In the early 2030s, ESAs new X-ray observatory, Athena, is scheduled to be launched. It will carry two main
instruments, one of which is the X-ray Integral Field Unit (X-IFU), an X-ray imaging spectrometer, which will consist of an array of several thousand transition-edge sensors (TESs) with a proposed energy resolution of 2.5 eV for photon energies up to 7 keV. At SRON we develop the backup TES
array based on Ti/Au bilayers with a transition temperature just below 100 mK. In this contribution we will give a broad overview of the properties and capabilities of these state-of-the-art detectors. Over the years we have fabricated and studied a large number of detectors with various
geometries, providing us with a good understanding of how to precisely control the properties of our detectors. We are able to accurately vary the most important detector properties, such as the normal resistance, thermal conductance and critical temperature. This allows us to finely tune our
detectors to meet the demands of various applications. The detectors have demonstrated excellent energy resolutions of below 1.8 eV for 5.9 keV X-rays. By tuning the properties of the devices, they can be optimally matched to various read-out schemes using both AC and DC biasing. The next step is to increase the size of our TES arrays from our current kilo-pixel arrays towards the
full-sized array for X-IFU.
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Transition-edge-sensor (TES) microcalorimeters and bolometers are used for a variety of applications.The sensors are based on the steep temperature-dependent resistance of the normal-to-superconducting transition, and are thus intrinsically sensitive to magnetic fields. Conventionally the detectors are shielded from stray magnetic fields using external magnetic shields. However, in particular for applications with strict limits on the available space and mass of an instrument, external magnetic shields might not be enough to obtain the required shielding factors or field homogeneity. Additionally, these shields are only effective for magnetic fields generated external to the TES array, and are ineffective to mitigate the impact of internally generated magnetic fields. Here we present an alternative shielding method based on a super-conducting groundplane deposited directly on the backside of the silicon nitride membrane on which the TESs are located. We demonstrate that this local shielding for external magnetic fields has a shielding factor of at the least approximately 80, and is also effective at reducing internal self-induced magnetic fields, as demonstrated by measurements and simulation of the eddy current losses in our ac-biased detectors. Measurements of 5.9-keV x-ray photons show that our shielded detectors have a high resilience to external magnetic fields, showing no degradation of the energy resolution or shifts of the energy-scale calibration for fields of several microtesla, values higher than expected in typical real-world applications.@en

In the frequency-domain multiplexing (FDM) scheme, transition-edge sensors (TESs) are individually coupled to superconducting LC filters and AC biased at MHz frequencies through a common readout line. To make efficient use of the available readout bandwidth and to minimize the effect of non-linearities, the LC resonators are usually designed to be on a regular grid. The lithographic processes, however, pose a limit on the accuracy of the effective filter resonance frequencies. Off-resonance bias carriers could be used to suppress the impact of intermodulation distortions, which, nonetheless, would significantly affect the effective bias circuit and the detector spectral performance. In this paper, we present a frequency shift algorithm (FSA) to allow off-resonance readout of TESs, while preserving the on-resonance bias circuit and spectral performance, demonstrating its application to the FDM readout of an x-ray TES microcalorimeter array. We discuss the benefits in terms of mitigation of the impact of intermodulation distortions at the cost of increased bias voltage and the scalability of the algorithm to multi-pixel FDM readout. We show that with FSA, in the multi-pixel and frequencies shifted on-grid, the line noises due to intermodulation distortion are placed away from the sensitive region in the TES response and the x-ray performance is consistent with the single-pixel, on-resonance level.

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Transition-edge sensors (TESs) are the selected technology for future spaceborne x-ray observatories, such as Athena, Lynx, and HUBS. These missions demand thousands of pixels to be operated simultaneously with high energy-resolving power. To reach these demanding requirements, every aspect of the TES design has to be optimized. Here we present the experimental results of tests on different devices where the coupling between the x-ray absorber and the TES is varied. In particular, we look at the effects of the diameter of the coupling stems and the distance between the stems and the TES bilayer. Based on measurements of the ac complex impedance and noise, we observe a reduction in the excess noise as the spacing between the absorber stem and the bilayer is decreased. We identify the origin of this excess noise to be internal thermal fluctuation noise between the absorber stem and the bilayer. In addition, we see an impact of the coupling on the superconducting transition in the appearance of kinks. Our observations show that these unwanted structures in the transition shape can be avoided with careful design of the coupling geometry. The stem diameter appears to have a significant effect on the smoothness of the TES transition. This observation is still poorly understood, but is of great importance for both ac and dc biased TESs.

