Transition-edge Sensor Microcalorimeters Research Articles

Nanoscale Three-Dimensional Imaging of Integrated Circuits Using a Scanning Electron Microscope and Transition-Edge Sensor Spectrometer.

X-ray nanotomography is a powerful tool for the characterization of nanoscale materials and structures, but it is difficult to implement due to the competing requirements of X-ray flux and spot size. Due to this constraint, state-of-the-art nanotomography is predominantly performed at large synchrotron facilities. We present a laboratory-scale nanotomography instrument that achieves nanoscale spatial resolution while addressing the limitations of conventional tomography tools. The instrument combines the electron beam of a scanning electron microscope (SEM) with the precise, broadband X-ray detection of a superconducting transition-edge sensor (TES) microcalorimeter. The electron beam generates a highly focused X-ray spot on a metal target held micrometers away from the sample of interest, while the TES spectrometer isolates target photons with a high signal-to-noise ratio. This combination of a focused X-ray spot, energy-resolved X-ray detection, and unique system geometry enables nanoscale, element-specific X-ray imaging in a compact footprint. The proof of concept for this approach to X-ray nanotomography is demonstrated by imaging 160 nm features in three dimensions in six layers of a Cu-SiO2 integrated circuit, and a path toward finer resolution and enhanced imaging capabilities is discussed.

Thermalization of Mesh Reinforced Ultra-Thin Al-Coated Plastic Films: A Parametric Study Applied to the Athena X-IFU Instrument.

The X-ray Integral Field Unit (X-IFU) is one of the two focal plane detectors of Athena, a large-class high energy astrophysics space mission approved by ESA in the Cosmic Vision 2015-2025 Science Program. The X-IFU consists of a large array of transition edge sensor micro-calorimeters that operate at ~100 mK inside a sophisticated cryostat. To prevent molecular contamination and to minimize photon shot noise on the sensitive X-IFU cryogenic detector array, a set of thermal filters (THFs) operating at different temperatures are needed. Since contamination already occurs below 300 K, the outer and more exposed THF must be kept at a higher temperature. To meet the low energy effective area requirements, the THFs are to be made of a thin polyimide film (45 nm) coated in aluminum (30 nm) and supported by a metallic mesh. Due to the small thickness and the low thermal conductance of the material, the membranes are prone to developing a radial temperature gradient due to radiative coupling with the environment. Considering the fragility of the membrane and the high reflectivity in IR energy domain, temperature measurements are difficult. In this work, a parametric numerical study is performed to retrieve the radial temperature profile of the larger and outer THF of the Athena X-IFU using a Finite Element Model approach. The effects on the radial temperature profile of different design parameters and boundary conditions are considered: (i) the mesh design and material, (ii) the plating material, (iii) the addition of a thick Y-cross applied over the mesh, (iv) an active heating heat flux injected on the center and (v) a Joule heating of the mesh. The outcomes of this study have guided the choice of the baseline strategy for the heating of the Athena X-IFU THFs, fulfilling the stringent thermal specifications of the instrument.

Kinetic Inductance Traveling Wave Amplifier Designs for Practical Microwave Readout Applications

A Kinetic Inductance Traveling Wave Amplifier (KIT) utilizes the nonlinear kinetic inductance of superconducting films, particularly niobium titanium nitride (NbTiN), for parametric amplification. These amplifiers achieve remarkable performance in terms of gain, bandwidth, and compression power and frequently approach the quantum limit for noise. However, most KIT demonstrations have been isolated from practical device readout systems. Using a KIT as the first amplifier in the readout chain of an unoptimized microwave SQUID multiplexer coupled to a transition-edge sensor microcalorimeter, we see an initial improvement in the flux noise [1]. One challenge in KIT integration is the considerable microwave pump power required to drive the non-linearity. To address this, we have initiated efforts to reduce the pump power by using thinner NbTiN films and an inverted microstrip transmission line design. In this article, we present the new transmission line design, fabrication procedure, and initial device characterization—including gain and added noise. These devices exhibit over 10 dB of gain with a 3 dB bandwidth of approximately 5.5–7.25 GHz, a maximum practical gain of 12 dB, and typical gain ripple under 4 dB peak to peak. We observe an appreciable impedance mismatch in the NbTiN transmission line, which is likely the source of the majority of the gain ripple. Finally, we perform an initial noise characterization and demonstrate system-added noise of three quanta or less over nearly the entire 3 dB bandwidth.

