CF_x radical production and loss in a CF₄ reactive ion etching plasma: Fluorine rich conditions

Space and time resolved laser induced fluorescence, combined with absolute calibration techniques, were used to probe the production and loss mechanisms of CF and CF₂ radicals in capacitively coupled 13.56 MHz plasmas in pure CF₄ at 50 and 200 mTorr. Under these conditions (pure CF₄, with no etched substrate) the gas-phase atomic fluorine concentration is high, minimizing polymer formation on the reactor surfaces. Fluorine-poor conditions will be considered in a following paper. Steady state axial concentration profiles show that, under many circumstances, the (aluminum) rf powered electrode is a net source for these radicals, whereas the grounded (aluminum) reactor surfaces are always a net sink. The summed fluxes of CF and CF₂ produced at this surface were found to be comparable to the incident ion flux. We propose therefore that CF_x radicals are produced by neutralization, dissociation, and reflection of the incident CF⁺_x ions under these conditions. This mechanism often predominates over the gas-phase production of these species by direct dissociation of CF₄, and accounts for the unexpectedly high concentrations observed. The difference in behavior between the powered and grounded electrode surfaces is explained by the difference in the incident ion energy and mass distributions.