Fabrication of ultrathin (∼100 nm), low-index nanoporous silica films for photonic devices: Role of substrate adhesion on the film thickness

Abstract

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 (∼1.458) or even of magnesium fluoride (∼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 (105nm thick, refractive index ∼1.24 at 632.8nm) 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.