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Relationship between infrared absorption and porosity in silica-based sol-gel films

Infrared absorption spectra were recorded for silica and silica-titania sol-gel films spin-coated on silicon substrates. Information of two types is derived from those spectra concerning the porosity of the films, which was independently calculated from ellipsometry results: (1) the IR spectra allow the determination of the `infrared thickness' of the porous films, i.e., the thickness that they will exhibit after full densification (independent of the film porosity), which is always less than the thickness obtained by mechanical profilometry (or ellipsometry); also, the volume percent porosity can be estimated from those two thickness values; (2) as the films are densified at increasing temperatures, the IR thickness remains constant, but the profilometer (and ellipsometer) thickness decreases, whereas the frequency of the dominant Si- O stretching mode near 1070 cm-1 passes through a minimum, at approximately equals 500 degree(s)C for silica films and at approximately equals 450 degree(s)C for silica-titania (20 mol% TiO2), suggesting a maximum porosity for those heat treatment temperatures; this was confirmed by a minimum in the refractive index, in the case of pure silica films only. The significance of these findings is discussed.