Abstract: Crack-free homogeneous nanoporous silica films on silicon wafers have been synthesized via supercritical drying of wet gel films that were obtained by spin coating the polymeric silica sol using sol-gel method with tetraethoxysilane(TEOS) as precursor. The silica film was composed of Si－O－Si and Si－OR characterized by FTIR and was hydrophobic. But the silica film became hydrophilic because of Si－OH group after it was heat treated above 250 ℃ in air. So the silica film was modified by trimethylchlorosilane(TMCS) in order to get hydrophobic, thermal stable film with low dielectric constant. The modified silica film was characterized by FTIR, AFM and spectroellipsometer. The modified silica film can keep hydrophobic and its nanoporous structure at temperature lower than 450 ℃ in nitrogen. The sizes of SiO2 primary particles and pores of silica film became larger after modification. The modified silica film had higher porosity and lower dielectric constant. Its dielectric constant could be reduced to below 2.5.

Key words: Nanoporous silica film, 　Sol-gel, 　TMCS, 　Low dielectric constant(lowκ)