High mass positive ions and molecules in capacitively-coupled radio-frequency CF4 plasmas

The positive ions and neutral radicals arriving at the earthed walls of a c apacitively-coupled radio-frequency pure CF4 plasma were analyzed using a q uadrupole mass spectrometer adapted for high masses. Experiments were perfo rmed at 50 and 200 mTorr, in an empty reactor and with Si and SiO2-coated S i substrates on the powered electrode. High mass ions and neutrals were det ected, up to 500 and 300 amu, respectively. The abundance of high-mass spec ies was greatest in the presence of silicon wafers and at higher pressure. The observed ion masses can be separated into distinct series, originating from different initial bases to which successive CF2 units have been added. We, therefore, propose that these high-mass species are the result of a ga s phase polymerization process consisting of CF2 addition reactions, in agr eement with a model proposed recently by our group. The influence of a sili con substrate derives primarily from the strong decrease that it induces in the concentration of F atoms, which otherwise limit the concentration of C F2 and of chain initiating species. (C) 1999 American Institute of Physics. [S0021-8979(99)05411-0].