Characterization of a hybrid array of single and multi-absorber transition-edge sensor microcalorimeters for the Line Emission Mapper

Characterization of a hybrid array of single and multi-absorber transition-edge sensor microcalorimeters for the Line Emission Mapper

Characterizations of the electrothermal parameters of a transition edge sensor microcalorimeter and its energy resolution

We developed a transition-edge sensor microcalorimeter (μ-calorimeter) using Mo/Au/Au thin films. We report the detector fabrication, the measurements of the electrothermal parameters and the noise characterizations of the transition-edge sensor μ-calorimeters. We estimated the energy resolution of the μ-calorimeter using the measured noise spectrum and the calculated power-to-current responsivity. Using a 55Fe radioactive X-ray source, we characterized the detector responses to incident photons at various working points. The best instrumental energy resolution achieved was 4.13 eV @ 5.9 keV, which agrees qualitatively with the resolution of 4.01 eV estimated from the noise spectrum.

Long Term Performance Stability of Transition-Edge Sensor Detectors

We are developing superconducting transition-edge sensor (TES) microcalorimeter arrays for a variety of applications such as ground-based laboratory astrophysics experiments and next generation space-based X-ray missions. These detectors can provide X-ray spectral information with an unprecedent resolution of ∼2 eV at 6 keV and have for instance been selected for the X-ray Integral Field Unit (X-IFU) instrument of ESA's large flagship mission Athena. To maintain detector performance over the lifetime of the mission, it is important to understand whether environmental conditions that the detector may be exposed to will affect its properties over time. This “aging” begins right after the array leaves the fabrication environment, with potential exposure to humidity, oxygen, or elevated temperatures which may affect the detector performance. In a few prior arrays we have observed increased fall times in the pulse shape and/or the introduction of anomalous low energy tails on the X-ray spectrum. This is thought to be an indication of “aging” on chips exposed to such conditions, causing e.g. changes in the absorber properties. In this contribution, we report on a systematic characterization of TES properties, before and after exposing the chip to various controlled temperature and humidity levels and assess the changes in the measured transition and pulse shapes, energy resolution, and spectral redistribution.

Demonstration of a Full-Scale Brassboard TES Microcalorimeter Array for the Athena X-IFU

We have characterized a full-scale transition-edge sensor (TES) microcalorimeter array as a milestone towards the demonstration of technology readiness level (TRL) 5 for the detector array for the X-ray Integral Field Unit (X-IFU) instrument on ESA's future flagship X-Ray Observatory called Athena. We fabricated a 90 mm full-scale prototype TES array and measured the properties and the performance in a newly developed platform in which up to 960 out of ∼3,200 pixels can be read out. In this paper, we report on measurements of the uniformity of the transition temperature and shape, the uniformity of the spectral energy resolutions at 7 keV and 10 keV, the thermal crosstalk for the first, the diagonal, and the second nearest neighbors, and the energy sensitivities to environment perturbations for the magnetic field, the TES bias voltage, and the heat bath temperature.

Two-Dimensional Position-Sensitive Transition-Edge Sensor Microcalorimeters for Gamma Rays

A transition-edge sensor (TES) microcalorimeter has been developed to measure high-energy gamma-rays in the range of up to several MeV. We improve the positional resolution of a position-sensitive TES (PoST) for gamma rays according to the energy resolution and plan to develop a two-dimensional (2D) PoST. Therefore, we devise a method to achieve 2D-position detection using a one-dimensional PoST as a feasible proposal given the current stage of technology.

