Optical diagnostics of radio-frequency plasmas containing CHF3 and CHF3/O2: Laser-induced fluorescence of CF2, CF, and O atoms, and optical emission from H, F, and O

Laser-induced fluorescence (LIF) has been used to measure absolute concentrations of CF2, CF, and O atoms in a parallel-plate capacitatively coupled radio-frequency-driven plasma containing CHF3 and its mixtures with Ar and O2 at pressures between 50 and 500 mTorr. In CHF3 the spatial distribution of CF2 peaks at the driven electrode, and shows the importance of surface processes for its production. Time-resolved studies show evidence for its homogeneous chemical removal. CF concentrations are an order of magnitude lower than those of CF2, and removal by reaction with H atoms is consistent with time-resolved data taken on plasma extinction. For both radicals the absolute concentrations are higher than those found for similar plasmas in CF4. In the presence of O2 the fluorinated radical concentrations drop to below the detection limit, and the influence of surface removal processes is again invoked to explain the increase in O atom concentration observed when a small amount of CHF3 is added to a dominantly O2 gas flow. Optical emission from excited F, H, and O atoms is observed, and even when corrected by actinometry, is shown to be an unreliable indicator of the relative concentration of the ground-state species because of contributions from dissociative excitation of stable species in the discharge. Excited H atoms are found to be translationally hot from measurements of their linewidths, and are clearly not all formed from excitation of H atoms. Time-resolved actinometry (TRA) can be used in some cases to remove the contribution to the emission from dissociative excitation, but in the case of O atoms where both LIF and TRA were compared, the influence of a time-dependent dissociative excitation step complicates the analysis and is attributed to the presence of other species such as O2(a1Δg) in the discharge