Bilayer Transition Edge Sensor Research Articles

Detection of low-energy electrons with transition-edge sensors

We present the detection of electrons with kinetic energy in the 100 eV range with transition-edge sensors (TESs). This has been achieved with a (100×100)-μm2 Ti/Au bilayer TES, with a critical temperature of about 84 mK. The electrons are produced directly in the cryostat by an innovative cold source based on field emission from vertically aligned multiwall carbon nanotubes. We obtain a Gaussian energy resolution between 0.8 and 1.8 eV for fully absorbed electrons in the (90–101)eV energy range, which is found to be compatible with the resolution of this same device for photons in the same energy range. This work opens possibilities for high-precision energy measurements of low-energy electrons. Published by the American Physical Society 2024

An optical transition-edge sensor with high energy resolution

Optical transition-edge sensors (TESs) have shown an energy resolution for resolving the number of incident photons at the telecommunication wavelength. However, a higher energy resolution is required for biological imaging and microscopic spectroscopy. In this study, we tested an Au/Ti (10/20 nm) bilayer TES that showed a high energy resolution. The high energy resolution was achieved by lowering the critical temperature to 115 mK, and the resultant energy resolution was 67 meV full width at half maximum (FWHM) at 0.8 eV. When was reduced to 115 mK, the theoretical resolution would scale up to 30 meV FWHM, considering that the typical energy resolution of optical TESs was 150 meV and was 300 mK. To investigate the difference between the theoretical expectation (30 meV) and the measured value (67 meV), we measured the complex impedance and current noise of the TES. We found excess Johnson noise in the TES; the excess Johnson noise term M was 1.5 at a bias point where the resistance was 10% of the normal resistance. For reference, the above mentioned TES was compared with a TES showing typical energy resolution (156 meV FWHM). We also discussed factors that improved or inhibited the energy resolution.

Diagnosing flux penetration condition of the Mo/Au bilayer transition-edge sensor

The magnetic-field-dependence critical current Ic(Ha) is measured for the Mo/Au bilayer transition-edge sensors. A model based on a surface barrier delaying the first penetration of magnetic flux into a flat superconducting strip, including both the geometrical and Bean–Livingston barrier, in combination with two Josephson junction models with different characteristic lengths, is found to ubiquitously fit the Ic(Ha) curves of devices with various geometry designs. The bulk penetration depth, London depairing current density, Josephson junction critical current, self-field coupling coefficient, and conditions of the first vortex entry are investigated throughout the superconducting transition. The evidences of Josephson effect are observed in both Fraunhofer interference pattern and Shapiro steps.

Excess Heat Capacity in Mo/Au Transition Edge Sensor Bolometric Detectors.

Excess heat capacity in a bolometric detector has the consequence of increasing or leading to multiple device time constants. The Mo/Au bilayer transition edge sensor (TES) bolometric detectors initially fabricated for the high resolution mid-infrared spectrometer (HIRMES) exhibited two response thermalization scales, one of which is a few times longer than estimates based upon the properties of the bulk materials employed in the design. The relative contribution of this settling time to the overall time response of the detectors is roughly proportional to the pixel area, which ranges between ~0.3 and 2.6 mm2. Use of laser ablation to remove sections of the silicon membranes comprising the pixels results in a detector response with a smaller contribution from the secondary time constant. Additional information about the nature of this excess heat capacity is gleaned from glancing incidence x-ray diffraction, which reveals the presence of molybdenum silicides near the silicon surface which is a consequence of the bi-layer deposition. Quantitative analysis of the concentration of excess molybdenum, estimated with secondary ion mass spectroscopy, is commensurate to the additional heat capacity needed to explain the anomalous time response of the detectors.

Modeling Low-TC Transition-Edge Sensors Made of NS Bilayers: The Specific Interface Resistance

One way of making a transition-edge sensor (TES) is by utilizing the proximity effect, in which the $$T_{C}$$ of a superconducting film is reduced with a normal metal film in metallic contact. The $$T_{C}$$ of a bilayer TES can be estimated by solving the Usadel equations with given boundary conditions. The classical boundary conditions of a bilayer include a specific interface resistance being temperature-independent. In this paper, we will introduce a temperature-dependent specific interface resistance. By fitting the measured $$T_{C}$$ data of Ir/Au bilayers from the literature to a $$T_{C}$$ calculation model, we will compare the fit parameters and fit errors with the temperature-dependent specific interface resistance described in this work and with the classical temperature-independent specific interface resistance.

