Development of Ultra-Low-Noise TES Bolometer Arrays

Abstract

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 $2\times 10^{-19}~\hbox {W}/\sqrt{{\text {Hz}}}$: 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 (${<}0.25~\upmu \hbox {m}$) and narrower (${<}1~\upmu \hbox {m}$) 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 $(0.20~\upmu \hbox {m}$), narrow (0.5–0.7 $\upmu \hbox {m}$), and long (340–460 $\upmu \hbox {m}$) SiN legs and show $T_{\mathrm {c}}$ of ${\sim }93~\hbox {mK}$ and $R_{\mathrm {n}}$ of ${\sim }158~\hbox {m}{\Omega }$. 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 $1.1\,\times \,10^{-19}~\hbox {W}/\sqrt{{\text {Hz}}}$ with a reasonable response speed (${<}1~\hbox {ms}$), which achieved the phonon noise limit. For TESs with the absorber, we confirmed a higher $\hbox {NEP}_{\mathrm {el}} ({\sim }5\,\times \,10^{-19}~\hbox {W}/\sqrt{{\text {Hz}}}$) 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.