Mo-Cu bilayers as Transition Edge Sensors for X-ray astrophysics

We are developing Transition Edge Sensor (TES) thermometers for X-ray astrophysics applications [1]. We are using molybdenum-copper bilayers to obtain a transition temperature around 60 mK. It is known that the shape of the device edges may affect the characteristics of the transition. We have therefore developed a process for undercutting the bottom layer of Mo that gives a sharp transition of the sample. Since we plan to fly these detectors in a magnetic refrigerator, we studied the effect of small magnetic fields on the bilayer transition. We investigated the dependence of the transition properties on the bias current. Studying the detector noise level as a function of bias current and sensor resistance at the working point, we found that the devices have a small excess noise that seems to be a function of both. In particular, this excess noise appears only at high current density.