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Uniform large transition-edge sensor (TES) arrays are fundamental for the next generation of x-ray space observatories. These arrays are required to achieve an energy resolution ΔE < 3 eV full width at half maximum (FWHM) in the soft x-ray energy range. We are currently developing x-ray microcalorimeter arrays for use in the future laboratory and space-based x-ray astrophysics experiments and ground-based spectrometers. In this contribution, we report on the development and the characterization of a uniform 32 × 32 pixel array with 140 × 30 μm2 Ti/Au TESs with the Au x-ray absorber. We report on extensive measurements on 60 pixels in order to show the uniformity of our large TES array. The averaged critical temperature is Tc = 89.5 ± 0.5 mK, and the variation across the array (∼1 cm) is less than 1.5 mK. We found a large region of detector's bias points between 20% and 40% of the normal-state resistance where the energy resolution is constantly lower than 3 eV. In particular, results show a summed x-ray spectral resolution ΔEFWHM = 2.50 ± 0.04 eV at a photon energy of 5.9 keV, measured in a single-pixel mode using a frequency domain multiplexing readout system developed at SRON/VTT at bias frequencies ranging from 1 MHz to 5 MHz. Moreover, we compare the logarithmic resistance sensitivity with respect to temperature and current (α and β, respectively) and their correlation with the detector's noise parameter M, showing a homogeneous behavior for all the measured pixels in the array.

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We report on the development and demonstration of MHz frequency domain multiplexing (FDM) technology to readout arrays of cryogenic transition edge sensor (TES) x-ray microcalorimeters. In our FDM scheme, TESs are AC biased at different resonant frequencies in the low MHz range through an array of high-Q LC resonators. The current signals of all TESs are summed at superconducting quantum interference devices (SQUIDs). We have demonstrated multiplexing for a readout of 31 pixels using room temperature electronics, high-Q LC filters, and TES arrays developed at SRON, and SQUID arrays from VTT. We repeated this on a second setup with 37 pixels. The summed x-ray spectral resolutions @ 5.9 keV are Δ E 31 pix MUX = 2.14 ± 0.03 eV and Δ E 37 pix MUX = 2.23 ± 0.03 eV. The demonstrated results are comparable with other multiplexing approaches. There is potential to further improve the spectral resolution, to increase the number of multiplexed TESs, and to open up applications for TES x-ray microcalorimeters.

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Superconducting transition-edge sensors (TESs) are highly sensitive detectors. Based on the outstanding performance on spectral resolution, the X-ray integral field unit (X-IFU) instrument on-board athena will be equipped with a large array of TES-based microcalorimeters. For optimal performance in terms of the energy resolution, it is essential to limit undesirable nonlinearity effects in the TES detector. Weak-link behavior induced on the TES by superconducting leads is such a nonlinearity effect. We designed and fabricated smart test structures to study the effect of the superconducting leads on the intrinsic transition curve of our TiAu-based TES bilayer. We measured and analyzed the resistance versus temperature transition curves of the test structures. We found relations of long-distance proximity effects with TES length and different lead materials. Based on these results, we can redesign and further optimize our TES-based X-ray detectors.

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Hot Universe Baryon Surveyor (HUBS), a Chinese space mission, is proposed to find a large fraction of the so-called missing baryons, which would help us to understand more about the structure formation and evolution of the universe. Both theoretical and experimental results show that developing a highly efficient soft X-ray spectrometer over a large field of view and with a high energy resolution is the key to detect the “missing baryons”. X-ray microcalorimeters based on a transition-edge sensor (TES) array is required for HUBS, which aims to have 1 deg² field of view (FoV) with 1' angular resolution and 2 eV energy resolution optimized around 0.6 keV. Taking the high throughput X-ray optical focusing system on HUBS into account, the TES array is designed to have 60 x 60 pixels with an area of 1 mm² for each pixel. The microcalorimeter consists of a TES, a weak thermal link to a heat bath, and a semi-metal or normal metal absorber to increase the X-ray absorption efficiency. When an X-ray photon with a given energy is absorbed, the temperature of the absorber increase, that can be monitored by measuring the resistance change of the TES. A bilayer of a superconductor and a normal metal is used to fabricate a TES with a critical temperature (Tc) of ~100 mK. The latter is set for the required energy resolution. For HUBS, both MoCu and TiAu TES technologies are considered in its development phase. Here we will focus on TiAu TES calorimeters designed and partially fabricated at SRON for HUBS. Recent demonstration of a resolution of 2.5 eV at 5.9 keV in an AC readout at SRON for X-IFU on board of Athena illustrates the promising of this technology. However, the challenging for the HUBS array is the large pixel size. We will report the design and fabrication of prototype HUBS calorimeters.

