Characterization of sputtered hafnium thin films for high quality factor microwave kinetic inductance detectors

Hafnium is an elemental superconductor which crystallizes in a hexagonal close packed structure, has a transition temperature TC 400 mK, and has a high normal state resistivity around 90 μΩ.cm. In Microwave Kinetic Inductance Detectors (MKIDs), these properties are advantageous since they allow for creating detectors sensitive to optical and near infra-red radiation. In this work, we study how sputter conditions and especially the power applied to the target during the deposition, affect the hafnium TC, resistivity, stress, texture and preferred crystal orientation. We find that the position of the target with respect to the substrate strongly affects the orientation of the crystallites in the films and the internal quality factor, Qi, of MKIDs fabricated from the films. In particular, we demonstrate that a DC magnetron sputter deposition at a normal angle of incidence, low pressure, and low plasma power promotes the growth of compressive (002)-oriented films and that such films can be used to make high quality factor MKIDs with Qi up to 600,000.