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

Abstract

The study of CF and CF2 radical production and loss mechanisms in capacitively-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 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 CF2 were strikingly different and more complex. This paper 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 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 CF2 concentration is high, causing the otherwise slow gas-phase concatenation reactions CXFY(CF2)n+CF2→CXFY(CF2)n+1 to occur. These processes produce 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 atom concentration, which simultaneously controls the concentrations of CF2, of chain initiating species such as CF3 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.