Change of TO and LO mode frequency of evaporated SiO2 films during aging in air

The microstructural modifications, induced by atmospheric water vapor, in t he silica network of porous amorphous SiO2 films, evaporated by electron gu n and capped by a nonporous TiO2 layer, have been investigated. We have tak en advantage of infrared ellipsometric measurements to extract the infrared complex dielectric function of the SiO2 films in the 600-5000 cm(-1) range , from which we deduce the actual values of the frequencies of the Si-O-Si stretching vibrational modes [transverse optic (TO) and longitudinal optic (LO)]. The TO and LO frequency shifts are studied experimentally and reprod uced by simulation. Experimental data show the increase as a function of ti me in air of both TO and LO frequencies, and of the refractive index in the visible range. The film is modeled as a mixture of two constituents: a sil ica matrix, with variable density, and pores, into which water can penetrat e. We show that the TO frequency is mainly sensitive to the silica matrix d ensity, while the LO frequency gives information about the pore volume frac tion. We first demonstrate that our deposition method leads to films having initially a dense silica matrix, characterized by a high visible refractiv e index (1.543 as compared to 1.458 for fused silica). Second, we show that the strong relaxation of the silica network (characterized by the decrease of the silica matrix refractive index from 1.543 to 1.475), due to the wat er penetration in the pores, is accompanied by a decrease of the pore volum e fraction (from similar to 30% before venting down to similar to 15% after air exposure). (C) 2000 American Institute of Physics. [S0021-8979(00)0361 0-0].