COMPARISON OF ELECTRON CONCENTRATIONS, ELECTRON TEMPERATURES, GAS KINETIC TEMPERATURES, AND EXCITATION TEMPERATURES IN ARGON ICPS OPERATED AT 27 AND 40 MHZ

Spatially resolved electron temperatures (T-e), electron number densit ies (n(e)) andgas kinetic temperatures (T-g) were measured for 27 and 40 MHz argon inductively coupled plasmas (ICPs) by means of Thomson an d Rayleigh scattering. The study used the same r.f. generator, impedan ce-matching network, load coil, torch, and operating conditions for bo th frequencies. The experiments were carried out at three r.f. forward power settings (1.00, 1.25 and 1.50 kW) and three central gas flow ra tes (0.80, 1.00 and 1.201 min(-1)). The results show that all three fu ndamental parameters drop when the r.f. frequency is increased from 27 to 40 MHz under all operating conditions used. The change in n(e) was the most significant. The relative change in each of the fundamental parameters depends mainly on the observation position in the plasma; t he largest drop is generally found in the central channel. Under the s ame operating conditions, the 40 MHz ICP shows a larger, clearer centr al channel than the 27 MHz ICP, offering ease of sample introduction. This beneficial plasma environment cannot be created in a 27 MHz ICP s imply by lowering the r.f. power, but could be produced by raising the central gas flow at the expense of shortening the sample residence ti me. The measured excitation temperature (T-exc) also declines with inc reasing r.f. frequency. The change in T-exc is comparable with those i n T-e and T-g. The argon ionization temperature (T-ion) obtained from measured n(e) values and the Saha equation is higher than T-e at both r.f. frequencies, indicating that the recombining mode is a common fea ture in the region above the load coil in an ICP. (C) 1997 Elsevier Sc ience B.V.