The development of large-scale, high-resolution transition-edge sensor (TES) arrays requires precise, wafer-scale control of the superconducting critical temperature (Tc). For microcalorimeters, this is often achieved with a superconductor-normal metal bilayer where the thickness ratio between the two materials determines Tc through the proximity effect. Reliably achieving a specific Tc across multiple wafers with good uniformity remains challenging as the fabrication process is highly sensitive to substrate quality, deposition parameters, and deposited thin-film thickness. Missing a Tc target results in an array with degraded or unusable performance and wastes fabrication/measurement resources. To help solve these limitations with TES microcalorimeter fabrication, we have developed a method of measuring 76.2 mm wafers after bilayer deposition to screen for Tc accuracy and uniformity. Four-wire resistance measurements of test structures across the wafer provide rapid feedback early in the fabrication process at temperatures down to 20 mK. We leverage the hybrid additive-subtractive TES process to screen TES arrays with multiple Tc values on a single wafer comprised of a single bilayer. After measurement, the bilayer normal metal thickness can be increased with an additional evaporation step to lower Tc or decreased with an ion mill step to raise Tc as needed. We also present a design to expand measurement capability to 150 mm wafers to screen both larger microcalorimeter arrays as well as NIST cosmic microwave background TES bolometer arrays.

We present a new instrument for soft x-ray absorption and emission spectroscopy based on the latest generation of transition edge sensor array optimized for soft x-ray detection, built on microwave superconducting quantum interference device multiplexing readout, and cooled with a dilution refrigerator. Its extreme collecting efficiency enables spectroscopy measurements on dilute systems. It also enables shorter acquisition time and large energy-range spectra. We describe the design and operation of the spectrometer and characterize its performance in terms of energy resolution, photon collecting efficiency, and stability. A resolution of 0.7-1.8 eV in the energy range 260-900 eV with a high detection rate of up to 10 000 photons per second across the array is achieved with minimal performance degradation. In addition, the use of the dilution refrigerator to cool the detector results in a robust and stable energy calibration over time. The spectrometer is attached to a dedicated ultra-high vacuum sample chamber equipped with a fully motorized sample cryostat for experiments at temperatures between 10 and 300 K, multi-sample mounting, and electric field-gated devices. The spectrometer-sample chamber is installed at a beamline providing full polarization control. We demonstrate the capabilities of the setup using two representative examples of XES on extremely low-concentration systems, namely monolayer hexagonal boron nitride and the K3[Fe(CN)6] molecular system at sub-millimolar concentration.

Superconducting microcalorimeter arrays with hundreds to thousands of pixels are enabling new measurements and capabilities in exotic atom spectroscopy, X-ray astronomy, nuclear materials analysis, and many other fields. We consider the prospects of multiplexing microcalorimeters with the kinetic inductance current sensor (KICS), an alternative to the superconducting quantum interference device (SQUID) historically used to read out cryogenic microcalorimeters. We show that the amplifier-limited noise of the KICS is determined by its dynamic range, which is a free design parameter and tuneable with a DC current bias. We predict that KICS can meet the requirements to read out prototype metallic magnetic calorimeter pixels designed for the Lynx X-ray satellite mission concept with the use of low-latency tone tracking. We propose a KICS design that has both high coupling efficiency and high isolation between the RF and DC components of the circuit. We argue that the KICS provides engineering tradeoffs that are better suited to the readout of microcalorimeters when compared to the analogous microwave SQUID multiplexer.

In this article, we discuss the development of molybdenum-gold bilayer transition-edge sensors on solid silicon substrate microcalorimeter designs suitable for real-time measurement of gamma-ray sources in the keV to MeV range. These novel detector designs incorporate tunable thermal conductance over a range from 1 & times;10(-9) to 4 & times;10(-7) W/K and are mechanically robust. We describe the microfabrication process and the gamma-ray absorber attachment in detail. Initial prototypes obtain detector response times twice as fast as our previous design while maintaining an energy resolution of less than 100 eV full-width-half-maximum at 100 keV.

It is often desirable to steer light within a cryostat and have a well focussed laser spot incident upon a detector. We demonstrate the operation of superconducting transition-edge sensor microcalorimeters with a fiber-coupled laser scanning device colocated on the 20 mK mixing plate of a dilution refrigerator. The scanner directs a 515 nm laser beam with spot size < 40 & micro;m over a range of +/- 14 mm in two orthogonal axes (covering 784 mm(2)). We present the optical elements of the system, show that the detector noise spectrum is unchanged when the scanner is operating or OFF, and observe less than 340 nW power dissipation in our use-case. This device will allow detector calibration up to 10 keV on a pixel-by-pixel basis with simultaneous X-ray measurements. Further, this device will enable more effective characterization of certain detector artifacts including crosstalk, position dependences, and drift corrections.