Study of Dissipative Losses in AC-Biased Mo/Au Bilayer Transition-Edge Sensors

We are developing kilo-pixel arrays of transition-edge sensors (TESs) for the X-ray Integral Field Unit on ESA’s Athena observatory. Previous measurements of AC-biased Mo/Au TESs have highlighted a frequency-dependent loss mechanism that results in broader transitions and worse spectral performance compared to the same devices measured under DC bias. In order to better understand the nature of this loss, we are now studying TES pixels in different geometric configurations. We present measurements on devices of different sizes and with different metal features used for noise mitigation and X-ray absorption. Our results show how the loss mechanism is strongly dependent upon the amount of metal in close proximity to the sensor and can be attributed to induced eddy current coupling to these features. We present a finite element model that successfully reproduces the magnitude and geometry dependence of the losses. Using this model, we present mitigation strategies that should reduce the losses to an acceptable level.

Characterization of Si Membrane TES Bolometer Arrays for the HIRMES Instrument

The high-resolution mid-infrared spectrometer instrument will fly onboard the National Aeronautics and Space Administration’s airborne stratospheric observatory for infrared astronomy in 2019. It will provide astronomers with a unique observing window (25–122 $$\upmu \hbox {m}$$ ) for exploring the evolution of protoplanetary disks into young solar systems. There are two focal plane detector arrays for the instrument: a high-resolution ( $$\lambda / {\varDelta }\lambda \,=\,100{,}000$$ ) $$8\times 16$$ detector array, with a target noise-equivalent power, $$\hbox {NEP} \le 3 \hbox { aW}/\sqrt{\mathrm{Hz}}$$ , and a low-resolution ( $$\lambda / {\varDelta }\lambda =600$$ –19,000) $$16\times 64$$ detector array with a target $$\hbox {NEP }\le 20\hbox { aW}/\sqrt{\mathrm{Hz}}$$ . The detectors for both of these arrays are superconducting Mo/Au bilayer transition-edge sensor bolometers on suspended single-crystal silicon membranes. We present detector characterization results for both arrays, including measurements of thermal conductance in comparison with phonon transport models, and measurements of saturation power and noise.

Design and Performance of Hybrid Arrays of Mo/Au Bilayer Transition-Edge Sensors

For future X-ray astrophysics missions, X-ray microcalorimeters can be optimized with different properties in different regions of the focal plane. This approach has the potential to improve microcalorimeter instrument capabilities with efficient use of instrument resources. For example a point-source array optimized for high angular resolution, high count-rate observations could be accompanied by a main array to expand the field of view for diffuse observations. In this approach, it is desirable to be able to simultaneously optimize different transition-edge sensor (TES) geometries on a single wafer design. The key properties of TESs such as transition temperature and shape are a strong function of size and geometry due to the complex interplay between the proximity effect from the superconducting bias electrodes and the normal metal features used for noise suppression and absorber contact. As a result, devices fabricated with the same deposited layer but with different sizes will have different transition temperatures and different response to X-ray events. In this paper, we present measurements of the transition temperature and properties of devices with different sizes and normal metal features, and discuss how by tuning the geometry we can achieve the desired pixel parameters for a given application. We also describe measurements of transition properties from large-format hybrid arrays containing three different pixel types.

Optical response of Al/Ti bilayer transition edge sensors

We report the optical response characteristics of Al/Ti bilayer transition edge sensors (TESs), which are mainly comprised of Al/Ti bilayer thermometers and suspended SiN membranes for thermal isolation. The measurement was performed in a 3He sorption refrigerator and the device's response to optical pulses was investigated using a pulsed laser source. Based on these measurements, we obtained the effective recovery time (τeff) of the devices at different biases and discussed the dependence of τeff on the bias. The device with a 940 μm × 940 μm continuous suspended SiN membrane demonstrated a fast response speed with τeff = 3.9 μs, which indicates a high temperature sensitivity (α = T/R · dR/dT = 326). The results also showed that the TES exhibits good linearity under optical pulses of variable widths.

Radiation Tolerance Evaluation of the Ti/Au Bilayer TES Microcalorimeter

We have developed Ti/Au bilayer transition-edge sensor (TES) microcalorimeters for future X-ray astrophysical satellite missions such as DIOS. One possible concern on the space use of TES microcalorimeters is its radiation tolerance. We have evaluated the performance of a Ti/Au bilayer (30/40 nm thick) TES microcalorimeter with 1.5 \(\upmu \)m thick Au absorber, before and after irradiation of 150 MeV proton beam with a total dose of 10 krad, corresponding to 10 years in the low Earth orbit. No significant changes on transition temperature, sensitivity, normal resistance, and critical current were observed. The energy resolution for 5.9 keV X-rays was 5.6 \(\pm \) 0.3 eV (FWHM) after the irradiation, which was slightly worse than 5.1 \(\pm \) 0.3 eV before the irradiation. We consider that our TES has sufficient radiation tolerance in orbit.

