Chemical beam deposition of titanium dioxide thin films

Abstract

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 thickness. To avoid this problem, SiO2 has been added so as to increase the temperature at which amorphous to anatase phase transition occurs, resulting in TiO2 amorphous phase deposition at 500°C substrate temperature with less than 10% SiO2 content. Alternating layers of TiO2 and TiO2-SiO2 as well as graded index films, both in the growth direction and perpendicular to the growth direction, have also been produced by thermal decomposition and their optical behaviour investigated. Laser assisted deposition has been used to improve optical properties of the film. A densification of the TiO2 anatase phase layer has been achieved resulting in higher refractive index n compared to pure thermal deposition even for films up to 400 nm thickness. A further densification with increasing laser fluence up to rutile phase transition is obtained. The band structure, extrapolated from spectral ellipsometry measurements, shows a progressive shift from the anatase to the rutile phase. Finally, the TiO2 selective deposition and the chemical composition variation in TiO2+SiO2 films as a function of laser repetition rate have been studied. In all cases the results obtained by means of the different characterization techniques are in mutual agreement and consistent with the global picture of the TiO2 growth resulting from the various studies reported in the literature.