Ultra-low noise TES bolometer arrays for SAFARI instrument on SPICA

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 the SAFARI instrument aboard the SPICA mission. We successfully fabricated TESs with very narrow (0.5-0.7 µm) and thin (0.25 µm) SiN legs on different sizes of SiN islands using deep reactiveion etching process. The pixel size is 840×840 µm 2 and there are variety of designs with and without optical absorbers. For TESs without absorbers, we measured electrical NEPs as low as <1×10 -19 W/√Hz with response time of 0.3 ms and reached the phonon noise limit. Using TESs with absorbers, we quantified the darkness of our setup and confirmed a photon noise level of 2×10 -19 W/√Hz. All bolometers are 50×50 µm 2 and made of Ti/Au (16/65 nm) bilayer on a 0.25 µm thick suspended SiN island. The absorbers are 200×200 µm 2 and made of 8 nm thick tantalum (Ta). The large SiN islands are 220×280 µm 2 and the small ones 70×70 µm 2 . The SiN legs are about 0.5 µm wide and 350 µm long for large devices and 460 µm for small ones.Fabrication of TES bolometers includes 14 lithography steps and finishes with a deep reactive-ion etching (DRIE) process that is explained in detail by Ridder et al [2]. *P.Khosropanah@sron.nl; phone +31 88777 5678; fax +31 88777 5601; sron.nl Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy VIII, edited by Wayne S. Holland, Jonas Zmuidzinas, Proc. of SPIE Vol. 9914, 99140B

The SAFARI detector system

“…SAFARI's TES bolometers are two orders of magnitude more sensitive than bolometers used for ground-based observations [19] and testing them requires careful attention to stray-light protection and shielding from electrical, magnetic, and mechanical disturbances [20] . The detector requirements are within reach: we have demonstrated the required NEP≤2×10 -19 W/√Hz [21] and optical efficiency ≥50% for single detectors [22] . More recent optical measurements of arrays fabricated at the University of Cambridge are described elsewhere in these proceedings [23] .…”

Origins Survey Spectrometer: revealing the hearts of distant galaxies and forming planetary systems with far-IR spectroscopy

“…Finally, we note that the speed of TES bolometers may be a concern when pushed to the lowest NEPs, since fundamentally, the time constant scales as the thermal conductance G, and G must be reduced as the square of desired NEP improvements (NEP ∝ ffiffiffiffi G p ). Khosropanah et al 38 indicated time constants of 0.2 to 0.3 ms for their low-NEP devices, which indicates that 3 ms is possible at the NEP target for OSS.…”

“…These programs have demonstrated NEPs as low as 1 × 10 −19 W Hz −1∕2 at both Jet Propulsion Laboratory (JPL) and Space Research Organization of the Netherlands. 37,38 Pushing to lower NEPs is possible though potentially cumbersome: either base temperatures must be reduced from the canonical 50 mK, and/or the leg isolation needs to be improved with more exotic microfabricated structures. Another challenge with a TES system in light of the formats required for OSS is the complexity of focal plane assembly, in particular the hybridization with superconducting quantum interference devices (SQUIDs).…”

SPICA—A Large Cryogenic Infrared Space Telescope: Unveiling the Obscured Universe

“…SAFARI employs the latest generation of ultra-sensitive TES to detect the incoming photons (Khosropanah et al 2016;Audley et al 2016;Suzuki et al 2016;Goldie et al 2012;Beyer et al 2012). TES bolometers fabricated at SRON (Ridder et al 2016) have already demonstrated the required NEP (Khosropanah et al 2016), as well as the required optical efficiency (Audley et al 2016). To achieve their performance, the detectors, their readout, and the first-stage amplifiers must be operated at 50-mK, which requires that both the sensors and the cold electronics be cooled and thermally isolated from the 1.8-K environment of the GM.…”