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IEEE Transactions on Applied Superconductivity

Thermal Design and Characterization of Transition-Edge Sensor (TES) Bolometers for Frequency-Domain Multiplexing

Martin Lueker, Bradford A. Benson, C. L. Chang, Hsiao-Mei Cho, M. Dobbs, W. L. Holzapfel, Trevor M. Lanting, Adrian Tae-Jin Lee, Jared Mehl, Tom Plagge, Erik D. Shirokoff, H. G. Spieler, Joaquin D. Vieira

Published Jun 1, 2009• 25 citations
Publisher

Abstract

In contemporary cosmic microwave background experiments, bolometric detectors are often background limited, and in this case the sensitivity of instruments can only be improved by increasing the number of background-limited detectors, and so contemporary TES receivers contain as many pixels as possible. Frequency-domain multiplexing (fMUX) is one strategy for reading out many detectors with one SQUID. For any readout system, it is important to carefully evaluate the thermal design of detector, in conjunction with the readout bandwidth, in order to ensure stable electro-thermal feedback (ETF). We demonstrate a novel technique for characterizing the thermal circuit of our detectors, using am AC-bias and the fMUX electronics. This technique is used to study the internal thermal coupling of a TES bolometer. We illustrate how the insights gathered by this technique have been instrumental in improving the stability of our multiplexed detectors for the south pole telescope (SPT).