Correcting Gain Drift in TES Detectors for Future X-Ray Satellite Missions

Changes in the operating environment of transition-edge sensor (TES) microcalorimeters can cause variations in the detector gain function over time. If not corrected, this can degrade the spectral resolution, and cause systematic errors in the knowledge of the absolute energy. The non-linear nature of the TES energy scale function and the potential for multiple, simultaneous sources of drift can make effective corrections extremely challenging. Satellite instruments typically employ an on-board calibration source to provide known reference X-ray lines. This allows real-time monitoring of the detector gain stability and provides information that can be used to correct for drifts. Here we discuss progress towards demonstrating that the energy scale requirements can be met for future instruments such as Athena X-IFU. We present measurements (from ∼ 1-12 keV) on ∼ 200 pixels in a prototype X-IFU array. We use a non-linear drift correction algorithm that uses two fiducial calibration lines (5.4 keV and 8.0 keV) to track gain and interpolate a new, corrected gain between a set of three pre-calibrated gain functions that span the anticipated range of induced drifts. We demonstrate this algorithm is effective at correcting the full gain scale in the presence of multiple sources of environmental drift.

511-CAM mission: a pointed 511 keV gamma-ray telescope with a focal plane detector made of stacked transition edge sensor microcalorimeter arrays

The 511 keV γ-ray emission from the galactic center region may fully or partially originate from the annihilation of positrons from dark matter particles with electrons from the interstellar medium. Alternatively, the positrons could be created by astrophysical sources, involving exclusively standard model physics. We describe here a new concept for a 511 keV mission called 511-CAM (511 keV gamma-ray camera using microcalorimeters) that combines focusing γ-ray optics with a stack of transition edge sensor microcalorimeter arrays in the focal plane. The 511-CAM detector assembly has a projected 511 keV energy resolution of 390 eV full width half maximum or better, and improves by a factor of at least 11 on the performance of state-of-the-art Ge-based Compton telescopes. Combining this unprecedented energy resolution with sub-arcmin angular resolutions afforded by Laue lens or channeling optics could make substantial contributions toward identifying the origin of the 511 keV emission through discovering and characterizing point sources and measuring line-of-sight velocities of the emitting plasmas.

Proof-of-Principle Experiment for Testing Strong-Field Quantum Electrodynamics with Exotic Atoms: High Precision X-Ray Spectroscopy of Muonic Neon.

To test bound-state quantum electrodynamics (BSQED) in the strong-field regime, we have performed high precision x-ray spectroscopy of the 5g-4f and 5f- 4d transitions (BSQED contribution of 2.4 and 5.2eV, respectively) of muonic neon atoms in the low-pressure gas phase without bound electrons. Muonic atoms have been recently proposed as an alternative to few-electron high-Z ions for BSQED tests by focusing on circular Rydberg states where nuclear contributions are negligibly small. We determined the 5g_{9/2}- 4f_{7/2} transition energy to be 6297.08±0.04(stat)±0.13(syst) eV using superconducting transition-edge sensor microcalorimeters (5.2-5.5eV FWHM resolution), which agrees well with the most advanced BSQED theoretical prediction of 6297.26eV.

Background rates of x-ray transition-edge sensor micro-calorimeters under a frequency domain multiplexing readout for solar axion-like particles’ detection

We report on the x-ray background rate measured with transition-edge sensors (TES) micro-calorimeters under frequency-domain multiplexing (FDM) readout as a possible technology for future experiments aiming at a direct detection of axion-like particles. Future axion helioscopes will make use of large magnets to convert axions into photons in the keV range and x-ray detectors to observe them. To achieve this, a detector array with high spectral performance and extremely low background is necessary. TES are single-photon, non-dispersive, high-resolution micro-calorimeters and represent a possible candidate for this application. We have been developing x-ray TES micro-calorimeters and an FDM readout technology in the framework of the space-borne x-ray astronomical observatories. We show that the current generation of our detectors is already a promising technology for a possible axion search experiment, having measured an x-ray background rate of 2.2(2) × 10−4 cm−2 s−1 keV−1 with a cryogenic demonstrator not optimized for this specific application. We then make a prospect to further improve the background rate down to the required value (<10−7 cm−2 s−1 keV−1) for an axion-search experiment, identifying no fundamental limits to reach such a level.

