Chemical beam deposition of titanium dioxide thin films

Titanium dioxide (TiO2) is a widely investigated material for its biological compatibility, high dielectric constant and refractive index, chemical and mechanical resistance, and catalytic activity. Several different techniques are available to produce TiO2 thin films, which are variously suited to provide the properties required for a specific application. Dense TiO2 coatings with high refractive index and low optical absorption are aimed at for optical applications. High thickness uniformity on large area coatings and a high, precisely controlled, growth rate are further advantages. A reactor for the production of optical coatings at 1% thickness uniformity on 150 mm diameter substrates has been designed according to a mathematical model, which has been extended to evaluate growth rates and precursor efficiency. The reactor design has been made suitable for light assisted TiO2 growth, so as to achieve selective or improved deposition or to locally modify the coating properties. Along this line, a light selective deposition of 3 µm resolution by mask projection is anticipated. The results obtained by operating the reactor in the Low Pressure Chemical Vapour Deposition (LPCVD) and Chemical Beam Deposition (CBD) indicate that the latter mode results in coatings with superior optical properties. Thickness uniformity better than 3% and a precursor impinging efficiency very close to the 5% calculated value have been achieved in the growth rate range from 50 to 1000 nm h-1. The causes of these small discrepancies are discussed and some reactor design improvements are proposed. The estimated resolution limit by mask projection has also been achieved by PET laser ablation, while for TiO2 a selective deposition of 30 µm structures has been obtained. Several characterization methods have been used to investigate the chemical composition (XPS, FTIR, Raman spectroscopy), the crystalline phase (XRD, GAXRD, Raman spectroscopy, TEM) and the morphology (XRR, TEM, SEM, AFM, RBS) of the thermally grown TiO2 films. Pure TiO2 with less than 1% carbon content with anatase crystalline phase is obtained in all the conditions explored (substrate temperature between 330 and 550°C). It is also shown that the anatase content increases with increasing substrate temperature and film thickness. The classical columnar growth related to the anatase crystalline phase is obtained with nanograins of smaller diameter than that of the columns (8-10 nm compared to 50-200 nm). Optical characterisation methods (photospectroscopy, spectral ellipsometry, and in situ Dynamic Optical reflectivity) have also been applied. Dense anatase has been obtained with refractive indexes n and k of 2.5 and 10-2 respectively for films up to 80-100 nm in thickness. For thicker films, an increasing porosity with increasing film thickness is observed resulting in enhanced Rayleigh scattering. A band gap blue shift is also observed when the Tauc model is applied due to increasing nanocrystallinity with th