Deposition of Niobium Films by Ultra-High-Vacuum Magnetron Sputtering

Due to its high superconducting transition temperature,niobium(Nb)is widely in superconducting electronics and quantum circuits. Nb films are usually deposited by conventional magnetron sputtering tools. To improve the film quality,it is important to use magnetron sputtering with ultra-high vacuum(UHV)background.Here the effects of UHV sputtering parameters on the surface roughness,grain size,transition temperature,and residual resistance ratio of the niobium films have been studied. Coplanar waveguide resonators made from a Nb film on silicon show intrinsic quality factor of one million at single-photon excitation under millikelvin temperature. It suggests that the Nb film deposited by UHV sputtering is of high quality and ultra-low microwave loss.