Frequency domain multiplexing readout for large arrays of transition-edge sensors

We report our most recent progress and demonstration of a frequency domain multiplexing (FDM) readout technology for transition-edge sensor (TES) arrays, both of which we have been developing in the framework of the X-IFU instrument on board the future Athena X-ray telescope. Using Ti/Au TES micro-calorimeters, high-Q LC filters and analog/digital electronics developed at SRON and low-noise two-stage SQUID amplifiers from VTT Finland, we demonstrated the feasibility of our FDM readout technology, with the simultaneous readout of 37 pixels with an energy resolution of 2.23 eV at an energy of 6 keV. We finally outline our plans for further scaling up and improving our technology in the future.

High-resolution Compton spectroscopy using x-ray microcalorimeters.

X-ray Compton spectroscopy is one of the few direct probes of the electron momentum distribution of bulk materials in ambient and operando environments. We report high-resolution inelastic x-ray scattering experiments with high momentum and energy transfer performed at a storage-ring-based high-energy x-ray light source facility using an x-ray transition-edge sensor (TES) microcalorimeter detector. The performance was compared with a silicon drift detector (SDD), an energy-resolving semiconductor detector, and Compton profiles were measured for lithium and cobalt oxide powders relevant to lithium-ion battery research. Spectroscopic analysis of the measured Compton profiles demonstrates the high-sensitivity to the low-Z elements and oxidation states. The line shape analysis of the measured Compton profiles in comparison with computed Hartree-Fock profiles is usually limited by the resolution of the semiconductor detector. We have characterized an x-ray TES microcalorimeter detector for high-resolution Compton scattering experiments using a bending magnet source at the Advanced Photon Source with a double crystal monochromator, providing monochromatic photon energies near 27.5keV. The momentum resolution below 0.16 atomic units (a.u.) was measured, yielding an improvement of more than a factor of 7 over a state-of-the-art SDD for the same scattering geometry. Furthermore, the lineshapes of narrow valence and broad core electron profiles of sealed lithium metal were clearly resolved using an x-ray TES compared to smeared and broadened lineshapes observed when using the SDD. High-resolution Compton scattering using the energy-resolving area detector shown here presents new opportunities for spatial imaging of electron momentum distributions for a wide class of materials with applications ranging from electrochemistry to condensed matter physics.

Absolute Energy Measurements with Superconducting Transition-Edge Sensors for Muonic X-ray Spectroscopy at 44 keV

Superconducting transition-edge sensor (TES) microcalorimeters have great utility in x-ray applications owing to their high energy resolution, good collecting efficiency and the feasibility of being multiplexed into large arrays. In this work, we develop hard x-ray TESs to measure the absolute energies of muonic-argon ($\mu$-Ar) transition lines around 44 keV and 20 keV. TESs with sidecar absorbers of different heat capacities were fabricated and characterized for their energy resolution and calibration uncertainty. We achieved ~ 1 eV absolute energy measurement accuracy at 44 keV, and < 12 eV energy resolution at 17.5 keV.

Development of an End-to-End Demonstration Readout Chain for Athena/X-IFU

The X-ray Integral Field Unit (X-IFU) of the Athena observatory, scheduled for launch in the mid 2030's, will provide X-ray spectroscopy data with unprecedented spectral and spatial resolution. This will be achieved with a 2 kilo-pixel array of transition-edge sensor (TES) microcalorimeters. The complete detection chain is under development by a large international collaboration. In order to perform an end-to-end demonstration of the X-IFU readout chain, a 50 mK test bench is being developed at IRAP in collaboration with CNES. The test bench uses a two-stage ADR cryostat from Entropy GmbH, a 1024-pixel array, and will initially be operated using a warm electronics chain from NIST and NASA Goddard Space Flight Center. We describe the complete system being installed in the cryostat and the current results obtained with these electronics. We also review the status of the integration of the digital readout electronics (DRE) prototype into the demonstration chain and the plan for integrating and testing the complete X-IFU readout chain.

