Preparation of nanoporous silica films with low dielectric constant

Crack-free homogeneous nanoporous silica films on a silicon wafer were synthesized through supercritical drying of wet gel films that were obtained by spin coating the polymeric silica sol, followed by aging in isopropanol(IPA) solution. Polymeric silica sol was prepared by adopting an acid/base two-step procedure using tetraethoxysilane(TEOS) as precursor. The viscosity-time curves of polymeric silica sol were analyzed by nearly linear growth model and fractal growth model, The structural evolution of silica sol was determined. The optimized viscosity of polymeric silica sol for spin coating was in the range of 9 mPa.s ∼ 15 mPa.s. The particle size of silica sol suitable for spin coating is dependent upon the sol concentration. The thickness of the nanoporous silica films was 400 nm∼ 1 000 nm; The refractive index was 1.09∼ 1.24 and the dielectric constant was 1.5∼ 2.5. The nanoporous silica films showed three-dimensioned network, cross-linked by the primary particles with the size distribution between 10 nm∼ 20 nm.