Arrays of hundreds or thousands of low temperature detectors have been deployed for many experiments, both bolometers for long wavelength applications and calorimeters for shorter wavelength applications. One challenge that is common to many of these arrays is the efficient use of focal plane area to achieve a large fill fraction of absorbers coupled to detectors. We are developing an integrated fabrication of soft X-ray transition edge sensors (TES) and microwave SQUID multiplexers ($\mu$MUX) with the goal of maximizing the fill fraction of the focal plane area on a scale of many thousand pixel detectors. We will utilize lithographically defined high density interconnects to circumvent limitations in existing solutions that use wirebonds or flip-chip bonds. Here we report the first demonstration of combining TES and $\mu$MUX processes into a single TES-System-on-a-Chip (TES-SoC) fabrication on a silicon wafer. The $\mu$MUX SQUIDs and TES electrothermal feedback circuits are microfabricated first and protected with passivating SiO$_{2}$, then the TES devices and TES-to-SQUID interconnects are fabricated, and finally the protective layer is removed before the fabrication of the microwave resonators. We show that the microwave SQUIDs are functional and have reasonable yield, and that we are able to read out the transition temperature of the connected TESs using those SQUIDs.

Hybrid photon counting detectors (HPCDs) have unlocked new capabilities for X-ray-based measurements at synchrotrons around the world in the last 30 years. By leveraging independently optimized sensor and readout layers, they offer high quantum efficiency (>80 %), ultra-low dark counts, sub-pixel point-spread function, and high count rates (>106 counts per pixel per second). Furthermore, their small pixel size and large active area endow them with excellent coverage and resolution for both real-space and reciprocal space imaging. Here, we demonstrate that HPCDs are also well-suited for laboratory-based nanoscale X-ray tomography (nano-xCT). We perform nano-xCT on an integrated circuit fabricated at the 130-nm node and produce a 3D reconstruction with over 40 times more photons collected more than 20 times faster than in this group's previous work, for an overall speedup of over 800×. We review the technical considerations of using an HPCD for tabletop tomography. We quantify our reconstruction image quality using well-established metrics, including the modulation transfer function (MTF), Fourier shell correlation (FSC), and contrast-to-noise (CNR), to validate our choice of experimental parameters that provide sufficient resolution and imaging speed. We determine that under current experimental conditions, 160-nm wiring features are reconstructed at 75-80 nm spatial resolution.

There are a number of considerations when designing a low-temperature detector for the best possible energy resolution. One that has been particularly challenging for thermal kinetic inductance detectors (TKIDs) is ensuring that the detector's response to deposited energy is constant across the active area of a single pixel. In this article, we directly probe the position-dependent response of our TKID devices at 100-μm-scale resolution through the use of a pulsed laser beam steered with a cryogenic microelectromechanical system mirror. We observe position-dependent response that could limit the energy resolution of our current devices to the part per ten scale. We find that the position-dependent response is strongly correlated with the local current density in the TKID inductor, although the thermal physics and geometry associated with the micromachined absorber also likely have a nonnegligible effect. Finally, we suggest analysis methods and design considerations that could reduce the severity of this position dependence, and we argue that there is significant potential for further improvement.

We demonstrate a robust and minimalist implementation of individually addressable optically actuated persistent current switches. We use a focused laser beam directed by a cryogenic laser scanner to drive a selected segment of a superconducting loop normal with a pulse of light. We address six distinct switches with a single electrical bias, and use superconducting quantum interference device (SQUID) readout to show that persistent current is trapped successfully. Based on the cryogenic scanner's performance, we argue that this method can address over 60 000 switches, with the full array being set in about one second. Persistent current switches are widely used in quantum circuits, and we expect the ability to set tens of thousands of persistent currents with a single bias line will be useful in many applications.

We formulate an energy resolution model for optical transition-edge sensors that combines aspects of existing models with the goal of better defining and constraining energy resolution optimization. The combined model is found to be in better agreement with experimental data, while also allowing for theoretical exploration of phonon trapping for energy resolution enhancement. Additionally, we present preliminary data from our recent low critical temperature (T-c= 50 mK to 100 mK) tungsten transition edge sensors (TESs) for the optical to near infra-red (NIR) regime. Our tungsten TES detectors are shown to exhibit curious "inverse" proximity effects compared to what is generally reported in the literature. A variety of wiring scheme test structures are analyzed under varying magnetic field shielding conditions in an effort to mitigate and characterize these spurious effects on the device T-c. We develop an electron beam lithography fabrication method in order to reduce the edge roughness of our tungsten TES devices, and demonstrate a more uniform T-c across various TES sizes when these e-beam lithography devices are measured within superconducting magnetic shielding.