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We are developing a transition edge sensor (TES) microcalorimeter array based on a Ti/Au superconducting bilayer, as a backup option for the X-IFU instrument on the Athena X-ray observatory. The array is read out by a frequency-division multiplexing readout system using a 1–5 MHz frequency band. Extensive research collaborations between NASA/Goddard and SRON have led to new design rules for microcalorimeters such as low resistivity of the superconductor bilayer, moderately high ohmic resistance of the TES by changing the aspect ratio and no extra metal strips. We have improved our detector fabrication process according to these design principles and produced TES arrays. Although single-pixel characterizations of these arrays are ongoing, the best energy resolution of 2.0 eV for 5.9 keV X-ray has been observed with a 120 × 20 μm² TES with a normal resistance of 150 mΩ, biased at 2.2 MHz frequency. This shows that our Ti/Au TES array has a potential to fulfill the detector requirements of the X-IFU instrument.

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We are developing X-ray microcalorimeters as a backup option for the baseline detectors in the X-IFU instrument on board the ATHENA space mission led by ESA and to be launched in the early 2030s. 5 × 5 mixed arrays with TiAu transition-edge sensor (TES), which have different high aspect ratios and thus high resistances, have been designed and fabricated to meet the energy resolution requirement of the X-IFU instrument. Such arrays can also be used to optimize the performance of the frequency domain multiplexing (FDM) readout and lead to the final steps for the fabrication of a large detector array. In this work, we present the experimental results from tens of the devices with an aspect ratio (length-to-width) ranging from 1-to-1 up to 6-to-1, measured in a single-pixel mode with a FDM readout system developed at SRON/VTT. We observed a nominal energy resolution of about 2.5 eV at 5.9 keV at bias frequencies ranging from 1 to 5 MHz. These detectors are proving to be the best TES microcalorimeters ever reported in Europe, intending to meet the requirements of the X-IFU instrument, but also those of other future challenging X-ray space missions, fundamental physics experiments, plasma characterization and material analysis.

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Frequency-division multiplexing is the baseline read-out system for large arrays of superconducting transition-edge sensors (TES’s) under development for the X-ray and infrared instruments like X-IFU (Athena) and SAFARI, respectively. In this multiplexing scheme, the sensors are ac-biased at different frequencies from 1 to 5 MHz and operate as amplitude modulators. Weak superconductivity is responsible for the complex TES resistive transition, experimentally explored in great detail so far, both with dc- and ac-biased read-out schemes. In this paper, we will review the current status of our understanding of the physics of the TES’s and their interaction with the ac bias circuit. In particular, we will compare the behaviour of the TES nonlinear impedance, across the superconducting transition, for several detector families, namely: high-normal-resistance TiAu TES bolometers, low-normal-resistance MoAu TES microcalorimeters and high-normal-resistance TiAu TES microcalorimeters.

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SRON is developing X-ray transition edge sensor (TES) calorimeters arrays, as a backup technology for X-IFU instrument on the ATHENA space observatory. These detectors are based on a superconducting TiAu bilayer TES with critical temperature of 100 mK on a 1 μm thick SiN membrane with Au or Au/Bi absorbers. Number of devices have been fabricated and measured using a Frequency Division Multiplexing (FDM) readout system with 1-5 MHz bias frequencies. We measured IV curves, critical temperature, thermal conductance, noise and also X-ray energy resolution at number of selected bias points. So far our best calorimeter shows 3.9 eV X-ray resolution at 6 keV. Here we present a summary of our results and the latest status of development of X-ray calorimeters at SRON.

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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 840x840 μm² and there are variety of designs with and without optical absorbers. For TESs without absorbers, we measured electrical NEPs as low as <1x10^-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 2x10^-19 W/√Hz.

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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 SAFARI aboard SPICA. We have two major concerns about realizing TESs with an ultra-low NEP of 2×10-19W/Hz: achieving lower thermal conductance and no excess noise with respect to the phonon noise. To realize TESs with phonon-noise-limited NEPs, we need to make thinner (c of ∼93mK and R_n of ∼158mΩ. These TESs were characterized under AC bias using our frequency-division multiplexing readout (1–3 MHz) system. TESs without the absorber show NEPs as low as 1.1×10-19W/Hz with a reasonable response speed (

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