Annealing-Induced Conductivity Enhancement of Electroplated Bismuth for Transition-Edge Sensors

Transition-edge sensors (TES) as a sensitive lowtemperature superconducting detector have gained increasing popularity in hard X-ray detection applications, prompting extensive research on suitable absorber materials. Bismuth (Bi) has emerged as a promising candidate for hard X-ray detection due to its low specific heat capacity, good thermal conductivity, high quantum efficiency, and easiness to be handled with microfabrication methods. Previous studies have demonstrated that electroplating generally yields larger grain sizes in the Bi films compared to evaporation methods, effectively mitigating energy trapping events. Furthermore, there have also been reports that annealing improves the thermal conductivity of Bi. In this study, we electroplate 5 μm Bi films on 140 nm gold (Au) seed layers, and conduct analysis on samples that are annealed with different durations. The comparison reveals that annealing at 230 °C for several hours can increase the Bi grain size by 3 to 4 times, and enhance the residual resistivity ratio (RRR) of the Au-Bi absorber nearly triples. This discovery will help us understand the performances of the annealed electroplated Bi films and provide guidance for future X-ray superconducting TES microfabrication and optimization.