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We are presenting the result of the microshutter arrays for multi-object spectroscopy. Microshutter arrays are MEMS technology devices that are 2D programmable field masks for object selection in the sparsely populated fields. This next generation microshutters are based on the first generation of the microshutter arrays developed for the James Webb Space Telescope Near-Infrared Spectrometer (JWST NIRSpec) we developed new fabrication process that allowed to build fully electrostatic microshutter arrays. The microshutter arrays based on this new development have been successfully demonstrated in the FORTIS project sounding rocket flight. We are currently in the process of expanding the fabrication process to large format microshutter arrays designed for the use on the future NASA flagship missions such as HabEx and LUVOIR.

We are developing a 2D programmable field masks for Mutli-Object Spectroscopy (MOS) in sparsely populated fields. The device is based on the microshutter array MEMS technology originally developed for JWST NIRSpec. A new fabrication process has been developed to actuate microshutter arrays electrostatically thus eliminating the need for the macroscopic mechanisms and improving the reliability and robustness of the device. The microshutters, made with silicon nitride membranes with a shutter pitch size of 100 μm x 200 μm, rotate on narrow torsion bars. The microshutters are actuated, latched, and addressed electrostatically by applying voltages to the electrodes on the microshutters and the adjacent walls of the array support grid. We have demonstrated the fabrication and operation of the pilot arrays and produced an integrated flight unit that was successfully used on the FORTIS (Far-UV Off Rowland-circle Telescope for Imaging and Spectroscopy) project for suborbital flight in 2019.

We present the first space flight use of the electrostatically actuated Next Generation MicroShutter Array systems (NGMSA) as multiple celestial object selectors developed for NASA space telescope missions at the NASA Goddard Space Flight Center. The NGMSA assembly was installed in the Next Generation Far-ultraviolet Off Rowland-circle Telescope for Imaging and Spectroscopy (NG-FORTIS) and successfully launched recently from the White Sands Missile Range in October 2019. We started to investigate this electrostatic NGMSA technology at the late stages of James Webb Space Telescope (JWST) in 2009 to strategically align with requirements of larger field of view for future telescopes in space. The NGMSA in NG-FORTIS is a 128 X 64 programable 2-D addressing microfabricated shutter array while the current NGMSA system under development is a 736 X 384 large format. The new microshutter array (MSA) features several unique designs aiming at performance improvements: electrostatic actuation, thinned microshutters, better electrical wall insulation and anti-stiction surface coating. To ensure reliable operation in the space environment, NGMSA systems have passed a series of critical environment tests including acoustic, random vibration, life cycle of operation, thermal cycling, and optical contrast tests. The successful launch of the NG-FORTIS sounding rocket equipped with NGMSA demonstrates our NGMSA technology have achieved the highest NASA Technical Readiness Level 9 for sounding rocket space applications.

The high-resolution mid-infrared spectrometer (HIRMES) is a high resolving power (R ~ 100,000) instrument operating in the 25–122 μm spectral range and will fly on board the Stratospheric Observatory for Far-Infrared Astronomy in 2019. Central to HIRMES are its two transition edge sensor (TES) bolometric cameras, an 8 × 16 detector high-resolution array and a 64 × 16 detector low-resolution array. Both types of detectors consist of Mo/Au TES fabricated on leg-isolated Si membranes. Whereas the high-resolution detectors, with a noise equivalent power (NEP) ~ 1.5 × 10−18 W/rt (Hz), are fabricated on 0.45 μm Si substrates, the low-resolution detectors, with NEP ~ 1.0 × 10−17 W/rt (Hz), are fabricated on 1.40 μm Si. Here, we discuss the similarities and differences in the fabrication methodologies used to realize the two types of detectors.