We present the electro-thermal characterization of transition-edge sensor (TES) detectors suspended on Si membranes fabricated using a silicon-on-insulator (SOI) wafer. The use of an all-silicon fabrication platform, in contrast to the more commonly used silicon nitride membranes, is compatible with monolithic fabrication of integrated TES and SQUID circuits. The all-silicon architecture additionally allows efficient use of focal plane area; the readout circuitry may be positioned out of the focal plane by bending a thinned portion of the chip. Compatibility with integrated fabrication and efficient use of focal plane area provide a path to an efficient soft X-ray spectrometer. This work is motivated by our goal to develop a 10,000-pixel TES spectrometer to overcome critical measurement limitations in catalysis research. The characterization of fragile, carbon-based intermediates via techniques like Resonant Inelastic X-ray Scattering (RIXS) is often precluded by the slow, high-flux nature of existing technologies. The new instrument will allow for fast RIXS measurements to be made without causing sample damage. We verify the detector models and measure the energy resolution using a pulsed optical laser, demonstrating the viability of this approach for the final instrument to be deployed at the National Synchrotron Light Source II (NSLS-II).

PAX (antiProtonic Atom X-ray spectroscopy) is a new experiment with the aim to test strong-field quantum electrodynamics (QED) effects by performing high-precision x-ray spectroscopy of antiprotonic atoms. By utilizing advanced microcalorimeter detection techniques and a low-energy antiproton beam provided by the ELENA ring at CERN, gaseous targets will be used for the creation of antiprotonic atoms, and the measurement of transitions between circular Rydberg states will be conducted with up to two orders of magnitude improved accuracy over previous studies using high-purity germanium detectors. Our approach eliminates the longstanding issue of nuclear uncertainties that have hindered prior studies using highly charged ions, thus enabling direct and purely QED-focused measurements. By precisely probing atomic systems with electric fields up to two orders of magnitude above the Schwinger limit, PAX will test vacuum polarization and second-order QED corrections, opening new frontiers in fundamental physics and uncovering potential pathways to physics beyond the Standard Model.

As the scale and complexity of integrated circuits (ICs) and advanced packaging grow larger while the individual features on a computer chip grow smaller, a clear need for novel metrology tools emerges. X-ray computed tomography (xCT) has proven to be an effective imaging approach in medicine, manufacturing, and the semiconductor industry, with state-of-the-art measurements capable of nondestructively producing a 3D reconstruction of ICs and resolving features as small as 10 nm buried deep within a chip. However, achieving these resolutions requires access to synchrotron radiation, which is not practical for many quality control and national security needs. Here, we present an update on our efforts to deliver a turnkey in-house xCT tool for producing 3D reconstructions of ICs at sufficient spatial resolution and imaging speeds to be useful to semiconductor failure analysis labs and for screening chips for hardware trojans. We explore the tradeoffs between using a NIST-developed 240-pixel transition-edge sensor (TES) spectrometer with excellent energy resolution that enables element-specific reconstruction and a $\mathbf{4}$-megapixel commercial hybrid photon counting (HPC) detector, which offers superior collection efficiency and measurement speeds at the cost of energy resolution. We review reconstruction results of an IC fabricated at the 130 nm node collected with a TES spectroscopic detector and discuss subsequent measurements performed with a commercial x-ray detector, which imaged the same circuitry 200 times faster than the TES array and is expected to lead to a full 3D reconstruction in the near future. Finally, we address artifacts of energy thresholding with a polychromatic source with the goal of preserving some degree of elemental sensitivity while achieving spatial resolutions and imaging speeds sufficient to make the tool useful in semiconductor manufacturing and hardware security.

Ultra-high energy resolution microcalorimeter gamma-ray spectroscopy—with energy resolution 5 to 10 times better than observed in spectra obtained by commercial-off-the-shelf high purity germanium detectors—is an enabling technology for ultra-precise isotope identification and quantification. Microcalorimeter gamma spectroscopy complements measurements requiring high-accuracy mass spectrometry, a costly, destructive analysis technique, and may offer benefits over mass spectrometry in the future. Microcalorimeter detectors are fabricated from superconducting materials and operate at ultra-low temperatures (<0.1 K), properties which permit measurement of spectra with peak full width half maximum (FWHM) of less than 100 eV at 100 keV. The microcalorimeter collaboration between Los Alamos National Laboratory, National Institute of Standards and Technology, and University of Colorado, Boulder has deployed three microcalorimeter gamma-ray spectrometers to nuclear facilities and analytical laboratories so far. These are the Spectrometer Optimized for Facility Integrated Applications (SOFIA), a portable system that can be moved to any facility, and two instruments called the High Efficiency and Resolution Microcalorimeter Spectrometers (HERMES) intended for permanent installation at Idaho National Laboratory and Pacific Northwest National Laboratory. Each spectrometer was customized to satisfy requirements for their specific applications. This work describes samples examined by microcalorimeter gamma-ray spectrometers, including recently irradiated materials, nuclear material from various stages of the fuel cycle, and medical isotope products. It also highlights useful signatures from actinide and fission product gamma-rays that are otherwise infeasible to observe or use for analysis without costly chemical separations and mass spectrometric assay. Microcalorimeter technology provides additional spectral signatures to existing techniques to better constrain the origin and intended use of nuclear and radioactive materials.

