Crystalline alumina deposited at low temperatures by ionized magnetron sputtering

Abstract

Ionized magnetron sputtering based on the work of Rossnagel and Hopwood [J. Vac. Sci. Technol. B 12, 449 (1994)] has been used to deposit alumina films containing orthorhombic κ alumina and monoclinic θ alumina at substrate temperatures of 370 to 430 °C. An inductively coupled rf Ar/O2/Al discharge between the sputter source and the heated substrate table was used to effectively ionize not only Ar but more importantly Al and O2. Both ion energy as well as the ion flux to the substrate influence the structure and properties of the coatings. The ion energy was controlled by the substrate bias potential, and the ion flux by means of the rf power supplied to the coil. The effect of the degree of ionization and therefore the ion flux to the substrate was studied at a constant substrate bias potential of −70 V. It was found that as the ion flux to the substrate was increased, the film crystallinity increased (i.e. the Bragg diffraction peaks were sharper and had higher intensity). It was shown that the formation temperature for κ alumina can be drastically decreased if high fluxes of reactive Al ions are employed to increase the energy at the substrate. Depending on the ion energy and ion flux, films with hardness values usually reported for chemical vapor deposition (CVD) κ alumina could be deposited at a maximum substrate temperature of 430 °C as opposed to ∼1000 °C by CVD. The process described allows for the first time the deposition of κ alumina onto metallic substrates such as hardened high speed steel without softening it.