Absolute fluorine atom concentrations in fluorocarbon plasmas determined from CF₂ loss kinetics

We present a simple technique for determining the absolute fluorine atom concentration in pure CF₄ capacitively coupled radio-frequency discharges. It is based on the measurement, by laser-induced fluorescence, of the loss rate k_loss of the CF₂ radical in the afterglow of a pulsed plasma. We first demonstrate that in our conditions, CF₂ is lost only by gas phase recombination with F atoms (with a known rate constant k_rec) and by recombination at the reactor walls at a rate k_wall, independent of the rf power injected. Hence, the total CF₂ loss rate, k_loss = k_wall+k_rec[F], varies linearly with [F] when the rf power is increased. By recording k_loss and the relative variation of the F atom concentration (by optical emission actinometry) as a function of rf power, k_wall and [F] can be determined. These measurements of [F] complement previous quantitative measurements of CF and CF₂ radicals [Booth et al., J. Appl. Phys. 85, 3097 (1999); and Cunge and Booth, J. Appl. Phys. 85, 3952 (1999)] made in the same reactor for the same plasma conditions.