In situ sputtering growth and characterization of MgB2 films for microwave applications

High-quality c-axis oriented MgB₂ superconducting thin films were grown on MgO and Al₂O₃ single crystal substrates by a d.c. planar magnetron sputtering technique. The films were obtained starting by Mg rich Mg-B precursor films, subsequently annealed in situ at 800 °C for 10 min in a In sealed Nb box in the presence of saturated Mg vapor. The process is highly reproducible and can be easily scaled to produce large area films. Resistive transition of the 1 μm thick resulting films showed a maximum critical temperature T_c of 35 K and a transition width lower than 0.5 K. The residual resistivity ratio was 1.6 for the best samples. The films were characterized by a variety of structural and electronic techniques including profilometry, XRD, EDS, and STM-AFM analyses, critical current, upper critical field and penetration depth measurements. In view of possible device applications, the microwave response has been studied in detail. The measured dependence of the surface impedance from temperature and r.f. field amplitude at 20 GHz via a dielectric resonator technique is reported and discussed.