Deposition of thick silica-titania sol-gel films on Si substrates

A process for production of thick (>10 μm) titania-doped silica films on Si substrates by repetitive spin-coating of sol-gel material and rapid thermal annealing for 10 s in the range 800–1200°C is described. The dependence of overall thickness and etch rate in buffered HF on annealing temperature is described, and it is shown that films annealed at low (< 1175°C) temperatures have a relatively large thickness and etch rate. However, films having the properties of fully densified material (minimum thickness and etch rate) can be produced by subsequent consolidation. The film stress characteristics are similar to those phosphosilicate glass formed by the same process: films annealed below a critical temperature (< 1075°C) are under tensile stress at the annealing temperature, and crack before a thick film can be built up. Refractive index data are given; these show that only fully consolidated films have the refractive index expected from their SiO₂ and TiO₂ compositions. Finally, discrepancies in results for thickness of unconsolidated single-layer and multilayer films are explained using a simple model that accounts for the effect of cumulative densification.