TES Bolometer Design and Testing for the Tomographic Ionized-carbon Mapping Experiment Millimeter Array

TL;DR

The TIME LIM paper details the development of silicon-nitride, leg-isolated Ti TES bolometers for 183–325 GHz line intensity mapping of [C II] and CO lines across z ~ 0.5–9. It reports lab results showing high LF optical efficiency (30–40%) up to 260 GHz and identifies HF yield challenges, prompting a redesigned HF module with improved cabling and a reduced backshort distance. Key upgrades include Kapton cable fabrication changes to reduce series impedance and a backshort optimization that boosts HF performance, with ongoing fabrication refinements and impending on-sky tests at the ARO 12 m telescope. The work aims to deploy ~1920 detectors across two wafer types to maximize S/N in baryon clustering measurements and to enable robust cross-checks with other LIM instruments. These developments are expected to enhance detector yield, time constants, and optical efficiency, advancing TIME’s capability to map [C II] and CO during the epoch of reionization and peak star formation.

Abstract

Transition Edge Sensor (TES) bolometers are a well-established technology with a strong track record in experimental cosmology, making them ideal for current and future radio astronomy instruments. The Tomographic Ionized-carbon Mapping Experiment (TIME), in collaboration with JPL, has developed advanced silicon nitride leg isolated superconducting titanium detectors for 200 to 300 GHz observations of the Epoch of Reionization. Compared to their MHz counterparts, bolometers operating in this frequency range are less common because of their large absorber size and fragility. TIME aims to fabricate a total of 1920 high frequency (HF) and low frequency (LF) detectors to fully populate the focal plane. TIME has successfully developed HF (230 to 325 GHz) and LF (183 to 230 GHz) wafers that are physically robust and perform well at cryogenic temperatures (300 mK). Recent laboratory tests have shown high optical efficiencies for the LF wafers (30 to 40%), but low device yield for the HFs. To address this, new HF modules have been designed with improved cabling and a reduced backshort distance, and are expected to perform similarly to LFs in a similar lab setting. We report on the development of these detectors as well as recent laboratory and on sky tests conducted at the Arizona Radio Observatory's (ARO) 12 meter prototype antenna at Kitt Peak National Observatory.

Figures (10)

• Figure 1: TIME will perform LIM of the Epoch of Reionization and the Peak of Cosmic Star Formation ($z\sim2$). LIM provides three dimensional information about the evolution of [C ii] and CO emission lines over time, revealing the growth of structure. The left image shows a simulation of individual galaxies at multiple redshifts, while the middle image shows the estimated TIME intensity map from these sources using a 30$"$ beam, multiple frequency channels, and no measurement noise. The right figure shows the power spectrum from the left image and the expected contributions from clustering and Poisson fluctuations.
• Figure 2: Amplifier SQUID module connected to the detector side through Kapton™ flex cables. The TIME architecture has 3 subarrays of Nyquist and SQUID chips (6 total), group in packets of 11 multiplexed rows for a total of 33. On the right is a close-up of a single Nyquist and SQUID chip. The Nyquist chips were designed and fabricated at NIST.
• Figure 3: The TIME spectrometers mounted to the 1K plate, and held in place using a rigid stage assembly. During installation, the feedhorns point towards the ground.
• Figure 4: The bottom figure shows an individual HF subarray, 4 of which are arranged in parallel to form a single HF module. The number of detectors varies across the high and low frequency subarrays. This HF wafer was designed by Jonathan Hunacek and created at the JPL microdevices laboratory by Clifford Frez and Anthony Turner. Above is a close-up of an individual TES Bolometer comprising a gold absorber, Ti and AL TES, and silicon nitride support legs which suspend the absorber.
• Figure 5: The bias is swept from high to low values for detectors within the same multiplexing column. Fabrication differences sometimes create detectors with diverse optimal transition bias locations. This can be a challenge for preserving large numbers of functional detectors at a single bias current.