The interest in amorphous SiO₂ thin films for various applications in microelectronics or in optical coatings persists. The evaporation by electron gun, usually used for optical coatings, is a deposition method which leads to porous films. When the films are exposed to air, the porous microstructure allows a penetration of water molecules, which is followed by a silica hydrolysis and a stress relaxation. Dense materials can be prepared using ion assisted deposition, but isolated pores in the silica network could remain.

In this work, for any pores connectivity we present a method for evaluating the pore volume fraction in silica films. A series of silica films, evaporated by electron gun with or without ion bombardment, has been studied using visible and infrared ellipsometry. From infrared ellipsometric data, we have calculated the film dielectric function and have deduced the actual TO and LO mode frequencies, which are, by this method, film thickness independent.

We show that the study of both TO and LO mode frequencies brings independent information on the film microstructure. It is widely accepted that the TO mode frequency (near 1075 cm^-1) varies mainly with the density of the silica matrix. On the other hand, the LO mode frequency (near 1245 cm^-1), whose behavior has not been often examined, is relatively insensitive to the densification of the silica matrix; its variation is essentially due to change in porosity. We have established a general relation between LO frequency and porosity in silica films. In the case of films with pores largely connected, the evaluation of the pore volume fraction from the LO frequency is compared with that obtained from the analysis of the large absorption band near 3300 cm^-1 (H-OH and Si-OH stretching absorption band).