Solid particle production in fluorocarbon plasmas II: Gas phase reactions for polymerization

Solid particles were observed in fluorocarbon (C4F8) plasmas applied to semiconductor plasma processing. In order to elucidate gas-phase reactions for particle formation, Fourier transform infrared (FTIR) spectroscopy was employed. The method showed that stable molecules (CF4, C2F6, and C2F4) were produced in the plasmas. Several absorption peaks found in a spectrum from a surface polymer were observed in the gas phase. The peaks were regarded to be from plasma-polymerized molecules in the plasmas. In particular, the absorption signals from species of -CF=CF- and -CF=CF2, which were produced from C2F4 molecules, were extinguished with the particle formation. Furthermore, densities of fluorocarbon radicals CFx (x = 1,2,3) were measured in CHF3 and C2F6 plasmas as well as in C4F8 plasmas with infrared laser absorption spectroscopy. The pressure dependence of the densities drastically changed with the presence of the particles. This was induced by sticking of the radicals onto gas-phase polymers including the particles. As the results obtained in the infrared spectroscopic methods, it was found that not these radicals but the C2F4 molecule as a reactive product played an important role in the formation of the particles. (C) 2002 American Vacuum Society.