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

TL;DR

This paper addresses the challenge of delivering reliable, high-yield Ultra-High-Frequency TES detector modules for the Simons Observatory. It reports the pre-deployment laboratory characterization of 25 production UHF modules, each with 1720 optically-coupled TES bolometers read out via microwave SQUID multiplexing and SMuRF electronics, tested in cryogenic conditions with a cold load. The key findings include an average TES operable yield of $83\%$, median $P_\text{sat}$ around $24$–$26$ pW, optical efficiency $\eta_\text{opt}$ near $0.62$–$0.63$, dark NEP around $40$ aW/$\sqrt{\mathrm{Hz}}$, and photon NEP projections of $64$ and $99$ aW/$\sqrt{\mathrm{Hz}}$ for the 220 and 280 GHz bands, respectively—indicating background-limited performance on the sky. With 19 modules selected for deployment (13 already in use and 6 scheduled for the LAT upgrade), the results demonstrate readiness for on-sky operation and inform ongoing commissioning and future instrument upgrades. The work establishes crucial performance baselines for resonator readout, TES stability, and optical coupling, enabling SO to achieve its science objectives across its six-frequency bands.

Abstract

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.

Figures (7)

• Figure 1: An assembled SO UHF detector module is shown in (a), containing a detector wafer, optical coupling components, and multiplexing circuitry. A bare detector wafer is shown in (b)¸ with insets depicting a single pixel (c) and one TES bolometer (d). The bolometer consists of a suspended, thermally-isolated island onto which power from the antenna is deposited via a lossy meander. The rectangular structure running down the center of the island is the TES itself.
• Figure 2: Histograms of resonator internal quality factor $Q_i$, bandwidth, and frequency spacing across all 25 modules before (gray) and after (red) coupling to detectors. Dashed black lines indicate parameter thresholds consulted during multiplexer chip screening. When TESs are added to the circuit, the peak of the $Q_i$ distribution shifts down by 9% and the peak of the bandwidth distribution shifts up by 4%, while the frequency spacings do not change.
• Figure 3: Readout white noise level (NEI) plotted against internal quality factor $Q_i$ for resonator channels across the 25 multiplexing modules. The blue dashed line indicates the baseline target of 65 pA$/\sqrt{\mathrm{Hz}}$, and the green dot-dash line indicates the goal target of 45 pA$/\sqrt{\mathrm{Hz}}$. The solid red line marks $Q_i=$50,000, a criterion consulted for accepting or rejecting sets of multiplexer chips. The distribution of NEI values peaks at 35 pA$/\sqrt{\mathrm{Hz}}$.
• Figure 4: Distributions of TES critical temperature $T_c$ and saturation power $P_\mathrm{sat}$. The top portion of each plot shows the distribution across all 25 modules. The lower portions of the plots show the parameter distributions for individual detector modules, where boxes indicate quartiles, and whiskers indicate the central 90% of the data. The shaded regions indicate the target $P_\mathrm{sat}$ ranges. The modules are ordered with the newest detector wafers on top and the oldest at bottom, with dashed lines separating fabrication batches.
• Figure 5: $I$-$V$ and corresponding $R$-$P$ curves as a function of bath temperature for one TES with $T_c=172$ mK on a high-$P_\mathrm{sat}$ detector module. The curves extend to the lowest $R_\mathrm{TES}/R_N$ value where the TES can be stably operated.