Nanocrystalline Alumina-Zirconia Thin Films Grown by Magnetron Sputtering

Abstract: Alumina-zirconia thin films have been deposited using dual magnetron sputtering. Film growth was performed at relatively low-to-medium temperatures, ranging from ~300°C to 810 °C. Different substrates were applied, including silicon (100), and industrially relevant materials, such as WC-Co hardmetal. Both radio-frequency sputtering and direct-current magnetron sputtering were utilised to achieve a range of film compositions. The influence of sputtering target was investigated; both ceramics and metals were used as sputtering sources. Microstructural characterisation was performed with a range of electron microscopy and x-ray diffraction techniques which show that the pure zirconia was deposited in the monoclinic phase. Reduced mobility of depositing species, as in the case of direct-current sputtering, yielded preferred crystallographic orientation in the {100} directions. The initial nucleation layer consisted of the metastable tetragonal zirconia phase. This phase could be grown over film thicknesses ~1 ?m through the addition of ~3 at.% Al under similar low mobility conditions. For cases of higher mobility, as obtained through radio-frequency sputtering, the metastable cubic zirconia phase formed in the film bulk for alumina-zirconia nanocomposites. A combination of two mechanisms is suggested for the stabilisation of metastable zirconia phases: oxygen-deficiency and aluminium segregations with resultant restraint on the zirconia lattice. The sputter deposition process was investigated through energy resolved mass spectrometry in the case of radio-frequency sputtering; the sputter deposition flux contained a mixture of metallic ions, metaloxygen clusters, and oxygen ions. The presence of metal-oxygen clusters was found to be important in oxygen-stoichiometry and thus the phase selection of the resultant film. The energy distributions were similar when comparing sputtering from ceramic and metallic targets. A mass-balance model has also been developed for the transport phenomena and reactions of particles in reactive sputtering of two targets in a two-gas scenario for the alumina-zirconia system. Addition of nitrogen to the working gas was found to eliminate the hysteresis in the target poisoning for oxygen reactive sputtering. The higher reactivity of oxygen contributed to a higher oxygen content in resultant films compared to the oxygen content in the oxy-nitride working gas. The model was thus shown to be successful for tuning depositions in the alumina-zirconia oxy-nitride system.