Behavior of excited argon atoms in inductively driven plasmas

Abstract

Laser induced fluorescence has been used to measure the spatial distribution of the two lowest energy argon excited states, 1s5 and 1s4, in inductively driven plasmas containing argon, chlorine and boron trichloride. The behavior of the two energy levels with plasma conditions was significantly different, probably because the 1s5 level is metastable and the 1s4 level is radiatively coupled to the ground state but is radiation trapped. The argon data are compared with a global model to identify the relative importance of processes such as electron collisional mixing and radiation trapping. The trends in the data suggest that both processes play a major role in determining the excited state density. At lower rf power and pressure, excited state spatial distributions in pure argon were peaked in the center of the discharge, with an approximately Gaussian profile. However, for the highest rf powers and pressures investigated, the spatial distributions tended to flatten in the center of the discharge while the density at the edge of the discharge was unaffected. The spatially resolved excited state density measurements were combined with previous line integrated measurements in the same discharge geometry to derive spatially resolved, absolute densities of the 1s5 and 1s4 argon excited states and gas temperature spatial distributions. Fluorescence lifetime was a strong function of the rf power, pressure, argon fraction and spatial location. Increasing the power or pressure resulted in a factor of 2 decrease in the fluorescence lifetime while adding Cl2 or BCl3 increased the fluorescence lifetime. Excited state quenching rates are derived from the data. When Cl2 or BCl3 was added to the plasma, the maximum argon metastable density depended on the gas and ratio. When chlorine was added to the argon plasma, the spatial density profiles were independent of chlorine fraction. While it is energetically possible for argon excited states to dissociate some of the molecular species present in this discharge, it does not appear to be a significant source of dissociation. The major source of interaction between the argon and the molecular species BCl3 and Cl2 appears to be through modification of the electron density.