Study of a Mo–Au TES deposited directly on a freestanding membrane

Transition edge sensor devices made with Mo–Au proximity bilayer thermometers have been fabricated. The bilayers were DC-sputter deposited directly on 250 nm thick pre-etched Si3N4 membranes. Mo was deposited at 700°C, whereas Au was deposited at room temperature in the same pumpdown in order to allow for a good proximity effect. The superconducting transition temperature of devices with 43 nm Mo and 200 nm Au was around 190 mK. The transition width of bilayers deposited on membranes was substantially larger than those measured on samples deposited on the bulk substrate in the same fabrication run (≈2 mK). We consider this difference to be the result of a large temperature gradient across the membrane during the deposition process. Devices were tested with a single stage SQUID array kept at 2.2 K. The resolution of a device that has a transition width of 30 mK is 30 eV at 6 keV. A detailed study shows that the detector noise roughly scales with αI0, confirming the importance of thermal fluctuation noise in the thermometer.