Vacuum-controlled growth of high-quality α-Ta superconducting films for quantum circuits

High-quality superconducting α-Ta films are grown on Si and Al 2 O 3 by dc magnetron sputtering with Nb buffer layers. At a sputtering pressure of 4 mTorr, pure α-Ta (110) phase is obtained. Reducing base pressure from 2 × 10 −6 to 5 × 10 −7 Torr raises the superconducting transition temperature from 3.63 to 4.05 K on Si and from 3.59 to 4.07 K on Al 2 O 3 , while decreasing normal-state resistivity. Improved vacuum suppresses structural disorder, yielding smoother surfaces and denser microstructures. These results demonstrate that vacuum engineering offers an effective route for optimizing Ta films for scalable quantum technologies.