Thermal Crosstalk of X-Ray Transition-Edge Sensor Micro-Calorimeters Under Frequency Domain Multiplexing Readout

We have measured and characterized the thermal crosstalk in two different arrays of transition-edge sensor (TES) X-ray micro-calorimeters with frequency-domain multiplexing (FDM) readout. The TES arrays are fabricated at SRON and are a 8$\times$8 and a 32$\times$32 "kilo-pixel" uniform array. The amount of crosstalk is evaluated as the ratio between the averaged crosstalk signal and the X-ray pulse amplitudes. The crosstalk ratios (CR) for our detectors are compliant with the requirements for future X-ray space missions, such as Athena X-IFU (CR$< 10^{-3}$ for first-neighbour pixels): we measured a nearest-neighbour thermal crosstalk ratio at a level of $10^{-4}$, with a highest crosstalk ratio of $4\times 10^{-4}$ for the kilo-pixel array (worst case, center of array) and $1\times 10^{-4}$ for the 8$\times$8 array, with a margin of improvement achievable by optimizing the Cu metallization and the width of the Si supporting structures (muntins) in the backside of the TES array chip. Based on the measured crosstalk ratios, we have estimated the impact on the spectral resolution by means of noise equivalent power (NEP) considerations and a Monte Carlo simulation, finding an average degradation in quadrature of less than 40~meV, compliant with the < 0.2~eV requirement for Athena X-IFU.

Calibration and Testing of Small High-Resolution Transition Edge Sensor Microcalorimeters with Optical Photons.

Pulses of narrow line-width optical photons can be used to calibrate and test sub-2 eV full-width at halfmaximum (FWHM) energy resolution transition-edge sensor (TES) microcalorimeters at low energies (< 1 keV), where it is very challenging to obtain X-ray calibration lines comparable to (or narrower than) the detector resolution. This scheme depends on the ability to resolve the number of 3 eV photons in each pulse, which we have recently demonstrated up to photon numbers of about 300. At LTD-18 we showed preliminary results obtained with this technique on a 0.25 eV baseline resolution TES microcalorimeter designed for the ultra-high-resolution subarray of the Lynx mission. The line-shape was well described by a simple Gaussian. However, the difficulty of delivering photons to the small 46 μm square absorbers resulted in a large thermal crosstalk signal, whose random nature is expected to rapidly degrade the observed energy resolution towards higher photon numbers/energies. We have since improved the coupling between the optical fiber and the TES absorber and report here our current results.

Deexcitation Dynamics of Muonic Atoms Revealed by High-Precision Spectroscopy of Electronic K X Rays.

We observed electronic K x rays emitted from muonic iron atoms using superconducting transition-edge sensor microcalorimeters. The energy resolution of 5.2eV in FWHM allowed us to observe the asymmetric broad profile of the electronic characteristic Kα and Kβ x rays together with the hypersatellite K^{h}α x rays around 6keV. This signature reflects the time-dependent screening of the nuclear charge by the negative muon and the L-shell electrons, accompanied by electron side feeding. Assisted by a simulation, these data clearly reveal the electronic K- and L-shell hole production and their temporal evolution on the 10-20fs scale during the muon cascade process.

Design, fabrication, and characterization of Ti/Au transition-edge sensor with different dimensions of suspended beams

For photon detection, superconducting transition-edge sensor (TES) micro-calorimeters are excellent energy-resolving devices. In this study, we report our recent work in developing Ti-/Au-based TES. The Ti/Au TES devices were designed and implemented with a thickness ratio of 1:1 and different suspended structures using micromachining technology. The characteristics were evaluated and analyzed, including surface morphology, 3D deformation of suspended Ti/Au TES device structure, I–V characteristics, and low-temperature superconductivity. The results showed that the surface of Ti/Au has good homogeneity and the surface roughness of Ti/Au is significantly increased compared with the substrate. The structure of Ti/Au bilayer film significantly affects the deformation of suspended devices, but the deformation does not affect the I–V characteristics of the devices. For devices with the Ti/Au bilayer (150 – m × 150 μm) and beams (100 μ m × 25 μm), the transition temperature (T c) is 253 mK with a width of 6 mK, and the value of the temperature sensitivity α is 95.1.