Ultra‐sensitive transition‐edge sensors (TESs) for far‐IR/submm space‐borne spectroscopy

We have built surface micromachined thin-fllm metallized Si_tN-y optical absorbers for transition-edge sensors (TESs) suitable for the Background-Limited far-IR/Submm Spectrograph (BLISS). BLISS is a broadband (38 ;Um-433 jxm ), grating spectrometer consisting of five wavebands each with a modest resolution of R -̂ 1000. Because BLISS requires the effective noise equivalent power (NEP) of the TES to be below 10^^^ W/Hz /̂̂ , our TESs consist of four long (1000 jxm), narrow (0.4 /im ), and thin (0.25 /im ) Si_tNy support beams that reduce the thermal conductance G between the substrate and the optical absorber. To reduce the heat capacity of the absorber and make the response time T fast enough for BLISS, our absorbers are patterned into a mesh geometry with a fill factor of less than 10%. We use a bilayer of Ti/Au to make the effective impedance of the absorber match the impedance of the incoming radiation for each band. Measurements of the response time of the metallized absorbers to heat pulses show that their heat capacity exceeds the predictions of the Debye model. Our results are suggestive that the surface of the low pressure chemical vapor deposition (LPCVD) Si_tNy used in the absorbers' construction is the source of microstates that dominate the heat capacity.