The Ar dilution effects on the ultrahigh-frequency (UHF) plasmas through C4F8 and CF4 are studied by optical emission spectroscopy and Langmuir probe measurement. For the C4F8 plasma, the Ar dilution is found to extend the electron energy distribution function (eedf) toward the higher energy and increase the electron density, n(e). The n(e)-normalized CF2 emission intensity is decreased proportionally with the partial pressure of the C4F8 feedstock gas. Thus, the Ar dilution increases the ratio of the ion density to the CF2 density, which changes the balance between the etching and the polymer deposition and affects the SiO2 etching characteristics significantly. The polymer deposited on the reactor wall is found to affect the emission spectra of the C4F8 plasma. For the CF4 plasma, the Ar dilution does not change the eedf probably because of approximately the same threshold energy for the ionization processes between Ar and CF4. However, the Ar dilution increases the n(e) in the CF4/Ar plasma. Since CF2 radicals are inferred to be the higher order dissociation products of CF4 molecules, the increased n(e) results in the enhanced production of CF2 radicals. Approximately the same ratio of the ion density to the CF2 density in the CF4/Ar plasma as that in the C4F8/Ar plasma suggests that in the SiO2 etching process by high-density, low-pressure plasmas, the CF4/Ar plasma chemistry can be a substitution fur the C4F8/Ar plasma chemistry.