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 capacitively-coupled radio-frequency pure CF₄ plasma were analyzed using a quadrupole mass spectrometer adapted for high masses. Experiments were performed at 50 and 200 mTorr, in an empty reactor and with Si and SiO₂-coated Si substrates on the powered electrode. High mass ions and neutrals were detected, up to 500 and 300 amu, respectively. The abundance of high-mass species 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 gas phase polymerization process consisting of CF₂ addition reactions, in agreement with a model proposed recently by our group. The influence of a silicon substrate derives primarily from the strong decrease that it induces in the concentration of F atoms, which otherwise limit the concentration of CF₂ and of chain initiating species.