Development of Mo/Au transition edge sensors for cryogenic x-ray detectors

Abstract

The so-called Transition Edge Sensors (TES) , whose basic element is a superconducting film operating at cryogenic temperatures (~100mK), constitute high performance detectors for a very wide radiation range. Thus, they are considered essential for extremely sensitive astronomic instruments, either on ground or in space, such as EDELWEISS, CRESST, BICEP2, SCUBA2, ACTPol, SAFARI and ATHENA’s X-IFU, as well as for other applications in science and industry. TES X-ray detectors with extremely high spectral resolution have been fabricated by NASA and SRON, using respectively Mo/Au or Ti/Au bilayers as the sensing element and Bi or Au as absorber. We report here our progress in the fabrication of TES X-ray detectors based on Mo/Au bilayers, within the framework of the initiative to develop a european backup for the X-IFU detector. Mo/Au TES are fabricated on SiN membranes in Ultra High Vacuum conditions through a two-step deposition process, using sputtering and electron-beam deposition, followed by dry etching photolithography. Superconducting Nb wiring is used. Electrodeposited Bi is being developed as absorber. An advanced specific characterization setup has been implemented, involving complex impedance and I-V curve measurements using superconducting electronics in a dilution refrigerator (base temperature <30mK). The first results of characterization are presented.