SIGNIFICANT DENSIFICATION OF SOL-GEL DERIVED AMORPHOUS SILICA FILMS BY VACUUM-ULTRAVIOLET IRRADIATION

Structural changes in sol-gel derived amorphous silica films were inve stigated using vacuum ultraviolet light emitted from an undulator. A s ignificant increase in the refractive index and a decrease in the thic kness for the silica films were induced by photons with energies above similar to 9 eV. The frequency shift of omega(4) (transverse optical mode) of infrared absorption to a lower wave number with the irradiati on indicates a decrease in the average Si-O-Si bridging angle. These r esults suggest that the silica films were densified by the irradiation with the energetic photons. The densification of silica gel films dri ed at 50 degrees C was accompanied by a decrease of OH groups in a man ner similar to thermal treatments. On the other hand, the structure of the irradiated silica was similar to that of silica glass densified u nder high pressures. Although silica gel films annealed at 600-1000 de grees C were also densified by the irradiation, the structural changes were relatively small for the films annealed at temperatures above 70 0 degrees C. The significant densification induced by the irradiation is presumed to be due to the electronic excitation of the distorted st ructure as well as residual OH groups in sol-gel derived amorphous sil ica. (C) 1996 American Institute of Physics.