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 2x10^-19 W/rt(Hz): achieving lower thermal conductance
and no excess noises with respect to the phonon noise. To realize TESs with
phonon-noise limited NEPs, we need to make thinner (<0.25 um) and narrower (<1 um)
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 thin (0.20 um), narrow (0.5--0.7 um),
and long (340--460 um) SiN legs, and show Tc of ~93 mK and Rn of ~158 m¦¸.

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 1.1x10^-19 W/rt(Hz) with a reasonable response speed (<1 msec)
, which achieved the phonon noise limit. For TESs with the absorber, we
confirmed a higher NEP (~5x10^-19 W/rt(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.

These results are based on my research activities at SRON, which were partly
supported by JAXA.