DOI
10.1109/tasc.2009.2018036

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In contemporary cosmic microwave background experiments, bolometric detectors are often background limited, and in this case the sensitivity of instruments can only be improved by increasing the number of background-limited detectors, and so contemporary TES receivers contain as many pixels as possible. Frequency-domain multiplexing (fMUX) is one strategy for reading out many detectors with one SQUID. For any readout system, it is important to carefully evaluate the thermal design of detector, in conjunction with the readout bandwidth, in order to ensure stable electro-thermal feedback (ETF). We demonstrate a novel technique for characterizing the thermal circuit of our detectors, using am AC-bias and the fMUX electronics. This technique is used to study the internal thermal coupling of a TES bolometer. We illustrate how the insights gathered by this technique have been instrumental in improving the stability of our multiplexed detectors for the south pole telescope (SPT).
For the next generation of Cosmic Microwave Background (CMB) experiments, kilopixel arrays of Transition Edge Sensor (TES) bolometers are necessary to achieve the required sensitivity and their science goals. We are developing read-out electronics for POLARBEAR-2 CMB experiment, which multiplexes 32-TES bolometers through a single superconducting quantum interface device (SQUID). To increase both the bandwidth of the SQUID electronics and the multiplexing factor, we are modifying cold wiring and developing LC filters, and a low-inductance superconducting cable. Using these components, we will show frequency domain multiplexing up to 3 MHz.
Key performance characteristics are demonstrated for the microwave SQUID multiplexer (μ\muμMUX) coupled to transition edge sensor (TES) bolometers that have been optimized for cosmic microwave background (CMB) observations. In a 64-channel demonstration, we show that the μ\muμMUX produces a white, input referred current noise level of 29~pA/Hz/\sqrt{\mathrm{Hz}}/Hz​ at -77~dB microwave probe tone power, which is well below expected fundamental detector and photon noise sources for a ground-based CMB-optimized bolometer. Operated with negligible photon loading, we measure 98~pA/Hz/\sqrt{\mathrm{Hz}}/Hz​ in the TES-coupled channels biased at 65% of the sensor normal resistance. This noise level is consistent with that predicted from bolometer thermal fluctuation (i.e., phonon) noise. Furthermore, the power spectral density exhibits a white spectrum at low frequencies (∼\sim∼~100~mHz), which enables CMB mapping on large angular scales that constrain the physics of inflation. Additionally, we report cross-talk measurements that indicate a level below 0.3%, which is less than the level of cross-talk from multiplexed readout systems in deployed CMB imagers. These measurements demonstrate the μ\muμMUX as a viable readout technique for future CMB imaging instruments.
Abstract Digital Frequency-Domain Multiplexing (DfMux) is a technique that uses MHz superconducting resonators and Superconducting Quantum Interference Device (SQUID) arrays to read out sets of transition edge sensors. DfMux has been used by several Cosmic Microwave Background experiments, including most recently POLARBEAR-2 and SPT-3 G with multiplexing factors as high as 68, and is the baseline readout technology for the planned satellite mission LiteBIRD. Here, we present recent work focused on improving DfMux readout noise, reducing parasitic impedance, and improving sensor operation. We have achieved a substantial reduction in stray impedance by integrating the sensors, resonators, and SQUID array onto a single-carrier board operated at 250 mK. This also drastically simplifies the packaging of the cryogenic components and leads to better-controlled crosstalk. We demonstrate a low readout noise level of 8.6 pA/Hz11/2\mathrm{pA/Hz}1^{1/2}pA/Hz11/2 pA / Hz 1 1 / 2 , which was made possible by operating the SQUID array at a reduced temperature and with a low dynamic impedance. This is a factor of two improvement compared to the achieved readout noise level in currently operating Cosmic Microwave Background experiments using DfMux and represents a critical step toward maturation of the technology for the next generation of instruments.
A technological milestone for experiments employing transition edge sensor bolometers operating at sub-Kelvin temperature is the deployment of detector arrays with 100s-1000s of bolometers. One key technology for such arrays is readout multiplexing: the ability to read out many sensors simultaneously on the same set of wires. This paper describes a frequency-domain multiplexed readout system which has been developed for and deployed on the APEX-SZ and South Pole Telescope millimeter wavelength receivers. In this system, the detector array is divided into modules of seven detectors, and each bolometer within the module is biased with a unique ∼MHz sinusoidal carrier such that the individual bolometer signals are well separated in frequency space. The currents from all bolometers in a module are summed together and pre-amplified with superconducting quantum interference devices operating at 4 K. Room temperature electronics demodulate the carriers to recover the bolometer signals, which are digitized separately and stored to disk. This readout system contributes little noise relative to the detectors themselves, is remarkably insensitive to unwanted microphonic excitations, and provides a technology pathway to multiplexing larger numbers of sensors.
Abstract A microwave SQUID multiplexer ( μ MUX) with a high multiplexing factor is a promising readout approach for large-scale superconducting transition-edge sensor (TES) bolometer arrays. This paper reports on the characteristics of 16-channel μ MUX prototype chips and a 60-channel readout module developed to read out TES bolometers for the Ali cosmic microwave background polarization telescope (AliCPT). We designed two different coupling structures between the resonator and the radio-frequency superconducting interference device: design A with a mutual inductance ( M T ) of 2.3 pH and design B with an M T of 6.5 pH. For design A, the mean resonator bandwidth ( Δ f BW ) and peak-to-peak frequency shift ( Δ f pp ) are approximately 610 kHz and 290 kHz, respectively. The measured mean open-loop current noise is less than 40 pA / Hz . Preliminary results indicate that crosstalk between adjacent channels is below 1% in our design A μ MUX chip. A measured channel with design B shows a Δ f BW of 586 kHz and a current noise of 33 pA / Hz , similar to the design A channels. In comparison, the Δ f pp of 803 kHz is much larger than the design A channels due to a larger M T . The readout module, consisting of two design A chips and two design B chips, shows 60 resonances, corresponding to a yield of 94%. In the future, we will place 32 design A channels within the same chip dimension to increase the channel density and build a larger readout module suitable for AliCPT TES arrays.
LiteBIRD is a satellite mission designed to map the polarization of the Cosmic Microwave Background (CMB) at degree and larger scales from 40 to 402 GHz. LiteBIRD will use 4,600 Transition-Edge Sensor (TES) bolometers biased and read out using Digital Frequency Domain Multiplexing (DfMux). The DfMux implementation for LiteBIRD uses sub-kelvin Superconducting Quantum Interference Device (SQUID) at the same 0.1 K thermal stage as the detectors, this allows for reduced parasitic impedances within the mK circuit and improved SQUID performance. Additionally it must work in the integrated system with the spacecraft’s wiring harnesses, which will be longer than is typical on similar ground based experiments, and therefore have more significant parasitic impedances which will impact readout performance. The properties of SQUID candidates at millikelvin temperatures and effects of the spacecraft-like meter scale wiring harness are investigated. Additionally, the possibility of inductively rather than resistively biasing our bolometers at the 0.1K stage, to reduce power dissipation in the bias element, is investigated. We will report progress on validating the cryogenic components of this readout system.