Dielectric property-microstructure relationship for nanoporous silica based thin films

Abstract

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 have a higher porosity, compared to the PECVD/CVD films, as measured by x-ray reflectivity, Rutherford back scattering, and ellipsometry measurements. The SOD films have lower dielectric constants compared to the PECVD/CVD films, as derived from electrical (1 MHz) and optical (5×1014 Hz) measurements. The correlation between the dielectric constant and the porosity for the SOD films fits well to the lower prediction of the Lorentz–Lorenz model, and the PECVD/CVD films agree with the higher prediction of the Rayleigh model. These results suggest that the dielectric constant of the inhomogeneous two phase nanoporous silica based films deposited by SOD is significantly lowered by forming air voids, whereas the PECVD/CVD films consist of a homogeneous low density loose microstructure originating from the bonding nature alone, and therefore their dielectric constant is lowered to a smaller extent.