Surface Kinetics of Titanium Isopropoxide in Chemical Vapor Deposition of Titanium Dioxide and Barium Titanate

All chemical vapor deposition (CVD) processes rely on the adsorption and decomposition of precursors on a substrate to deposit the desired material. The growth rate of the film is determined by the surface kinetics of the utilized precursor molecules and generally dependent on precursor concentration and substrate temperature. Investigation of surface kinetics is challenging in CVD techniques as precise quantification of the precursor concentration (or precursor flux) is difficult. Furthermore gas-phase reactions can influence the film growth. In this thesis we develop a method to characterize precursor surface kinetics during deposition processes using high vacuum chemical vapor deposition (HV-CVD). In a high vacuum environment precursor transport takes place in the molecular flow regime and gas phase reactions are efficiently suppressed. Furthermore precursor impinging rates can be calculated using a relatively simple mathematical model. Relating the number of impinging precursor molecules to the absolute amount of deposited material allows investigating the deposition kinetics in a very precise manner. We apply this method to characterize titanium isopropoxide and water in chemical vapor deposition conditions over large parameter range. Fitting a surface kinetic model to experimental data enabled us to derive activation energies for desorption, hydrolysis and pyrolysis. The surface kinetic model is then applied to characterize the TTIP based HV-CVD and ALD process of titanium dioxide in detail. Furthermore, we have studied the surface kinetics of TTIP when the substrate is additionally exposed to barium precursors aiming to deposit barium titanate. We demonstrate that HV-CVD is capable of growing epitaxial barium titanate films on magnesium oxide, strontium titanate and strontium titanate buffered silicon substrates at 400°C process temperature; this is the lowest reported substrate temperature for epitaxial growth of barium titanate by CVD. Finally, we investigate two experimental techniques to selectively grow titanium dioxide: a lift-off technique based on shading masks and a surface passivation technique, based on the local modification of surface reactions. We will demonstrate, that the surface kinetics of TTIP are not optimal for the local deposition of titanium dioxide in the first case, but that HV-CVD is capable of enhancing selectivity compared to previously reported values in the latter approach.

Surface Kinetics of Titanium Isopropoxide in High Vacuum Chemical Vapor Deposition

Patrik Hoffmann, Yury Kuzminykh, Michael Oliver Reinke

Understanding the surface kinetics of precursor decomposition during thin film formation represents a key aspect in the understanding and engineering of chemical vapor deposition (CVD) and atomic layer deposition (ALD) processes. The determination of activation energies of the surface reaction steps, however, is often challenging because it requires precise knowledge of precursor impinging rates. Afterward, the kinetics can be investigated by comparing the amount of deposited material with the absolute precursor flow. Ideally, the experimental equipment allows a distinction between gas phase and surface reactions. Both are difficult to achieve in conventional CVD processes. A high vacuum environment, however, enables the quantitative prediction of precursor impinging rates due to the ballistic nature of precursor transport; additionally precursor gas phase reactions do not occur. We investigated the surface reaction kinetics of titanium isopropoxide (TTIP) in the high vacuum CVD of titanium dioxide. Additionally, we have investigated the addition of water as a reactani to the deposition process. In this way, even surface kinetics relevant for ALD processes could be investigated. Here, we propose a comprehensive surface kinetic model of titanium isopropoxide surface reactions including hydrolytic and pyrolytic reactions. The surface kinetic model was fitted to 363 data points taken from combinatorial experiments covering a wide range of deposition parameters (substrate temperature, 175-610 degrees C; TTIP impinging rate, 0.1 x 10(15)-6.0 x 10(15)cm(-2) s(-1); water impinging rate, 4.5 x 10(16)cm(-2) s(-1)), which enabled us to derive activation energies for desorption, hydrolysis, and pyrolysis.

American Chemical Society2015

Combinatorial Characterization of TiO2 Chemical Vapor Deposition Utilizing Titanium Isopropoxide

Patrik Hoffmann, Yury Kuzminykh, Evgeniy Ponomarev, Michael Oliver Reinke

The combinatorial characterization of the growth kinetics in chemical vapor deposition processes is challenging because precise information about the local precursor flow is usually difficult to access. In consequence, combinatorial chemical vapor deposition techniques are utilized more to study functional properties of thin films as a function of chemical composition, growth rate or crystallinity than to study the growth process itself. We present an experimental procedure which allows the combinatorial study of precursor surface kinetics during the film growth using high vacuum chemical vapor deposition. As consequence of the high vacuum environment, the precursor transport takes place in the molecular flow regime, which allows predicting and modifying precursor impinging rates on the substrate with comparatively little experimental effort. In this contribution, we study the surface kinetics of titanium dioxide formation using titanium tetraisopropoxide as precursor molecule over a large parameter range. We discuss precursor flux and temperature dependent morphology, crystallinity, growth rates, and precursor deposition efficiency. We conclude that the surface reaction of the adsorbed precursor molecules comprises a higher order reaction component with respect to precursor surface coverage.

ACS Publications2015

Selective Growth of Titanium Dioxide by Low-Temperature Chemical Vapor Deposition

Patrik Hoffmann, Yury Kuzminykh, Michael Oliver Reinke

A key factor in engineering integrated optical devices such as electro-optic switches or waveguides is the patterning of thin films into specific geometries. In particular for functional oxides, etching processes are usually developed to a much lower extent than for silicon or silicon dioxide; therefore, selective area deposition techniques are of high interest for these materials. We report the selective area deposition of titanium dioxide using titanium isopropoxide and water in a high-vacuum chemical vapor deposition (HV-CVD) process at a substrate temperature of 225 °C. Here—contrary to conventional thermal CVD processes—only hydrolysis of the precursor on the surface drives the film growth as the thermal energy is not sufficient to thermally decompose the precursor. Local modification of the substrate surface energy by perfluoroalkylsilanization leads to a reduced surface residence time of the precursors and, consequently, to lower reaction rate and a prolonged incubation period before nucleation occurs, hence, enabling selective area growth. We discuss the dependence of the incubation time and the selectivity of the deposition process on the presence of the perfluoroalkylsilanization layer and on the precursor impinging rates—with selectivity, we refer to the difference of desired material deposition, before nucleation occurs in the undesired regions. The highest measured selectivity reached (99 ± 5) nm, a factor of 3 superior than previously reported in an atomic layer deposition process using the same chemistry. Furthermore, resolution of the obtained patterns will be discussed and illustrated.

Amer Chemical Soc2015