Tuning the superconducting critical temperature of a four-layer Ti-Au-Ti-Au thin film stack for transition edge sensor detector applications

We have developed superconducting Ti transition edge sensors with Au protection layers on the top and bottom. The bottom Au layer (deposited on a thin Ti glue layer) isolates the Ti from any substrate effects; the top Au layer protects the Ti from oxidation during processing and subsequent use of the sensors. We control the transition temperature of the sensors by varying the relative thicknesses of the Ti and Au layers. Films derived from these studies have been successfully deployed to the South Pole Telescope as part of the SPT-3G experiment. Here, we present new work exploring the underlying physics associated with these films. We show that our measured films are well-described by a N-S-N model derived from the Usadel equations (where we ignore the bottom Ti glue layer). From this model we infer that the transition temperature is roughly six times more sensitive to the thickness of the bottom Au layer than to that of the top Au layer. We present preliminary results investigating this asymmetry.