Paper SE-TuM9
Effect of Thermal Oxidation and Annealing on the Structure and Morphology of Glancing Angle Deposited TiO_x Films

Tuesday, October 16, 2007, 10:40 am, Room 617

The typical Glancing Angle Deposited (GLAD) structure is an isolated, self-similar construct on a nanometer scale. Therefore, each structure can react to external influences independently of the other structures, as a separate, albeit similar system. This small scale system independence allows the nanostructures to exhibit different material characteristics than conventional thin films. As with other nanoscale structures, the large surface-volume ratio enhances surface effects, but the isolated GLAD structures are also effectively pinned on the substrate suppressing cross-structure grain growth and other microstructure propogation phenomena. Therefore, they provide an excellent surface engineering template to investigate the effects of annealing and thermal oxidation on thin film structure and morphology. Oxidation of titanium, with the existence of the metastable anatase and stable rutile polymorphs within the titania system, is significantly influenced by the microstructure obtained by GLAD films. Simple GLAD structures (pillars and chevrons) were deposited using reactive e-beam evaporation with a titanium source at various partial pressures of oxygen and argon. Additional process variables included deposition rate, flux angle of incidence, rotation speed, and substrate. Post deposition anneals were conducted in oxidizing, inert, and reducing atmospheres from 150-1200 °C. XRD patterns and SEM images served as the primary forms of film characterization. The combination of reactive evaporation at higher oxygen partial pressures and lower temperature anneals formed films with the highest percentage of the anatase phase. Depositions at lower oxygen partial pressures yielded films with a higher percentage of the rutile phase, which supports the role of oxygen vacancies as nucleation centers for the anatase to rutile transformation. Higher temperature annealing produced rutile films as expected from the thermodynamics of the titanium dioxode system, however the thermal stability of anatase phase structures was significantly enhanced by the GLAD microstructure. The titanium films also exhibit highly ordered crystallographic textures dependent on the flux angle of incidence for both ss-deposited films and after subsequent thermal oxidation. Finally, SEM images provide an interesting perspective on the dynamics of oxidation, grain growth, and sintering for a surface reaction dominated oxidation process.