Study on reactive sputtering of yttrium oxide: Process and thin film properties

This paper investigates the influence of deposition conditions on the properties of yttrium oxide thin films. The paper focuses on the texture, optical and mechanical properties. With this objective, a series of yttrium oxide thin films with different thicknesses were deposited by direct current (DC) unbalanced reactive magnetron sputtering at high and low pumping speed. By changing the oxygen flow, depositions were performed in the three characteristic deposition modes for reactive magnetron sputtering, i.e., metallic, transition and poisoned mode. By using an oxygen flow directed to the substrate, full oxidation of the samples, as shown by X-ray photoelectron spectroscopy (XPS), in the three modes is obtained. Crystallographic characterization by X-ray diffraction (XRD) shows that films crystallize in the cubic phase with a strong (222) out-of-plane orientation at low oxygen flow. As the oxygen flow increases a mixture of cubic and monoclinic phase is obtained. In poisoned mode, the films consist of the cubic phase with preferred (420) orientation. Scanning electron microscopy (SEM) cross sections show, with increasing oxygen flow, a loss of the columnar structure. As the oxygen flow rates increase through the metallic, transition, and the poisoned mode, the grain size becomes gradually smaller. An overview diagram of all experimental results uncovers that the textural changes are closely linked to the oxygen partial pressure rather than the oxygen flow. The optical properties of films were investigated by spectroscopic ellipsometry (SE). The films with a columnar structure demonstrate superior hardness and modulus as well as the high plasticity.