One solution to signal delay in Very large-scale integrated circuits is to use a low dielectric constant interlayer film, such as F-doped silicon dioxide (SiO2). By adding CF4 to SiH4/N2O plasma-enhanced chemical vapor deposition, we obtained F-doped SiO2 films with a dielectric constant as low as 2.6. We studied the mechanism behind this decrease in the dielectric constant by estimating the constants due to each polarization component (ionic, electronic, and orientational) using capacitance-voltage (C-V) measurements, Fourier transform infrared spectroscopy spectra combined with the Kramers-Kronig relation, and spectroscopic ellipsometry. The Kramers-Kronig calculations showed that ionic polarization decreased with increasing CF4 concentration, whereas the electronic polarization remained almost constant. However, the low dielectric constant obtained by the C-V measurements could not be completely explained by the reduction in ionic and electronic polarizations but probably resulted from a decrease in the orientational polarization. Orientational polarization may be caused by Si-OH bonds and is mainly decreased by adding CF4. Therefore, an effective way to reduce the dielectric constant in F-doped SiO2 films is to reduce the orientational polarization.