Standardization and control of a dip-coating procedure for optical thin films prepared from solution

Abstract

The properties of dip-coated thin films prepared from colloidal silica and titania solutions have been examined. The effects of withdrawal rate, heat treatment, and solution concentration on film thickness, refractive index, and other properties have been studied. Results show that extremely uniform and reproducible thin films with refractive indices between 1.46 and 2.08 and thicknesses up to 0.25 μm can be produced with a single dip. The refractive index of the SiO2:TiO2 composite films varies linearly with the volume fraction of TiO2, and the thickness varies linearly with withdrawal rate. The effects of drying on the final thin film properties are minimal; however, baking conditions have a significant effect, with films shrinking up to 50% and the refractive index increasing 40% during baking. The resulting films are hard, durable, and of good optical quality. We have studied these films for use as planar optical waveguides; however, the results are also applicable to films made for various uses such as optical coatings, filters, or protective layers.