Low dielectric constant silica based films which incorporate a large amount of nanometer sized pores are attractive candidates as interlayer dielectrics in future gigascale integrated circuits chip technology.
Nanoporous silica based films were deposited by surfactant templated self-assembly spin-on deposition (SOD). Other low-k materials with relatively low density silica based films were deposited by plasma enhanced chemical vapor deposition (PECVD), and some silica films were deposited by a CVD process. The SOD films showed higher porosity compared with the PECVD and CVD films, as measured by x-ray reflectivity, Rutherford back scattering, and ellipsometry measurements. These SOD films had lower dielectric constant compared with the PECVD and CVD films, as derived from electrical and optical measurements.
Most commonly, a porous silica thin film is modeled as a parallel or serial connection of two capacitors, one with air and the other with SiO2 as the dielectric medium. Our results of the correlation between the dielectric constant and the porosity for the SOD films fit well the Lorentz-Lorenz model, which is close to the lower limit serial model. For the PECVD and CVD films they agree with the Rayleigh model, which is close to the upper limit parallel model. These results suggest that the dielectric constant of the nonhomogeneous two phase (silica and air) nanoporous silica based films deposited by SOD technique is significantly lowered by forming closed air voids. The silica based films deposited by PECVD and CVD, consist of a homogeneous low density loose structure originating from the bonding nature, i.e., Si-CH3 bonds and the six-fold rinds of the Si-O bond, and therefore their dielectric constant is lowered to a smaller extent.