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

Abstract

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 CFx radicals are produced by neutralization, dissociation, and reflection of the incident CFx+ 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.