New radical control method for high-performance dielectric etching with nonperfluorocompound gas chemistries in ultrahigh-frequency plasma

A new method for controlling radical generation in the etching of silicon dioxide is described. In an ultrahigh-frequency (UHF) plasma, the mean electron energy is about 2-3 eV and there are a small number of high-energy electrons. The plasma mainly dissociates C-I (2.4 eV), C=C (2.8 eV), C-C (4.3 eV), and C-Br (3.0 eV) bonds in the CF₃I, C₂F₄, CF₂Br₂, C₂F₆, and C₄F₈ plasmas, and it mainly generates CF₃ and CF₂ radicals because the bond energies of these bonds are lower than the bond energies of C-F (5.6 eV in CF₄) bonds. We found that the densities of these radicals were inversely proportional to the bond dissociation energy in these gases. That is, we found that C-I and C=C bonds are ideal for selective radical generation in the UHF plasma. The ratio of each radical density can be precisely controlled by changing the ratio of the mixture of these gases. As a result, etching selectivity and etching rate are improved considerably. From an environmental viewpoint, CF₃I and C₂F₄ are also good alternatives to perfluorocarbon chemistries because they are believed to have a very short life in the atmosphere.