Implications of weak-link behavior on the performance of Mo/Au bilayer transition-edge sensors

Understanding the physical properties of the superconducting-to-normal transition is fundamental for optimizing the design and performance of transition-edge sensors (TESs). Recent critical current IC measurements of square Mo/Au bilayer structures show that they act as weak superconducting links, exhibiting oscillatory, Fraunhofer-like behavior with applied magnetic field. In this paper, we investigate the implications of this behavior for TES x-ray detectors operated in the resistive transition. These devices include normal metal features used for absorber attachment and suppression of detector noise. We present extensive measurements of IC as a function of temperature T and field B, which show a complex temperature and current evolution when compared with the behavior expected from a simple geometry. We introduce a resistively shunted junction model for describing the TES resistive transition as a function of current I, temperature T, and magnetic field B. From this model, we calculate the R(T,I,B) transition and the logarithmic resistance sensitivity with respect to T and I (α and β, respectively), as a function of applied magnetic field and operating point within the resistive transition. Different examples are presented to illustrate the role of critical current on the transition parameters, and results are qualitatively compared with measurements. Results show that the important device parameters α and β exhibit oscillatory behavior with applied magnetic field due to the modulation of the critical current. This in turn affects the signal responsivity and noise, and the predicted energy resolution. These results show the significance of the critical current in determining the performance of TESs and how externally applied and self-induced magnetic fields can affect the transition and, thus, hold promise for future optimization.

Mo/Au Bilayer Superconducting Transition Edge Sensor Tuning With Surface Modification Structures

We have conducted an experimental study tuning the critical temperature, transition width, normal resistance, and critical current of Mo/Au bilayer transition edge sensors (TES) using Nb stripes on the TES surface. The Nb stripes modify the TES parameters through the lateral proximity effect. We summarize the dependence of the shift in transition temperature and the broadening of the transition width on the separation length between the Nb stripes and leads. We calculate the Ginzburg-Landau coherence length of the TES in two methods. One takes advantage of the dependence of the TES normal resistance on the number of the TES and Nb contacts. Another utilizes the temperature dependence of the TES critical current. The two methods are in good agreement with a coherence length ξ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ≈ 0.4 μm.

An Absorber-coupled TES Bolometer for Measuring CMB Polarization

We report an absorber-coupled superconducting Transition Edge Sensor (TES) bolometric polarimeter for measuring Cosmic Microwave Background (CMB) B-mode polarization. The polarimeter consists of a dipole-like PdAu absorber and a Mo/Au bi-layer TES on a suspended silicon nitride rectangle. Its design, fabrication, thermal properties, and optical performance are presented.

Proximity effects and nonequilibrium superconductivity in transition-edge sensors

We have recently shown that normal-metal/superconductor (N/S) bilayer TESs (superconducting Transition-Edge Sensors) exhibit weak-link behavior.1 Here we extend our understanding to include TESs with added noise-mitigating normal-metal structures (N structures). We find TESs with added Au structures also exhibit weak-link behavior as evidenced by exponential temperature dependence of the critical current and Josephson-like oscillations of the critical current with applied magnetic field. We explain our results in terms of an effect converse to the longitudinal proximity effect (LoPE)1, the lateral inverse proximity effect (LaiPE), for which the order parameter in the N/S bilayer is reduced due to the neighboring N structures. Resistance and critical current measurements are presented as a function of temperature and magnetic field taken on square Mo/Au bilayer TESs with lengths ranging from 8 to 130 {\mu}m with and without added N structures. We observe the inverse proximity effect on the bilayer over in-plane distances many tens of microns and find the transition shifts to lower temperatures scale approximately as the inverse square of the in- plane N-structure separation distance, without appreciable broadening of the transition width. We also present evidence for nonequilbrium superconductivity and estimate a quasiparticle lifetime of 1.8 \times 10-10 s for the bilayer. The LoPE model is also used to explain the increased conductivity at temperatures above the bilayer's steep resistive transition.

