[formula omitted] production and loss mechanisms in fluorocarbon discharges: Fluorine-poor conditions and polymerization

The study of CF and [formula omitted] radical production and loss mechanisms in capacitively-coupled 13.56 MHz [formula omitted] plasmas has been extended to [formula omitted] plasmas with an Si substrate, and to [formula omitted] plasmas, conditions where the atomic fluorine concentration is lower and where more polymer deposition 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 [formula omitted] were strikingly different and more complex. This paper focuses on the [formula omitted] 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 [formula omitted] concentration is high, causing the otherwise slow gas-phase concatenation reactions [formula omitted] to occur. These processes produce high-mass neutrals (and ions) which are the real polymer precursors. The [formula omitted] 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 [formula omitted] of chain initiating species such as [formula omitted] 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.