Investigation of Superconducting Ti/Ti-Au/Au Tri-Layer Films With a Co-Sputtering Process for Transition-Edge Sensors

The critical temperature (T_c) of superconducting Ti/Au bilayer films is crucial for the performance of transition-edge sensors. We use a co-sputtering technique to insert a Ti-Au mixture layer as an artificial diffusion layer between a superconducting Ti film and a normal Au bilayer. The Ti-Au mixture layers have different thicknesses and component ratios. The cross-section and element information of thin films was characterized by a high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectroscopy (EDS). The variation of T_c is measured for a series of Ti/Ti-Au/Au films. T_c is related to both the thickness and Ti-Au ratio in the mixture layer. We attempt to model the T_c variation based on the Usadel theory with an equivalent thickness ratio.