We have demonstrated in the laboratory multiple, fully functional, kilopixel, bolometer arrays for the upgraded instrument, the High-resolution airborne wideband camera plus (HAWC+), for the stratospheric observatory for infrared astronomy (SOFIA). Each kilopixel array consists of three individual components assembled into a single working unit: (1) a filled, Transition Edge Sensor (TES) bolometer array, (2) an infrared, back-termination, and (3) an integrated, two-dimensional superconducting quantum interference device (SQUID) multiplexer readout. Kilopixel TES arrays are directly indium-bump-bonded to a 32 $$\times $$ 40 SQUID multiplexer (MUX) circuit. In order to provide a fully superconducting pathway from the TES to the SQUID readout, numerous superconductor-to-superconductor interfaces must be made. This paper focuses on the fabrication techniques needed to create the superconducting path from the TES, out of the detector membrane, through the wafer, and to the SQUID readout.

The calorimeter array of the JAXA Astro-H (renamed Hitomi) Soft X-ray Spectrometer (SXS) was designed to provide unprecedented spectral resolution of spatially extended cosmic x-ray sources and of all cosmic x-ray sources in the Fe-K band around 6 keV, enabling essential plasma diagnostics. The SXS has a square array of 36 microcalorimeters at the focal plane. These calorimeters consist of ion-implanted silicon thermistors and HgTe thermalizing x-ray absorbers. These devices have demonstrated a resolution of better than 4.5 eV at 6 keV when operated at a heat-sink temperature of 50 mK. We will discuss the basic physical parameters of this array, including the array layout, thermal conductance of the link to the heat sink, resistance function, absorber details, and means of attaching the absorber to the thermistorbearing element. We will also present the thermal characterization of the whole array, including thermal conductance and crosstalk measurements and the results of pulsing the frame temperature via alpha particles, heat pulses, and the environmental background. A silicon ionization detector is located behind the calorimeter array and serves to reject events due to cosmic rays. We will briefly describe this anti-coincidence detector and its performance.

We have developed a process to integrate microstripline wiring with transition edge sensors (TES). The process includes additional layers for metal-etch stop and dielectric adhesion to enable recovery of parameters achieved in non-microstrip pixel designs. We report on device parameters in close-packed TES arrays achieved with the microstrip process including R(sub n), G, and T(sub c) uniformity. Further, we investigate limits of this method of producing high-density, microstrip wiring including critical current to determine the ultimate scalability of TES arrays with two layers of wiring.

Transition-edge sensor microcalorimeter arrays in compact geometries and large formats experience local heating from bias power and x-ray hits that must be dissipated in the frame. For devices on solid, non-perforated silicon substrates, we have introduced an underlying embedded copper heatsinking layer to enhance the ability of the frame to remove this heat. In particular, such a layer can mitigate thermal crosstalk between nearby pixels within the array. Further improvements in array performance, such as decreased magnetic field sensitivity and stray inductance, are possible by turning the heatsinking layer into a superconducting ground plane. In this presentation, we report on the development of heatsinking layers consisting of a 1-2 μm thick high-quality copper layer which is sandwiched between two thin refractory metal-based diffusion barriers. These diffusion barriers are designed to avoid copper migration into the surrounding material over time, especially during our high temperature TES fabrication process which takes place in excess of 400°C . A 0.3-0.5 μm thick PECVD SiO2 cover layer isolates the heatsinking layer from the detector circuit. We present first results on our attempt to tailor the materials forming the diffusion barrier to fabricate both well defined superconducting ground planes and non-superconducting layers with the desired barrier characteristics.

We report on the development of position-sensitive transition-edge sensors (PoST's) for future X-ray astronomy missions such as the International X-ray Observatory (IXO), under study by NASA and ESA. PoST's consist of multiple absorbers each with a different thermal coupling to one or more transition-edge sensors (TESs). This results in a characteristic pulse shape for each absorber element and allows position discrimination. PoST development is motivated by a desire to achieve maximum focal-plane area with the fewest number of readout channels. We report detailed characterization of our single TES PoST's or Hydras, which consist of four electroplated Au/Bi absorbers coupled to a low noise Mo/Au TES. Using a numerical model of the Hydra we fit to measured complex impedance curves and determine device parameters that allow us to accurately reproduce the measured pulse shapes and noise spectra. Results from Hydras with different internal thermal conductances reveal the trade-offs in optimizing for energy resolution or position-sensitivity. We report a best achievable energy resolution of < 6.0 eV across all pixels for a device with transition temperature of 86 mK, coupled with straightforward position discrimination by rise-time.

