Effect of bias voltages on the microstructures and superconductive properties of Nb films deposited via HiPIMS

Background The Nb/Cu film superconducting cavities offer several enhancements over traditional bulk niobium RF cavities, including improved mechanical and thermal stability, as well as a reduced sensitivity to DC magnetic fields. Despite these advantages, Nb/Cu film cavities produced via DC magnetron sputtering often exhibit a pronounced Q-slope issue, potentially due to the low-energy deposition process. In contrast, high-power impulse magnetron sputtering (HiPIMS) allows for greater peak power by employing a small duty cycle, to generate a higher ionization rates of target atoms, which, in turn, can control the deposition energy of the particles through the substrate bias voltage adjustments, thereby improving film quality.
Methods Therefore, utilizing HiPIMS, we deposited Nb coatings on a 1.3 GHz dummy cavity and examined the impact of varying bias voltages on the films’ crystal structure, microstructure, and superconductivity. Superior superconducting Nb coatings were achieved, as confirmed by X-ray diffractometry (XRD) for crystal structure, scanning electron microscopy (SEM) for surface and cross-section morphologies, four-probe measurement for low-temperature resistivity, and vibrating sample magnetometry (VSM) for superconducting properties.
Results and Conclusion HiPIMS-deposited Nb coatings consistently displayed lattice constants approximating those of bulk Nb, tightly packed surface morphologies, and dense cross-sectional structures, with superconducting transition temperatures (Tc) reaching the theoretical value of 9.2 K. Notably, at a bias voltage of − 100 V, the coatings in the cavity’s cell region transitioned from a polycrystalline to a single-crystalline structure, exhibiting increased density and enhanced surface continuity, with a predominance of the body-centered cubic (BCC) Nb (100) phase. These findings underscore the potential of HiPIMS in fabricating superconducting cavities with superior qualities.