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 2×10-19W/Hz\documentclass[12pt]{minimal}
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\begin{document}$$2\times 10^{-19}~\hbox {W}/\sqrt{{\text {Hz}}}$$\end{document}: 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 (<0.25μm\documentclass[12pt]{minimal}
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\begin{document}$${<}0.25~\upmu \hbox {m}$$\end{document}) and narrower (<1μm\documentclass[12pt]{minimal}
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\begin{document}$${<}1~\upmu \hbox {m}$$\end{document}) 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 (0.20μm\documentclass[12pt]{minimal}
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\begin{document}$$(0.20~\upmu \hbox {m}$$\end{document}), narrow (0.5–0.7 μm\documentclass[12pt]{minimal}
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\begin{document}$$\upmu \hbox {m}$$\end{document}), and long (340–460 μm\documentclass[12pt]{minimal}
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\begin{document}$$\upmu \hbox {m}$$\end{document}) SiN legs and show Tc\documentclass[12pt]{minimal}
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\begin{document}$$T_{\mathrm {c}}$$\end{document} of ∼93mK\documentclass[12pt]{minimal}
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\begin{document}$${\sim }93~\hbox {mK}$$\end{document} and Rn\documentclass[12pt]{minimal}
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\begin{document}$$R_{\mathrm {n}}$$\end{document} of ∼158mΩ\documentclass[12pt]{minimal}
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\begin{document}$${\sim }158~\hbox {m}{\Omega }$$\end{document}. 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 1.1×10-19W/Hz\documentclass[12pt]{minimal}
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\begin{document}$$1.1\,\times \,10^{-19}~\hbox {W}/\sqrt{{\text {Hz}}}$$\end{document} with a reasonable response speed (<1ms\documentclass[12pt]{minimal}
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\begin{document}$${<}1~\hbox {ms}$$\end{document}), which achieved the phonon noise limit. For TESs with the absorber, we confirmed a higher NEPel(∼5×10-19W/Hz\documentclass[12pt]{minimal}
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\begin{document}$$\hbox {NEP}_{\mathrm {el}} ({\sim }5\,\times \,10^{-19}~\hbox {W}/\sqrt{{\text {Hz}}}$$\end{document}) 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.
