under some circumstances EEDFs can be derived. In addition, emission from the trace rare gases can be used to determine the relative number density of the positive ions in the plasma, and the number density of other species (e.g. Cl/sub 2/, or O-atoms) by taking the ratio of the emission from a selected level of the species (e.g. Cl/sub 2/ 305 nm band, or O 844.6 nm line) to an emission from one of the rare gases that most closely matches its electron energy dependence. This actinometry method has been widely used to determine species concentrations. We have more recently obtained a more detailed accounting for other pathways for excitation of emission, as well as for mismatches in relative electron energy dependencies, allowing the accuracy of these methods to be improved.