Measurement of absolute CO number densities in CH₃F/O₂ plasmas by optical emission self-actinometry

CH₃F/O₂ inductively coupled plasmas at 10 mTorr were investigated using optical emission spectroscopy. A ‘self-actinometry’ method was developed to measure the absolute number density of CO that formed in reactions following dissociation of CH₃F and O₂ in the plasma. In this method, small amounts of CO were added to the plasma, leading to small increases in the CO emission intensity. By carefully accounting for small perturbations to the plasma electron density and/or electron energy distribution, and by showing that very little of the CO added to the plasma was decomposed by electron impact or other reactions, it was possible to derive absolute number densities for the CO content of the plasma. With equal fractions (0.50) of CH₃F and O₂ in the feed gas, the CO mole fraction as a function of plasma power saturated at a value of 0.20–0.25. As O₂ in the feed gas was varied at a constant power of 100 W, the CO mole fraction went through a maximum of about 0.25 near an O₂ feed gas fraction of 0.5. The relative CO number densities determined by ‘standard’ actinometry followed the same functional dependence as the absolute mole fractions determined by self-actinometry, aided by the fact that electron temperature did not change appreciably with power or feed gas composition. (paper)