Ti/Au Transition-edge Sensor Research Articles

Improvement of energy resolution of x-ray transition-edge sensor using K-means algorithm and Wiener filter

We develop an x-ray Ti/Au transition-edge sensor (TES) with an Au absorber deposited on the center of TES and improved its energy resolution using the K-means clustering algorithm in combination with Wiener filter. We firstly extract the main parameters of each recorded pulse trace, which are adopted to classify these traces into several clusters in the K-means clustering algorithm. Then real traces are selected for energy resolution analysis. Following the baseline correction, the Wiener filter is used to improve the signal-to-noise ratio. Although the silicon underneath the TES has not been etched to reduce the thermal conductance, the energy resolution of the developed x-ray TES is improved from 94 eV to 44 eV at 5.9 keV.

Ti/Au TES 32 × 32 Pixel Array: Uniformity, Thermal Crosstalk and Performance at Different X-Ray Energies

Large format arrays of transition edge sensor (TES) are crucial for the next generation of X-ray space observatories. Such arrays are required to achieve an energy resolution of ΔE <; 3 eV full-width-half-maximum (FWHM) in the soft X-ray energy range. We are currently developing X-ray microcalorimeter arrays as a backup option for the X-IFU instrument on board of ATHENA space telescope, led by ESA and foreseen to be launched in 2031. In this contribution, we report on the development and the characterization of a uniform 32 × 32 pixel array with (length × width) 140 × 30 μm$^2$ TiAu TESs, which have a 2.3 μm thick Au absorber for X-ray photons. The pixels have a typical normal resistance R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sub> = 121 mΩ and a critical temperature T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ~ 90 mK. We performed extensive measurements on 60 pixels out of the array in order to show the uniformity of the array. We obtained an energy resolutions between 2.4 and 2.6 eV (FWHM) at 5.9 keV, measured in a single-pixel mode at AC bias frequencies ranging from 1 to 5 MHz, with a frequency domain multiplexing (FDM) readout system, which is developed at SRON/VTT. We also present the detector energy resolution at X-ray with different photon energies generated by a modulated external X-ray source from 1.45 keV up to 8.9 keV. Multiplexing readout across several pixels has also been performed to evaluate the impact of the thermal crosstalk to the instrument's energy resolution budget requirement. This value results in a derived requirement, for the first neighbour, that is less than 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> when considering the ratio between the amplitude of the crosstalk signal to an X-ray pulse (for example at 5.9 keV).

AC/DC Characterization of a Ti/Au TES with Au/Bi Absorber for X-ray Detection

Transition-edge sensors (TESs) are used as very sensitive thermometers in microcalorimeters aimed at detection of different wavelengths. In particular, for soft X-ray astrophysics, science goals require very high resolution microcalorimeters which can be achieved with TESs coupled to suitable absorbers. For many applications there is also need for a high number of pixels which typically requires multiplexing in the readout stage. Frequency Domain Multiplexing (FDM) is a common scheme and is the baseline proposed for the ATHENA mission. FDM requires biasing the TES in AC at MHz frequencies. Recently there has been reported degradation in performances under AC with respect to DC bias. In order to assess the performances of TESs to be used with FDM, it is thus of great interest to compare the performances of the same device both under AC and DC bias. This requires two different measurement setups with different processes for making the characterization. We report in this work the preliminary results of a single pixel characterization performed on a TiAu TES under AC and afterwards under DC bias in different facilities. Extraction of dynamical parameters and noise performances are compared in both cases as a first stage for further AC/DC comparison of these devices.

Characterization of High Aspect-Ratio TiAu TES X-ray Microcalorimeter Array Under AC Bias

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$\times$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 optimise 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, being able to meet not only 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.

A frequency domain multiplexing system to readout the TES bolometers on the LSPE/SWIPE experiment

LSPE/SWIPE is a balloon-borne experiment aimed at measuring the B-mode polarization of the cosmic microwave background (CMB) exploiting the reionization peak at ℓ < 10 , with the primary target of improving the tensor-to-scalar ratio limit down to r = 0 . 03 at 99.7% confidence level. SWIPE will use 326 spider-web Ti/Au transition-edge sensor (TES) bolometers , realized at INFN Genova, hosted on two focal planes cooled down at 300 mK and readout by superconducting quantum interference devices (SQUIDs), with multi-mode horns to couple the instrument’s optical stages to the detectors. We present our design and demonstration of a 16-channel frequency domain multiplexing (FDM) system to readout SWIPE’s TES bolometers.

