Thin films of silica aerogel on a silicon wafer have been synthesized via supercritical drying of wet gel films that were obtained by spin coating the polymeric silica sol, followed by aging in an isopropanol (IPA) or tetraethoxysilane/isopropanol (TEOS/IPA) solution. The viscosity of the sol and the spin rate required to form uniform coating layers were optimized. The film thickness and microstructures could be controlled by adjusting the concentration of the sol, the spin rate, and the aging conditions. The porosities and the dielectric constants of the thin films were in the range of 76%-90% (densities of 059-0.22 g/cm(3)) and 2.0-1.5, respectively. The degree of planarization in the aerogel thin film was similar to 100%, and the gap-filling capability on a 0.2 mu m tungsten patterning wafer was excellent. In particular, aging the wet gel film in the TEOS/IPA solution was very effective in improving the properties of the aerogel films.