Low noise transition edge sensor (TES) for the SAFARI instrument on SPICA

Abstract

We fabricated and characterized a low thermal conductance (G) transition edge sensor (TES). The device is based on a superconducting Ti/Au bilayer deposited on a suspended SiN membrane. The critical temperature of the device is 78 mK. The low thermal conductance is realized by using narrow SiN supporting structures. All measurements were performed having the device in a light-tight box, which to a great extent eliminates the loading of the background radiation. We measured the current-voltage (IV) characteristics of the device at different bath temperatures and determine the thermal conductance (G) to be equal to 0.13 pW/K. The current noise and complex impedance are also measured at different bias points at a bath temperature of 30 mK. The measured electrical (dark) NEP is 4.2×10-19 W/√Hz, which is about a factor of 3 higher than what is expected from the thermal conductance that comes out of the IV curves analysis. We also measured the complex impedance of the same device at several bias points. Fitting a simple first order thermal-electrical model to the measured data, we find an effective time constant of about 1-4.5 ms and a heat capacity of 2-3 fJ/K.