Characterization of silica xerogel films by variable-angle spectroscopic ellipsometry and infrared spectroscopy

Silica xerogel films with low dielectric constants were prepared using a sol-gel spin coating method. The as-prepared films were further treated by hexamethyldisilazane to achieve the hydrophobization of the pore surfaces, by replacing hydrophilic silanol groups with hydrophobic trimethylsilyl (TMS) groups. The thickness and optical constants of the films were derived from variable-angle spectroscopic ellipsometry measurements. The determined refractive index decreases from 1.271 +/- 0.008 to 1.188 +/- 0.003 (values at 632.8 nm) while the porosity increases from 40.4 to 58.6% with the process parameters used. The Maxwell-Garnet approximation was used to relate the ellipsometric data to porosity. The IR absorption bands of CH species in TMS groups reveal that the surface area of the pores is larger in the samples with lower porosity.