REASONS FOR LOWER DIELECTRIC-CONSTANT OF FLUORINATED SIO2-FILMS

The electronic and ionic contributions to the overall dielectric const ant of fluorinated SiO2 films deposited in SiF4 and O-2 plasmas were q uantified 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 spe ctroscopy. The Kramers-Kronig dispersion relations facilitated the com putation 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-O-2 ratio (R) in the feed gas mixture revealed th at F incorporation leads to a decrease in both electronic and ionic co ntributions, thus reducing the overall dielectric constant. The electr onic component, for instance, comprised 1/3 of the total dielectric co nstant above the vacuum level and decreased with increasing F content until SiF4-rich plasma resulted in a-Si incorporation. The rate of dec rease, however, showed a sudden change at R = 0.25. Below the ratio, t he Si-O-Si bond angle relaxation in the SiO2 matrix and the subsequent density reduction were largely responsible for a moderate rate of dec rease in the electronic contribution. Above this ratio, inclusion of v oids caused a more pronounced decrease in the electronic contribution. The ionic component, which comprised less than 1/3 of the total diele ctric constant, similarly decreased with increasing F content. This de crease 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, constit uted the remaining 1/3 and remained constant at 1, independent of fluo rine concentration. Based on these observations, we propose a method t o predict the total dielectric constant of SiOF films deposited in SiF 4/O-2 plasmas from a combination of ellipsometric and infrared absorpt ion measurements. (C) 1998 American Institute of Physics.