Characterisation of Step Coverage by Pulsed-Pressure Metalorganic Chemical Vapour Deposition: Titanium Dioxide Thin Films on 3-D Micro- and Nano-Scale Structures.
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
An examination of the possibility of applying pulse pressure metalorganic chemical vapour deposition (PP-MOCVD) to conformal coating and an investigation of PP-MOCVD processing parameters were undertaken using the deposition of thin, conformal titanium dioxide (TiO₂) on 3-D featured and non-featured substrates. The characterisation of the conformality and wettability analysis of thin TiO₂ was carried out using titanium tetraisopropoxide (TTIP) dissolved in toluene as a precursor and featured silicon (Si) and silicon nitride (Si₃N₄) as substrates. The features on the substrates were in micro- and nano-scale with the aspect ratio up to 2:1. The processing parameters investigated were temperatures between 400 and 600°C, reactor base pressures from 50 to 200 Pa, injection volumes between 50 and 250 µl, precursor concentrations in the range of 0.15 to 0.50 mol% and pulsing times from 10 to 20 sec. The surface morphology and thickness were examined using a scanning electron microscope (SEM). The composition of the films was qualitatively identified by energy dispersive X-ray spectroscopy (EDS). X-ray diffraction (XRD) and Raman spectroscopy were used to analyse the phase and grain size. The surface roughness and grain size were evaluated using atomic force microscopy (AFM). The optical properties were characterised using UV-VIS light spectroscopy. The anti-sticking characteristic was examined by wettability analysis, measuring the contact angle of the film with water. The research examined the relationships between processing parameters and growth rate, conformality, surface roughness, grain size, phase and water contact angle. A new measurement for thin film conformality was derived based on a statistical analysis of a large number of film thickness measurements on a fracture surface over the lithographed features. The best conformality of 0.95 was obtained for micro-scale features at the lowest temperature in the range of investigation, 400℃, with pulse exposure characterised by a base pressure of 100 Pa, TTIP concentration of 0.50 mol%, injection volume of 50 µl and pulsing time of 10 sec. Conformality for micro-scale features was in the range of 0.82 to 0.97 over a wide range of deposition temperatures. Conformality was as low as 0.45 over nano-scale structures at the higher exposure rate. The conformality decreased as the temperature and precursor concentration increased. The precursor injection volume was found to have minor influences on conformality. The growth rate increased as the temperature increased and reached the maximum at the deposition temperature of 450℃ with the precursor concentration of 0.50 mol% and injection volume of 100 µl. The base pressure and relaxation time had slight influences on the growth rate over the deposition temperature range of 400 to 500℃. The growth rate was increased as the precursor concentration and precursor injection volume increased. The deposited TiO₂ films exhibited columnar growth and anatase phase. The base pressure and pulsing time had no obvious effects on grain size and surface roughness. The grain size decreased as the deposition temperature increased. The surface roughness increased as the deposition temperature increased. Contact angles of over 100° were found with conformality of over 0.80. The variation in contact angle was related to the surface morphology of the deposited films. The contact angle increased as the grain size decreased. High wettability was found for films in the mid-range of pulse exposure, in this study at pulse exposure of 53, or at high deposition temperature, in this case at 600°C. The as-deposited TiO₂ thin films were hydrophobic depending on the surface morphology, surface roughness and grain size.