DETERMINATION OF ELECTRON TEMPERATURES IN PLASMAS BY MULTIPLE RARE-GAS OPTICAL-EMISSION, AND IMPLICATIONS FOR ADVANCED ACTINOMETRY

A method is described for determining the electron temperature of a lo w pressure plasma of the type used in microelectronics materials proce ssing. A small amount of an equal mixture of He, Ne, Ar, Kr, and Xe is added to the process gas (in this example Cl-2) and the intensities o f optical emission lines from the Paschen 2p levels of the rare gases are recorded. The observed emission intensities are compared with thos e computed from a model that includes electron impact excitation from the ground state, as well as two-step electron impact excitation throu gh intermediate metastable levels. This latter route is shown to be th e dominant one for nearly half of the levels. Using adjusted, publishe d electron impact excitation cross sections and assuming a Maxwellian electron energy distribution, the electron temperature (T-e), the only adjustable parameter, was determined from the best match between the observed and computed intensities. For a high density, helical resonat or Cl-2 plasma at 10 mTorr, T-e = 2.2+/-0.5 eV was determined from thi s method. This value is about 1 eV lower than the typical values repor ted for high density inductively coupled Cl-2 plasmas at similar press ures. While the precision of the method is expected to be high, the ac curacy of T-e determined from optical emission would likely be improve d with more accurate cross section data. Implications for actinometric determination of species concentrations in plasmas are also discussed . (C) 1997 American Vacuum Society.