CF 2 production and loss mechanisms in fluorocarbon discharges: Fluorine-poor conditions and polymerization

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Abstract
The study of CF and CF 2 radical production and loss mechanisms in capacitively-coupled 13.56 MHz CF 4 plasmas has been extended to CF 4 plasmaswith an Si substrate, and to C 2 F 6 plasmas, conditions where the atomic fluorine concentration is lower and where more polymerdeposition occurs on the reactor surfaces. Processes in the gas phase and at the reactor surfaces were investigated by time resolved axial concentration profiles obtained by laser induced fluorescence, combined with absolute calibration techniques. The results for CF were similar to those observed in the fluorine rich case, whereas the results for CF 2 were strikingly different and more complex. This paper focuses on the CF 2 radical, which, under these conditions is produced at all of the surfaces of the reactor, apparently via a long-lived surface precursor. The results can only be explained if large polymeric ions and/or neutrals are produced by polymerization in the gas phase. The gas-phase CF 2 concentration is high, causing the otherwise slow gas-phase concatenation reactions C X F Y ( CF 2 ) n + CF 2 →C X F Y ( CF 2 ) n+1 to occur. These processes produce high-mass neutrals (and ions) which are the real polymer precursors. The CF 2 radical therefore circulates in a closed cycle between the surface and the gas phase. The degree of polymerization is controlled by the fluorine atom concentration, which simultaneously controls the concentrations of CF 2 , of chain initiating species such as CF 3 and of dangling bonds on the growing oligomers. This model appears to apply to fluorocarbon discharges in general, and agrees well with other results presented in the literature.