Low dielectric constant nanoporous SiO2 films formed by twice-modification processing

Silica sol synthesized by a two-step acid-base catalyst procedure was deposited on Si (100) substrate by spin coating. Then the wet gel film was modified twice with trimethylchlorosilane (TMCS). The proper annealing temperature of the film, determined by Fourier transform infrared spectroscopy (FTIR) and Thermogravimetric and Differential thermal analysis (TG-DTA), is 300 °C. The twice-modified film doesn't shrink and can keep continuous while drying. The differences brought by surface modification mainly depend on the compositional changes (hydrophobic -CH3 groups substituted for hydrophilic -OH species) of the film. The network structure of the twice-modified SiO2 porous film was clearly observed by scanning electron microscopy (SEM). The pore size about 70-80 nm can be seen from SEM and transmission electron microscope (TEM). As for the films with no modification, only 23.6% porosity can be obtained and its corresponding dielectric constant (k) is 3.29. However, with twice modification, the porosity increases to 73.5% and k reaches 1.8.