Properties of silica and silica-titania layers fabricated from silica sols containing fumed silica

Abstract

Silica layers fabricated by the sol-gel method are usually amorphous and exhibit a relatively low refractive index (n∼1.46 at about 0.6 μm), high optical transmittances at UV-VIS and near-IR ranges, as well as high thermal stability. Such layers can be used as high reflection or antireflection coatings, dielectric mirrors, bandpass filters, etc. However, crack-free layers with a maximum thickness of about 400 nm can be prepared in one coating cycle from sols of simple silicon alkoxides. This paper presents an approach for the preparation of crack-free silica layers that allows us to increase this maximum thickness. The approach is based on the dip-coating technique used for applying stable silica sols containing fumed silica. These sols were obtained from input silica sols based on tetraethyl orthosilicate (TEOS) with concentrations 1 and 2 mol/l, ethanol, HCl and water (RW 1.75). Fumed silica was dispersed in water/TritonX-100 solution and added into some of the input sols. Zeta potentials, average dimensions and diffusion coefficients of silica agglomerates in sols containing fumed silica are presented. Both types of sols were deposited onto silica slides and silicon wafers by the dip-coating technique using different withdrawing velocities 100,200 and 300 mm/min. Prepared silica gel layers were thermally treated at 450 or 900 °C. One of silica sols with fumed silica has also been used for the preparation of a silica-titania sol with a molar ratio Ti/Si = 3/7. Both these sols have been employed for the preparation of a Bragg mirror consisting of three pairs of high-index silica-titania and low index silica layers on a silica slide.The appearance and thickness of prepared single silica and silica-titania layers have been characterized by optical microscopy and scanning electron microscopy. Optical properties of layers have been determined by spectral ellipsometry and UV-VIS-NIR transmission spectrometry. The measurements have shown that characteristics of layers fabricated from both types of the sols differ and that one can obtain stable transparent silica layers with thicknesses up to 650 nm and refractive indices in a range from 1.41 to 1.44 at 600 nm using TEOS sols containing fumed silica. It has been found that larger layer thicknesses can be achieved by using silica sols with fumed silica, by increasing the alkoxide concentration, by using increased withdrawing velocities, and by decreasing heat-treatment temperatures.