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 densities (n(e)) andgas kinetic temperatures (T-g) were measured for 27 and 40 MHz argon inductively coupled plasmas (ICPs) by means of Thomson and Rayleigh scattering. The study used the same r.f. generator, impedance-matching network, load coil, torch, and operating conditions for both 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 rates (0.80, 1.00 and 1.201 min(-1)). The results show that all three fundamental 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; the largest drop is generally found in the central channel. Under the same operating conditions, the 40 MHz ICP shows a larger, clearer central channel than the 27 MHz ICP, offering ease of sample introduction. This beneficial plasma environment cannot be created in a 27 MHz ICP simply by lowering the r.f. power, but could be produced by raising the central gas flow at the expense of shortening the sample residence time. The measured excitation temperature (T-exc) also declines with increasing r.f. frequency. The change in T-exc is comparable with those in 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 feature in the region above the load coil in an ICP. (C) 1997 Elsevier Science B.V.