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.