Reasons for lower dielectric constant of fluorinated SiO2 films

Sang M. Han, Eray S. Aydil

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72 Scopus citations

Abstract

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.

Original languageEnglish (US)
Pages (from-to)2172-2178
Number of pages7
JournalJournal of Applied Physics
Volume83
Issue number4
DOIs
StatePublished - Feb 15 1998

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