Heat Capacity of Absorbers for Transition-Edge Sensors Suitable for Space-Borne Far-IR/Submm Spectroscopy

Using a noise thermometry technique, we measured the temperature dependence of the heat capacity C(T) of suspended Si_xN_y membranes for two different geometries from 30 mK to 300 mK. We determined a volumetric specific heat of the Si_xN_y on the order of 10 aJ/(mum³K) at 0.1 K for both absorber geometries. This value for the specific heat is comparable to a typical metal and is more than a factor of 10 larger than typical glassy materials. This result affects the design of highly efficient optical absorbers for membrane-isolated transition-edge sensors (TESs) for the Background-Limited far-IR/Submillimeter Spectrograph (BLISS), a proposed grating spectrometer that could fly on the next generation of large, cryogenic space-borne telescopes. We show that it is possible to reduce the thermal mass of the absorber of TESs so the effective response time tau_e is less than 100 ms-a requirement for all the wavelength bands for BLISS.