Changes in orientational polarization and structure of silicon dioxide film by fluorine addition

Use of F-doped silicon dioxide (SiO₂) film as a low dielectric constant intermetal film for ultralarge scale integrated circuits (ULSI) is useful from the viewpoint of product cost and compatibility with present processing technologies. 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. The mechanism behind this decrease was investigated by estimating the dielectric constants due to the polarization components using capacitance-voltage measurements, Kramers-Kronig transformation, and spectroscopic ellipsometry. Fluorine addition lowers the orientational polarization by decreasing the number of -OH bonds, and this decrease in Si-OH concentration is a key factor which is responsible for the lower dielectric constant in the F-doped SiO₂ film. Orientational polarization due to the Si-OH bond disappears in the far infrared with increasing frequency, thus the dielectric constant is composed only of ionic and electronic polarization components in the frequency range beyond the far infrared. The quantitative analysis of fluorine and OH shows that excess Si-F bonds replace Si-O and form Si-F₂. We suggest that the water absorption for F-doped SiO₂ film can be minimized by suppressing fluorine addition at the time that the Si-OH bonds vanish in order to avoid forming Si-F₂ and destroying the threefold rings.