Impact of Electrical Contacts Design and Materials on the Stability of Ti Superconducting Transition Shape

The South Pole Telescope SPT-3G camera utilizes Ti/Au transition edge sensors (TESs). A key requirement for these sensors is reproducibility and long-term stability of the superconducting (SC) transitions. Here, we discuss the impact of electrical contacts design and materials on the shape of the SC transitions. Using scanning electron microscope, atomic force microscope, and optical differential interference contrast microscopy, we observed the presence of unexpected defects of morphological nature on the titanium surface and their evolution in time in proximity to Nb contacts. We found direct correlation between the variations of the morphology and the SC transition shape. Experiments with different diffusion barriers between TES and Nb leads were performed to clarify the origin of this problem. We have demonstrated that the reproducibility of superconducting transitions can be significantly improved by preventing diffusion processes in the TES–leads contact areas.

Fabrication and Test of Large Area Spider-Web Bolometers for CMB Measurements

Detecting the primordial ’B-mode’ polarization of the cosmic microwave background is one of the major challenges of modern observational cosmology. Microwave telescopes need sensitive cryogenic bolometers with an overall equivalent noise temperature in the nK range. In this paper, we present the development status of large area (about 1 cm\(^{2})\) spider-web bolometer, which imply additional fabrication challenges. The spider-web is a suspended Si\(_{3}\)N\(_{4}\) 1 \(\upmu \)m-thick and 8-mm diameter with mesh size of 250 \(\upmu \)m. The thermal sensitive element is a superconducting transition edge sensor (TES) at the center of the bolometer. The first prototype is a Ti–Au TES with transition temperature tuned around 350 mK, new devices will be a Mo–Au bilayer tuned to have a transition temperature of 500 mK. We present the fabrication process with micro-machining techniques from silicon wafer covered with SiO\(_{2}\) - Si\(_{3}\)N\(_{4}\) CVD films, 0.3 and 1 \(\upmu \)m- thick, respectively, and preliminary tests.

Distributed TES Model for Designing Low Noise Bolometers Approaching SAFARI Instrument Requirements

Transition edge sensors (TES) are the chosen detector technology for the SAFARI imaging spectrometer on the SPICA telescope. The TES are required to have an NEP of 2–3 × 10?19 W/? Hz to take full advantage of the cooled mirror. SRON has developed TiAu TES bolometers for the short wavelength band (30–60 ?m). The TES are on SiN membranes, in which long and narrow legs act as thermal links between the TES and the bath. We present a distributed model that accounts for the heat conductance and the heat capacity in the long legs that provides a guideline for designing low noise detectors. We report our latest results that include a measured dark NEP of 4.2×10?19 W/? Hz and a saturation power of about 10 fW.

Characterization of a High-Performance Ti/Au TES Microcalorimeter with a Central Cu Absorber

We are developing X-ray microcalorimeters based on Ti/Au transition-edge sensors (TES). Among sensors we have fabricated, one with a Cu absorber at the center of the TES shows a particularly good X-ray energy resolution: 1.56 eV at 250 eV and 2.5 eV at 5.9 keV. In this paper, a detailed study of its impedance and noise is presented. The noise is not explained by a sum of known sources. The magnitude of unexplained noise is largest when the sensitivity of the TES on temperature (α) and on current (β) are the highest. The observed relation between the noise level and sensitivity suggests a source of thermal fluctuations inside the TES or between the TES and the absorber. We also found that β is linearly correlated to the product of α and current, which limits the effective sensitivity that is expressed as α/(1+β).

A waveguide-coupled millimetre-wave TES bolometer suitable for 2-D arrays

We are developing a single-pixel antenna-coupled bolometric detector for use in low-background millimetre-wave astronomy. The device consists of a radial probe in a rectangular waveguide coupled to a Ti/Au transition edge sensor (TES). The signal propagates from the antenna along a superconducting microstrip line into a metal resistor on a thermally isolated Si 3N 4 island, where the TES is also located. We have designed and built prototype pixels optimised for 150 GHz band measurements. We discuss our progress in the development as well as the first results from dark measurements.

