Development of Ultra-Low-Noise TES Bolometer Arrays

SRON is developing ultra-low-noise transition edge sensors (TESs) based on a superconducting Ti/Au bilayer on a suspended SiN island with SiN legs for SAFARI aboard SPICA. We have two major concerns about realizing TESs with an ultra-low NEP of : achieving lower thermal conductance and no excess noise with respect to the phonon noise. To realize TESs with phonon-noise-limited NEPs, we need to make thinner () and narrower () SiN legs. With deep reactive-ion etching, three types of TESs were fabricated in combination with different SiN island sizes and the presence or absence of an optical absorber. Those TESs have a thin ), narrow (0.5-0.7 ), and long (340-460 ) SiN legs and show of and of . These TESs were characterized under AC bias using our frequency-division multiplexing readout (1-3 MHz) system. TESs without the absorber show NEPs as low as with a reasonable response speed (), which achieved the phonon noise limit. For TESs with the absorber, we confirmed a higher ) than that of TESs without the absorber likely due to stray light. The lowest NEP can make the new version of SAFARI with a grating spectrometer feasible.