CF2 production and loss mechanisms in fluorocarbon discharges: Fluorine-poor conditions and polymerization

The study of CF and CF2 radical production and loss mechanisms in capacitiv ely-coupled 13.56 MHz CF4 plasmas has been extended to CF4 plasmas with an Si substrate, and to C2F6 plasmas, conditions where the atomic fluorine con centration is lower and where more polymer deposition occurs on the reactor surfaces. Processes in the gas phase and at the reactor surfaces were inve stigated by time resolved axial concentration profiles obtained by laser in duced fluorescence, combined with absolute calibration techniques. The resu lts for CF were similar to those observed in the fluorine rich case, wherea s the results for CF2 were strikingly different and more complex. This pape r focuses on the CF2 radical, which, under these conditions is produced at all of the surfaces of the reactor, apparently via a long-lived surface pre cursor. The results can only be explained if large polymeric ions and/or ne utrals are produced by polymerization in the gas phase. The gas-phase CF2 c oncentration is high, causing the otherwise slow gas-phase concatenation re actions CXFY(CF2)(n) +CF2-->CXFY(CF2)(n+1) to occur. These processes produc e high-mass neutrals (and ions) which are the real polymer precursors. The CF2 radical therefore circulates in a closed cycle between the surface and the gas phase. The degree of polymerization is controlled by the fluorine a tom concentration, which simultaneously controls the concentrations of CF2, of chain initiating species such as CF3 and of dangling bonds on the growi ng oligomers. This model appears to apply to fluorocarbon discharges in gen eral, and agrees well with other results presented in the literature. (C) 1 999 American Institute of Physics. [S0021-8979(99)04908-7].