Origin of low dielectric constant of carbon-incorporated silicon oxide film deposited by plasma enhanced chemical vapor deposition

Dielectric properties of carbon-incorporated silicon oxide (SiOC) films deposited by plasma enhanced chemical vapor deposition (PECVD), using a bis-trimethylsilylmethane precursor, were compared with the dielectric properties of silicon oxide thermally grown in a furnace (thermal oxide) and also with oxide deposited by PECVD using a tetraethoxysilane (TEOS) precursor (PE-TEOS oxide). The electronic contribution to the dielectric constant of the three oxide films was calculated from their refractive indices measured at 632.8 nm by ellipsometry. Ionic contributions were computed from their IR reflection spectra, measured at 650–4000 cm−1, by using the Kramers–Kronig relation. The dipolar contribution was qualitatively analyzed from temperature dependence of the polarizability, on a per unit volume basis. The dielectric constant of the SiOC films, which was measured at 1 MHz, decreased from 4.2 to 2.3 as the carbon content increased from 0 to 19.6 at. %. Although there was a significant reduction of the dielectric constant, the electronic contribution was only slightly changed from 2.10 to 1.90, whereas the ionic polarization noticeably changed from 1.86 to 0.25. SiOC films showed considerable dipolar contribution when compared to thermal oxide and PE-TEOS oxide films, but the dipolar contribution in the SiOC films became negligible as the carbon content was increased. The variation of each contribution with the carbon content shows that carbon incorporation leads to a decrease in the electronic and ionic contributions. The reduction of the ionic contribution was the predominant factor leading to a decrease in the overall dielectric constant.