Controlling T c of iridium films using the proximity effect

A superconducting Transition-Edge Sensor (TES) with low-Tc 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 Tc 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-Tc films. In the first approach, an Ir film sandwiched between two Au films, a Au/Ir/Au trilayer, has a tunable Tc 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 Tc in the same range. We present a detailed study of fabrication and characterization of Ir-based low-Tc 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.