Current-Dependent Resistance in TES Wiring Superimposed Nb Striplines

During the characterization of the demonstration model of the Cryogenic AntiCoincidence (CryoAC) Detector (ACS-10), a current-dependent parasitic resistance was found in series with the TES network on board the detector. Analysis was possible because the resistance rises for currents above 11 A, and is therefore not observed at low bias excitation. A comparison of measurements of the TES across its Nb wiring at different temperatures suggested that the source of resistance was in the wiring and not in the TES network. After several analysis of the wiring fabrication steps, FIB-FE-SEM studies of film sections and tests of niobium film quality, we understood that the parasitic resistance was due to point contact in the Nb step coverage caused by film cracks. The fracture was due to the wall steepness and thickness of the films, since rapid step coverage is less mechanically stable and the stress on the films is proportional to the fourth power of the thickness. Therefore, all thicknesses in the wiring were reduced to the minimum optimum step coverage values and the first negative lithography parameters were optimized to reduce the wall film angle. The samples after this optimization showed no current-dependent series resistance to TES.