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 , 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. (C) 2001 American Institute of Physics.