Optimization of RF- and DC-sputtered NbTiN films for integration with Nb-based SIS junctions

NbTiN is one of the most promising materials for use in the tuning circuits of Nb-based SIS milers far operating frequencies above the gap frequency of Nb (approximate to 700 GHz), We examine the properties of NbTiN films obtained using an unbalanced sputtering source in both rf and de operating regimes. It is found that the properties of NbTiN films are strongly affected by the total pressure of the sputtering process. Films obtained under lower pressures have higher compressive stresses and lower resistivities. The best NbTiN films are obtained by de sputtering and have a transition temperature of 14.4 K, a resistivity of 90 mu Omega.cm at 20 K, and a compressive stress of -1 GPa. Films with a resistivity of 110 mu Omega.cm at 20 K and a compressive stress of -0.5 GPa have been successfully used as a stripline material for Nb/Al-AlOx/Nb SIS junctions on fused quartz substrates.