Thermal characterization and noise measurement of NbSi TES for future space experiments

The principal observational demonstration of the theory of the Big Bang, the cosmic microwave background (CMB), has its maximum of intensity to the millimeter-length wavelengths. Instrumental progress allowed the development of bolometric detectors adapted to these wavelengths. Superconducting transition-edge sensors (TESs) are currentlyunder heavy development to be used as ultra sensitive bolometers. In addition to good performance, the choice of material depends on long term stability (both physical and chemical) along with a good reproducibility and uniformity in fabrication. For this purpose we are investigating the properties of NbSi thin films. NbSi is a well-known alloy for use in resistive thermometers. We are co-evaporating Nb and Si simultaneously. We present a full low temperature characterization of the NbSi films. In order to tune the critical temperature of the NbSi thermometers down to the desired range, we have to adjust the concentration of niobium in the NbSi alloy. In this experiment, we set for a Niobium concentration of 15%, to be able to run tested at a convenient temperature of 300mK. Tests are made using 4He-cooled cryostats, 300mK 3He mini-fridges, resistance bridge and a commercial SQUID. Parameters being measured are: critical temperature, resistance, sharpness of the transition and noise measurements.