CF and CF₂ radical kinetics and transport in a pulsed CF ₄ ICP

The kinetics of CF and CF₂ radicals (in their electronic ground states) was investigated using space and time resolved laser induced fluorescence (LIF) in an inductively coupled discharge in CF₄ gas at 33 mTorr, both in the steady state and in pulsed plasmas. The gas temperature was determined from the rotationally resolved LIF excitation spectra of the CF radical, reaching about 800 K in the centre of the chamber. The densities were put on an absolute scale with the aid of broad-band UV absorption spectroscopy. The density profiles were used to deduce the radical diffusive transport, and therefore the localized (gas-phase and surface) net production or destruction rates. However, when gas heating is significant, the radical transport becomes more complex due to gas density gradients and thermo-diffusion. In the afterglow, gas convection induced by gas cooling and contraction can play a significant role. The CF and CF₂ axial fluxes estimated from measured density and temperature profiles indicate that these species are emitted from the top and bottom surfaces, and are destroyed in the gas phase at high rates (≈900 s^-1 and 1500 s^-1, respectively): for CF₂ this process appears to be electron-impact induced. In the afterglow, the CF₂ density at the reactor centre increases abruptly, reaching four times the steady-state value after 3 ms, followed by a slow decay (100 s^-1). This phenomenon is the combined result of the return of CF₂-rich gas as the central region cools and contracts, along with gas-phase reactions producing CF₂. The central CF density shows only a slight initial increase in the afterglow, followed by a rapid decay (600 s^-1), due to gas-phase chemical reactions, e.g. with F₂ or CF₃, converting it to CF₂, processes which are also likely to be important in the steady state.