Dielectric constant and stability of fluorine doped PECVD silicon oxide thin films

As device geometry shrinks to 0.35 mm and below, the parasitic capacitance between closely spaced metal lines becomes important in terms of RC time delay in device switching. In this study, we investigated the use of fluorine doped plasma enhanced chemical vapour deposition (PECVD) grown silicon oxide thin film as a low dielectric constant inter-metal dielectric (IMD) material. We evaluated C2F6 and an organometallic liquid source as the F dopant. It was found that the dielectric constant generally decreased as the F concentration increased. However, we also found that above certain F concentration, the F-doped oxide film would become unstable and absorb moisture from ambient. At similar F concentrations, the films deposited with the liquid source are always more stable than those deposited with C2F6. This, in addition to the different stress versus low frequency rf power behaviour between the two types of film, suggests that the already present Si-F bond in the liquid precursor results in a denser and more stable film comparing to the C2F6, doped film where interstitial F may be present.