Superconducting/metal bilayers for radiation detectors

Abstract

Thin bilayers of superconducting/metal materials allow the development of very sensitive thermometers with tunable Tc in the range of 100mK with outstanding spectroscopic capabilities for radiation detectors. These Transition Edge Sensors (TES) are in use in a wide range of applications and have been proposed for future experiments and missions like ESA’s X-ray telescope ATHENA. Ti/Au and Mo/Au are typical choices of materials for the fabrication of these bilayers. We report on the present status and recent results of the development of Mo/Au based sensors in our group as part of the ATHENA collaboration. A new fabrication site with respect to previous efforts has been selected and new protocols, with performance and reproducibility in mind, developed. The sensors are sputtered on a Si substrate with a SiN membrane. The trilayer approach is followed which implies the sputtering of 15nm of Au over the Mo layer for passivation and the completion of the desired Au thickness by e-beam deposition, which improved Au conductivity. The sputtering process was optimized with respect Ar pressure and applied power. Different approaches were also tested which include, DC sputtering, RF sputtering and a novel DC+RF co-sputtering. A lithographic process develops the final size and shape of the sensor with Nb wires and contacts for signal reading and polarization. Design was optimized with respect sensor size, membrane size,stability, resolution, noise and time response. In parallel, specific studies of the physics of the bilayers have been carried out. Detailed analysis of critical currents and fields has been performed. Besides, the dependence of Tc on the widths of the bilayers have been obtained and compared with theoretical models. Also important in these studies have been the precise measurement of resistivities of materials as a function of width. As a result, independent tuning of Tc and Rn for ours sensors can be obtained, which can be of interest to optimize the detectors for different applications. The development of the TES has been completed with the optimization of the growing process of Bismuth absorbents and the integration of them into the sensors in order to produce fully operational pixels for X-ray detection.