CVD of titanium oxide coatings: Comparative evaluation of then-nal and plasma assisted processes

Deposition of titanium oxide (TiO2) films has been investigated in a pulsed DC plasma assisted CVD process (PACVD) using titanium isopropoxide, Ti(OPri)(4), as the metal-organic source. The coatings were compared with TiO2 films obtained by a thermally activated decomposition of Ti(OPri)(4) in a LPCVD process and characterized with respect to their morphology, microstructure, chemical composition and mechanical properties by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), photoelectron spectroscopy (XPS) and micro-indentation studies, respectively. While nanostructured deposits of crystalline TiO2 could be obtained in the LPCVD process at temperatures as low as 350 degrees C, the PACVD films were identified to be amorphous under different deposition conditions. On the other hand, the plasma assisted CVD produced dense and mechanically stable TiO2 films at temperatures below 200 degrees C showing less residual carbon contamination when compared to LPCVD coatings. Despite the similar chemical composition, the precursor fragmentation and surface reactions (diffusion, nucleation and grain growth) were different in the two cases and found to be sensitive to the energy mode. Whereas high substrate temperatures forced nucleation and crystallization in thermally activated process, incomplete decomposition of organic ligands caused high carbon contamination in the films. The fragmentation of precursor was found to be more effective in PACVD process as indicated by low residual carbon contamination in the films. Nevertheless, the PACVD films possessed an amorphous character and a post-annealing step was necessary to obtain crystalline deposits, which showed hydrophilic behaviour stimulated by UV irradiation. While the deposition temperature was found to be a key parameter for the LPCVD process which affects morphology and microstructure, the chemical and mechanical properties in the PACVD process were strongly influenced by plasma power variations, e.g. discharge voltage and gas composition. (c) 2006 Elsevier B.V. All rights reserved.