Evaluation of 256-pixel TES microcalorimeter arrays with electrodeposited Bi absorbers

We succeeded in fabricating 256-pixel arrays of Ti/Au transition edge sensor (TES) microcalorimeters with electrodeposited microabsorbers made of Bi, and report on their test results. We confirmed all pixels tested so far were alive, however most of the microabsorbers were stripped off during cooling cycles. We investigated some pixels by irradiating with 5.9 keV X-rays, and confirmed that they actually worked as microcalorimeters. The X-ray pulses are represented by a sum of two components with different decay times. The ratio of the two components fluctuates from pulse to pulse, which limits the energy resolution to ∼ 200 eV . We discuss the pulse shape of two decay-time components considering thermal conductivities, i.e., inside microabsorbers, between microabsorbers and TES.

Performance of a bridge-type TES microcalorimeter, excess noise characteristics and dependence of sensitivity on current

Fully utilizing the benefit of strong electrothermal feedback, we achieved the energy resolution of 6.6 eV (FWHM) at 5.9 keV and a fast response time of 74 μs with our bridge-type Ti/Au transition-edge sensor (TES) microcalorimeter. The energy resolution of this device was limited by a noise that was larger than the intrinsic noise and the readout noise. This noise only appeared when the current through the TES was large (≳10 μA) , and its level, defined as a fluctuation amplitude of the current through the TES, was in proportion to the inverse of the TES resistance. We also found that the TES sensitivity depended on the current through the TES, normalized with the critical current of the TES. When the current exceeded about 1% of the critical current, the TES sensitivity was significantly degraded. The critical current clearly correlated with the TES sensitivity, and hence the signal-to-noise ratio at the optimal operating point, among devices with different TES size and thickness. Thus, the critical current is one of the key parameters to determine the performance of the TES microcalorimeter.

Performance analyses of TES microcalorimeters with mushroom shaped X-ray absorbers made of Sn or Bi

Performance of microcalorimeters with a Ti/Au transition-edge sensor (TES) and an electrodeposited absorber has been investigated. The absorbers are in a mushroom-shape made of Sn or Bi. With an Sn absorber we observed a long time scale component of 1.8ms for X-ray pulses, which is due presumably to recombination of quasiparticles into phonons. As a result, the energy resolution is not good with the Sn absorber. Better energy resolution of 28±1eV was obtained with an incomplete-shaped Bi absorber, although the long component was also seen with smaller level. A calorimeter with only a neck of the Au seed layer for the electrodeposition had the best energy resolution of 6.3±0.4eV in spite of saturated pulses and relatively high transition temperature of 210mK.

Analyses on the operating point dependence of the energy resolution with a Ti/Au TES microcalorimeter

We examined the performance of a single pixel Ti/Au transition-edge sensor (TES) calorimeter for incident X-ray energies of Al-Kα, Cr-Kα, and Fe-Kα, as a function of the TES resistance. We find that the energy resolution does not always degrade with increasing energy. The best energy resolution of 5.7±0.9eV at 6.4keV is obtained, which is possibly even better than the baseline width of 6.5±0.2eV. Assuming that the noise level is determined by the noise spectrum NS(f;R∼R+dR(E)) considering the resistance change of dR(E), instead of NS(f;R) at the operating point, these results may be explained by the fact that the noise decreases at the higher TES resistance. The pulse variation appears to have a minimum at a certain resistance of R+dR(E)≃48mΩ, and the best energy resolution for each line is obtained at such an operating point, respectively. The pulse variation could be enhanced when the fluctuation of the TES sensitivity α is large at R+dR(E).

The non-equilibrium response of a high-resolution Ti/Au X-ray microcalorimeter

We have studied the response of a high resolution Ti/Au transition edge sensor (TES) microcalorimeter with an energy resolution of 6.2±0.7eV at 1.5keV and 7.8±0.9eV at 6.4keV. We find that the sensitivity α≡R/TdR/dT of the TES varies significantly along the superconducting-to-normal transition that the bias point traverses during a pulse. Furthermore, α is reduced significantly during a pulse compared to its equilibrium value calculated from the DC characteristics. This leads to an overestimate of the expected energy resolution when basing the prediction on the equilibrium sensitivity.