Effects of geometry and hardware on the stress of Nb thin films

It has been widely reported that the intrinsic stress of Nb films is an important parameter with respect to film quality and electrical characteristics of superconducting-insulating-superconducting (SLS) junctions. In particular, the quality of electrical characteristics for small functions is critically dependent on film stress unless a lift-off process is used to pattern the trilayer films. We have discovered that the measured total stress of our Nb films, calculated with a laser based wafer curvature measurement system, is very dependent on the method employed to hold the wafer to the substrate table. Special care must be taken to avoid hardware effects on the stress results. Using two guns of different manufacture in the same vacuum system, we also compared the stress characteristics and the dependence of film stress on gun to wafer distance, angle of deposition, substrate and gun temperature and substrate material.