A highly charged muonic ion is a unique few-body atomic system where a negatively charged muon and a few electrons are simultaneously bound to a single nucleus. We report the first state-selective observation of highly charged muonic Ar (mu Ar) by electronic K x-ray spectroscopy using an array of transition-edge sensor microcalorimeters. The high-precision K x-ray spectra provide a clear signature of the presence of muonic atoms with one, two, and three electrons, i.e., H-like, He-like, and Li-like mu Ar. With the aid of theoretical calculations, we confirmed that the peak positions are consistent with the x-ray energies from highly charged Cl ions, and the intensities reflect deexcitation dynamics of highly charged mu Ar.

The X-ray spectroscopy of the muonic atom has attracted atomic, nuclear, and particle physicists since its discovery. The properties of a muonic atom, such as its binding energy or atomic radius, are different from an ordinary atom because of the difference in the mass between the muon and electron. Our collaboration has employed superconductor transition-edge sensor (TES) microcalorimeters for the x-ray spectroscopy of the muonic atom. Thanks to the recent detector development, the 44-keV lines from muonic Ar, which is important for the precision test of bound-state quantum electrodynamics, and the 76-keV lines from muonic Si, which is of interest from the viewpoint of the measurement of nuclear radii, have been reached by the dynamic range of the state-of-art TES microcalorimeters. An accelerator facility that can produce a high-intensity muon beam is necessary for such spectroscopic experiments. We performed a commissioning experiment of the hard x-ray and gamma-ray TES microcalorimeter at the J-PARC MLF MUSE muon beam line. The energy resolution, gain stability, and performance of timing selection of the pulses were evaluated in the environment of a large-scale accelerator facility.

A Super-Conducting ENergetic x-ray Telescope (ASCENT) is a concept for a future balloon-borne high-energy X-ray telescope in the energy range 60–85 keV to study gamma-ray emissions of 67.87 keV and 78.32 keV from the radioactive isotope 44Ti. For the focal plane instrumentation, ASCENT will use Mo-Cu/Mo-Au bilayer transition edge sensor (TES) microcalorimeter gamma-ray detectors with tin (Sn) absorbers. ‘Spectrometer to Leverage Extensive Development of Gamma-ray TESs for Huge Arrays using Microwave Multiplexed Enabled Readout’ (SLEDGEHAMMER), a detector development project at the National Institute of Standards and Technology, acts as the basis for the detector arrays for ASCENT. SLEDGEHAMMER has tin (Sn) absorbers attached to the SU-8 epoxy posts, lithographically placed on the detectors, but we are also considering other geometries for the chips where the absorbers are attached to the chips separated from the TESs, which could help to avoid parallel path for a current flow around the detectors with these BiSn sphere attachments. In this work, we are reporting on developing a method to attach Tin (Sn) absorbers to the transition edge sensors (TESs) with 0.2 mm diameter BiSn solder spheres replacing epoxy. The goal is to improve the thermal conductivity between the absorbers and the TESs compared to what was achieved using epoxy, potentially reducing the presence of an athermal component in the tails of signal pulses. We describe our efforts toward finding optimal temperature and pressure conditions for making this contact and the progress toward contact resistance measurements of these joints.

Naturally occurring radiation backgrounds cause correlated decoherence events in superconducting qubits. These backgrounds include both gamma rays produced by terrestrial radioisotopes and cosmic rays. We use the particle-transport code Geant4 and the PARMA summary of the cosmic-ray spectrum to model both sources of natural radiation and to study their effects in the typical substrates used in superconducting electronics. We focus especially on three rates that summarize radiation's effect on substrates. We give analytic expressions for these rates, and how they depend upon parameters including laboratory elevation, substrate material, ceiling thickness, and wafer area and thickness. The modeled rates and the distribution of event energies are consistent with our earlier measurement of radiation backgrounds using a silicon thermal kinetic-inductance detector.