Reactive HiPIMS of oxides for optical coatings

ZrO2, TiO2 and Al2O3 thin films were deposited by reactive High Power Impulse Magnetron Sputtering (HiPIMS) by the use of a linear double magnetron. For reactive HiPIMS deposition from metallic targets, a partial pressure gas control was used to deposit films at different setpoints. A superimposed process with additional mid-frequencies was realized to overcome the problem of arcing induced by low-frequency HiPIMS sputtering. However, for reactive alumina sputtering arcing is still a problem due to the different erosion profiles on the target of HiPIMS and of standard pulsed magnetron sputtering, respectively. ZrO2 films deposited in the oxide mode showed a degradation when the process is driven in the HiPIMS mode. This is explained by the role of negatively charged oxygen ions bombarding the growing film. Partial pressure control significantly improved the optical and structural properties. TiO2 layers with high refractive index of up to 2.7 at 550 nm could be demonstrated with HiPIMS processes. The oxygen partial pressure influences the deposition rate, but had only a minor influence on the refractive index. Al2O3 layers with a high refractive index of 1.72 could be deposited with HiPIMS at moderate peak current densities. To our knowledge, this is the highest value found for an unheated, unbiased sputter process. X-ray diffraction measurements show that the alumina films are amorphous