Controlling T-C Of Iridium Films Using The Proximity Effect

A superconducting Transition-Edge Sensor (TES) with low-T-c is essential in high resolution calorimetric detection. With the motivation of developing sensitive calorimeters for applications in cryogenic neutrinoless double beta decay searches, we have been investigating methods to reduce the T-c of an Ir film down to 20 mK. Utilizing the proximity effect between a superconductor and a normal metal, we found two room temperature fabrication recipes for making Ir-based low-T-c films. In the first approach, an Ir film sandwiched between two Au films, a Au/Ir/Au trilayer, has a tunable T-c in the range of 20-100 mK depending on the relative thicknesses. In the second approach, a paramagnetic Pt thin film is used to create the Ir/Pt bilayer with a tunable T-c in the same range. We present a detailed study of fabrication and characterization of Ir-based low-T-c films and compare the experimental results to the theoretical models. We show that Ir-based films with a predictable and reproducible critical temperature can be consistently fabricated for use in large scale detector applications.