Development of a Gamma-Ray Detector With Iridium Transition Edge Sensor Coupled to a Pb Absorber

We have recently started to develop a high-resolution gamma-ray spectrometer for material defect analysis. Our gamma-ray detector is a microcalorimeter consisting of an iridium/gold bilayer transition edge sensor (TES) thermometer and a bulk Pb absorber, which is directly coupled to a TES with Stycast 2850FT epoxy. This paper describes our TES based gamma-ray detector and the first experimental results for 662 keV and 1173 keV incident gamma-rays. We report an energy resolution of 4.7 keV at 662 keV and 2.9 keV at 1173 keV.

Design of a Transition-Edge Hot-Electron Microbolometer for Millimeter-Wave Astrophysical Observations

We are developing a Transition-edge Hot-electron Microbolometer (THM) with the capacity to make sensitive and broadband astrophysical observations over frequencies ranging from 30-300 GHz (10-1 mm). This micron-sized bolometer consists of a superconducting bilayer Transition-Edge Sensor (TES) and a thin-film absorber. The THM employs the decoupling between electrons and phonons at low temperatures (below 300 mK) to provide thermal isolation. The devices are fabricated photolithographically and read out with Superconducting Quantum Interference Devices (SQUIDs). We present the details of a thermal model for a THM detector and the design for new thermally optimized antenna-coupled THMs for illumination by a RF source at 40 and 100 GHz.

TES Development for a Frequency Selective Bolometer Camera

We discuss the development, at Argonne National Laboratory (ANL), of a four-pixel camera with four spectral channels centered at 150, 220, 270, and 360 GHz. The scientific motivation involves photometry of distant dusty galaxies located by Spitzer and SCUBA, as well as the study of other millimeter-wave sources such as ultra-luminous infrared galaxies, the Sunyaev-Zeldovich effect in clusters, and galactic dust. The camera incorporates Frequency Selective Bolometer (FSB) and superconducting Transition-Edge Sensor (TES) technology. The current generation of TES devices we examine utilizes proximity effect superconducting bilayers of Mo/Au, Ti, or Ti/Au as TESs, located along with frequency selective absorbing structures on silicon nitride membranes. The detector incorporates lithographically patterned structures designed to address both TES device stability and detector thermal transport concerns. The membrane is not perforated, resulting in a detector which is comparatively robust mechanically. In this paper, we report on the development of the superconducting bilayer TES technology, the design and testing of the detector thermal transport and device stability control structures, optical and thermal test results, and the use of new materials.

Characterizing the Superconducting-to-Normal Transition in Mo/Au Transition-Edge Sensor Bilayers

We are developing arrays of Mo/Au bilayer transition-edge sensors (TES’s) for future X-ray astronomy missions such as NASA’s Constellation-X. The physical properties of the superconducting-to-normal transition in our TES bilayers, while often reproducible and characterized, are not well understood. The addition of normal metal features on top of the bilayer are found to change the shape and temperature of the transition, and they typically reduce the unexplained ‘excess’ noise. In order to understand and potentially optimize the properties of the transition, we have been studying the temperature, widths and current dependence of these transitions. We report on the characterization of devices both deposited on silicon substrates and suspended on thin silicon nitride membranes. This includes key device parameters such as the logarithmic resistance sensitivity with temperature α, and the logarithmic resistance sensitivity with current β, and their correlation with excess noise.

Sensitivity Measurements of a Transition-Edge Hot-Electron Microbolometer for Millimeter-Wave Astrophysical Observations

Future experiments to probe the Cosmic Microwave Background (CMB) polarization will need arrays of 1000s of sensitive bolometers. We are developing a Transition-edge Hot-electron Microbolometer (THM) to fill this need. This small-volume bolometer consists of a superconducting bilayer Transition-Edge Sensor (TES) with a thin-film absorber. Unlike traditional monolithic bolometers which make use of micromachined structures, the THM employs the decoupling between electrons and phonons at milliKelvin temperatures to provide thermal isolation. The devices are fabricated photolithographically and are easily integrated with antennas via microstrip transmission lines, and with SQUID readouts. We present the results of noise, responsivity, and thermal conductance measurements in which electrical power is dissipated in the absorber, and confirm a thermal model for a test THM with a Mo/Au TES and Bi/Au absorber.

Reducing Excess Noise in Au/Ti Transition-Edge Sensors

We have studied the noise spectra of Au/Ti bilayer transition-edge sensors with transition temperatures around ∼100 mK. The sensors did not include an absorber element, and were patterned into either a circular Corbino geometry, or a Corbino disk divided into four parallel sections. Both sensor types contained noise in excess of the thermodynamic phonon noise and the equilibrium Johnson noise of the sensor. However, the divided Corbino detector had a significantly lower noise level than the undivided Corbino disk, especially at bias points below the mid-point of the transition.