Fabrication of Mo/Au-based Transition Edge Sensor

Abstract

Transition Edge Sensors (TESs) are devices which exploit the sharp transition of superconducting materials to use them as extremely sensitive detectors. Within the framework of the ESA mission ATHENA -an X-ray telescope to be launched in 2032- our group is developing X-ray detectors with Mo/Au TESs coupled to bismuth absorbers, aimed at detecting soft X-rays (1-10keV band). The TES sensor is a superconductor-normal metal bilayer with critical temperature typically between 50 and 400 mK. The bilayer takes the advantage of the proximity effect between both materials to control the critical temperature and thus the operation temperature of the device. Sensor contacts and wires are constituted by a superconductor material with a higher critical temperature than that of the bilayer, in order to avoid any heat load to the detector. In this contribution we describe the fabrication of TESs. This is very crucial and demanding, since there is a strong dependence of the Mo critical temperature on impurities and stress, and also because the critical temperature of proximity bilayers depends on the interface quality and individual layers’ thickness. Thus, sharp superconducting transitions require outstanding control of all the steps of the fabrication. The Mo/Au bilayers are deposited at room temperature on Si wafers covered by low stress Si3N4 in a so called trilayer design. First, Mo and a thin Au layer are sputtered in a UHV chamber. Afterwards, Au is deposited exsitu by electron-beam up to the desired thickness. The sensors are fabricated through several optical lithography steps. Nb/Mo wires are used. We have found that different etching techniques (wet, ion milling, RIE) produce different Mo/Au edge profiles, which have significant impact on the superconducting transition width. We have achieved a fabrication procedure with excellent yield and homogeneity, producing transition widths of less than 2 mK for TEsS with critical temperatures of 100 mK. With this process we have fabricated square and rectangular devices from 240x240 mm2 down to 10x10 mm2, over 4’’ wafers.