Reasons for lower dielectric constant of fluorinated SiO₂ films

The electronic and ionic contributions to the overall dielectric constant of fluorinated SiO₂ films deposited in SiF₄ and O₂ plasmas were quantified from the refractive index measured by in situ spectroscopic ellipsometry in the visible-to-UV range and the infrared spectra taken by in situ attenuated total reflection Fourier transform infrared spectroscopy. The Kramers-Kronig dispersion relations facilitated the computation of ionic contribution to the dielectric constant from the IR absorbance spectra. The dependence of the dielectric response of SiO₂ films on the SiF₄-to-O₂ ratio (R) in the feed gas mixture revealed that F incorporation leads to a decrease in both electronic and ionic contributions, thus reducing the overall dielectric constant. The electronic component, for instance, comprised 1/3 of the total dielectric constant above the vacuum level and decreased with increasing F content until SiF₄-rich plasma resulted in a-Si incorporation. The rate of decrease, however, showed a sudden change at R = 0.25. Below the ratio, the Si-O-Si bond angle relaxation in the SiO₂ matrix and the subsequent density reduction were largely responsible for a moderate rate of decrease in the electronic contribution. Above this ratio, inclusion of voids caused a more pronounced decrease in the electronic contribution. The ionic component, which comprised less than 1/3 of the total dielectric constant, similarly decreased with increasing F content. This decrease was attributed to the replacement of more ionic Si-O bonds with Si-F bonds. The ionic contributions, whose characteristic vibrational frequencies appear below our experimentally observable range, constituted the remaining 1/3 and remained constant at 1, independent of fluorine concentration. Based on these observations, we propose a method to predict the total dielectric constant of SiOF films deposited in SiF₄/O₂ plasmas from a combination of ellipsometric and infrared absorption measurements.