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 o f Nb (approximate to 700 GHz), We examine the properties of NbTiN films obt ained using an unbalanced sputtering source in both rf and de operating reg imes. 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 be st NbTiN films are obtained by de sputtering and have a transition temperat ure of 14.4 K, a resistivity of 90 mu Omega.cm at 20 K, and a compressive s tress 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 m aterial for Nb/Al-AlOx/Nb SIS junctions on fused quartz substrates.