Low thermal conductance transition edge sensor (TES) for SPICA

Abstract

We fabricated and characterized low thermal conductance transition edge sensors (TES) for SAFARI instrument on SPICA. The device is based on a superconducting Ti/Au bilayer deposited on suspended SiN membrane. The critical temperature of the device is 113 mK. The low thermal conductance is realized by using long and narrow SiN supporting legs. 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 in different bath temperatures and determine the thermal conductance (G) to be equal to 320 fW/K. This value corresponds to a noise equivalent power (NEP) of 3x10-19 W/√HZ. The current noise and complex impedance is also measured at different bias points at 55 mK bath temperature. The measured electrical (dark) NEP is 1x10-18W/ √HZ, which is about a factor of 3 higher than what we expect from the thermal conductance that comes out of the IV curves. Despite using a light-tight box, the photon noise might still be the source of this excess noise. 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 fmd an effective time constant of about 2.7 ms and a thermal capacity of 13 fJ/K in the middle of the transition. © 2009 American Institute of Physics.