The Simons Observatory: Characterization of the 220/280 GHz TES Detector Modules

The Simons Observatory (SO) is a new suite of cosmic microwave background telescopes in the Chilean Atacama Desert with an extensive science program spanning cosmology, Galactic and extragalactic astrophysics, and particle physics. SO will survey the millimeter-wave sky over a wide range of angular scales using six spectral bands across three types of dichroic, polarization-sensitive transition-edge sensor (TES) detector modules: Low-Frequency (LF) modules with bandpasses centered near 30 and 40 GHz, Mid-Frequency (MF) modules near 90 and 150 GHz, and Ultra-High-Frequency (UHF) modules near 220 and 280 GHz. Twenty-five UHF detector modules, each containing 1720 optically-coupled TESs connected to microwave SQUID multiplexing readout, have now been produced. This work summarizes the pre-deployment characterization of these detector modules in laboratory cryostats. Across all UHF modules, we find an average operable TES yield of 83%, equating to over 36,000 devices tested. The distributions of (220, 280) GHz saturation powers have medians of (24, 26) pW, near the centers of their target ranges. For both bands, the median optical efficiency is 0.6, the median effective time constant is 0.4 ms, and the median dark noise-equivalent power (NEP) is ~40 aW/rtHz. The expected photon NEPs at (220, 280) GHz are (64, 99) aW/rtHz, indicating these detectors will achieve background-limited performance on the sky. Thirty-nine UHF and MF detector modules are currently operating in fielded SO instruments, which are transitioning from the commissioning stage to full science observations.

💡 Research Summary

The Simons Observatory (SO) will map the cosmic microwave background (CMB) over a wide range of angular scales using six frequency bands from 30 GHz to 280 GHz. To achieve this, SO employs three types of dichroic, polarization‑sensitive transition‑edge sensor (TES) detector modules: low‑frequency (LF) at 30/40 GHz, mid‑frequency (MF) at 90/150 GHz, and ultra‑high‑frequency (UHF) at 220/280 GHz. This paper presents a comprehensive laboratory characterization of the 25 production‑design UHF modules that will populate the SO instruments.

Each UHF module contains 430 feed‑horn‑coupled orthomode transducers (OMTs), each feeding four AlMn TES bolometers (two polarizations, two frequencies), for a total of 1,720 optical TESs plus 36 dark TESs used for calibration. The TESs are read out with a microwave SQUID multiplexing (µmux) architecture, employing two RF readout chains per module that span 4–6 GHz. The µmux chips are fabricated at NIST, and the readout electronics are provided by the SLAC Microresonator RF (SMuRF) system.

Testing was performed in dilution‑refrigerator cryostats equipped with a variable‑temperature black‑body (“cold load”). One‑third of the detectors on each module were exposed to the cold load while the remainder were masked, allowing simultaneous measurement of dark and optical properties. The primary measurement technique involved stepping the TES bias voltage and recording the resulting current to generate I‑V curves at bath temperatures from 60 mK to 200 mK and cold‑load temperatures from 9 K to 20 K. From these data the authors extracted critical temperature (Tc), normal resistance (RN), saturation power (Psat), optical efficiency (ηopt), effective time constant (τeff), and noise‑equivalent power (NEP). Additional square‑wave bias modulation was used to determine responsivity and τeff.

Key results:

• Resonator performance: Across all 25 modules, 98 % of resonators were identified in VNA scans, with a minimum yield of 95 % after chip replacement. After applying SMuRF frequency‑spacing and SQUID‑response cuts, the usable resonator channel yield settled at 86 %. The median internal quality factor Q_i dropped from 1.30 × 10⁵ (pre‑detector) to 1.18 × 10⁵ (post‑detector), a modest 9 % reduction that had negligible impact on readout noise. The median bandwidth was ~110 kHz, close to the 100 kHz target. Readout white‑noise (NEI) peaked at 35 pA/√Hz, with 87 % of channels below the baseline target of 65 pA/√Hz and 68 % below the more aggressive goal of 45 pA/√Hz.

• TES yield and basic parameters: The average operable TES yield was 83 % (minimum 71 %), corresponding to over 36,000 TESs measured. The median critical temperature was 163 mK (±7 mK), close to the design goal of 160 mK, though batch‑to‑batch variations were observed. Saturation powers were centered at 24 pW (220 GHz) and 26 pW (280 GHz), sitting near the middle of the specified ranges. 91 % of detectors had Psat above the lower bound of the target range, and 77 % were below the upper bound, ensuring sufficient headroom for typical on‑sky loading while avoiding excess phonon noise.

• Stability considerations: High‑Psat detectors exhibited a reduced stable bias region; the TES could be operated down to only ~0.36 RN at the coldest bath temperature before electrothermal feedback became unstable. Empirically, this instability disappeared when the electrical bias power fell below ~15 pW, a condition expected to be met under normal sky loading for all but the very best‑weather days.

• Noise performance: Dark NEP medians were 39 aW/√Hz (220 GHz) and 42 aW/√Hz (280 GHz), comfortably below the baseline requirements of 53 aW/√Hz and 64 aW/√Hz, respectively. Approximately 89 % of detectors met the NEP requirement. The predicted photon‑noise contributions for on‑sky operation are 64 aW/√Hz (220 GHz) and 99 aW/√Hz (280 GHz), which lie above the measured dark NEP, indicating that the detectors will be photon‑noise limited in the field.

• Optical efficiency: Median ηopt values were 0.62 (220 GHz) and 0.63 (280 GHz), with interquartile ranges of 0.57–0.67 and 0.54–0.68, respectively. While no strict ηopt requirement exists, an array‑averaged ηopt ≈ 0.5 is considered adequate for meeting the overall noise‑equivalent temperature goals. The majority of detectors (89 % at 220 GHz, 81 % at 280 GHz) exceed this threshold. A subset of detectors showed low ηopt (<0.4), notably in module “Uv63”, which suffered from extensive post‑fabrication thermal re‑annealing—a process used to raise Tc that appears to degrade optical coupling, though the exact mechanism remains under investigation.

• Module selection: Of the 25 tested modules, 23 satisfied all performance criteria (yield, Psat, NEP, ηopt). Since only 19 UHF modules are required for the deployed instruments, the production run comfortably exceeds the need. Thirteen modules have already been installed in one small‑aperture telescope (SAT) and the large‑aperture telescope (LAT), with six additional modules slated for installation as part of the Advanced SO upgrade.

In summary, the laboratory characterization demonstrates that the UHF TES detector modules meet or surpass all design specifications for yield, saturation power, noise, and optical efficiency. The measured dark NEP is well below the expected photon noise, confirming that the detectors will operate in a background‑limited regime on the sky. These results validate the readiness of the UHF modules for scientific observations and support the Simons Observatory’s capability to deliver high‑precision, multi‑frequency CMB measurements.