Low refractive index, nanoporous silica films can enhance the reflectivity of multilayer omnidirectional reflectors by decreasing the refractive index of the low-index layer below that of dense silicon dioxide (similar to 1.458) or even of magnesium fluoride (similar to 1.4). Precise thickness and refractive index control of the low-index dielectric layer are required to achieve maximum benefit. In this article, we demonstrate successful processing and integration of quarter wavelength nanoporous silica films (105 nm thick, refractive index similar to 1.24 at 632.8 nm) for applications in omnidirectional reflectors. The low-index film's thickness was found to depend strongly on the choice of underlying substrate and for identical processing conditions, the film thickness decreased in the order Si > GaAs > GaSb. The thickness variation on these substrates was related to liquid-solid adhesion during spin coating and final film thicknesses were well correlated with the contact angle and spreading coefficient for the sol on the substrate. Two different models were evaluated to simulate the dependence of film thickness on the underlying substrate. The spin coating model proposed by Yanagisawa [J. Appl. Phys. 61, 1035 (1987)] introduces liquid slip at the solid-liquid interface and the model of [Adrienko et al., J. Chem. Phys. 119, 13106 (2000)] proposes the formation of an interfacial vapor layer that provides an effective slip at the interface. Calculated film thickness values using both models agree well with those obtained from the experiments. (c) 2006 American Vacuum Society.