In a large close-packed array of x-ray microcalorimeters, sufficient heat sinking is important to minimize thermal crosstalk between pixels and to make the bath temperature of all the pixels uniform. We have measured crosstalk in our 8 times 8 pixel arrays. The shapes of the thermal crosstalk pulses are reproduced well as a convolution of heat input from the source pixel and the thermal decay in the receiver pixel. The amount of the thermal crosstalk is clearly dependent on the degree of electrothermal feedback. We have compared the magnitude of thermal crosstalk with and without a heat-sinking copper layer on the backside of the silicon frame as a function of distance between the source and receiver pixels. Using the results obtained, we have estimated the degradation of energy resolution that is expected as a function of count rate. We have also studied the temperature distribution within an array due to continuous heating from the TES bias to estimate impacts on the uniformity of the pixel performance.

We have developed a new calorimeter array to increase our collecting area by a factor of four. The 6×6 pixel device has a total area of 144 mm2, making it one of the largest X-ray microcalorimeter arrays yet constructed. A relatively thin high-z absorber consisting of a 0.7 μm HgTe layer supported on 15 μm high-purity silicon provides good efficiency up to photon energies of 1.5 keV. The heat capacity of this composite is low enough to obtain an energy resolution of ∼6 eV FWHM on the 2 mm×2 mm pixels when operated at a base temperature of 50 mK. The infrared blocking filters have also been improved. Room temperature radiation must be attenuated by about 9 orders of magnitude between 2 μm and 2 cm to avoid having photon shot noise dominate the detectornoise. Accomplishing this while maintaining a high transmission for very soft X-rays that can penetrate only a few μg cm−2 is a problem common to all soft X-ray calorimeters that observe external targets. We are constructing monolithic silicon two-layer support meshes with a 350 μm pitch front layer on a 5 mm pitch backing layer. These are 98% open and have >95% effective transmission over a 60° field of view, while providing robust support for 38 mm diameter filters consisting of 20 nm of aluminum on 50 nm of polyimide. Five of these filters in series provide the necessary infrared attenuation. Integral deicing heaters are ion implanted in the fine mesh to remove contamination when necessary.

We have successfully modeled our new TES (transition-edge sensor) X-ray microcalorimeters with a novel X-ray absorber design that is suitable for close-packed array with high quantum efficiency. We have determined device parameters that reproduce complex impedance curves and noise spectra throughout the transition. Observed pulse height, decay time and baseline energy resolution were in good agreement with simulated results using the same parameters.

We have studied crosstalk on our 8×8 transition-edge sensor (TES) x-ray microcalorimeter arrays. The shapes of thermal crosstalk pulses are well reproduced as a convolution of heat input from the source pixel and decay in the receiver pixel. We have measured the amount of thermal crosstalk as a function of distance between the source and receiver pixels. Using the results, we have estimated the degradation of energy resolution due to the thermal crosstalk as a function of count rate to determine the level of thermal crosstalk that is needed to satisfy the requirements of NASA’s Constellation-X mission.

We describe our current fabrication process for arrays of superconducting transition edge sensor microcalorimeters, which incorporates superconducting Mo/Au bilayers and micromachined silicon structures. We focus on materials and integration methods for array heatsinking with our bilayer and micromachining processes. The thin superconducting molybdenum bottom layer strongly influences the superconducting behavior and overall film characteristics of our molybdenum/gold transition-edge sensors (TES). Concurrent with our successful TES microcalorimeter array development, we have started to investigate the thin film properties of molybdenum monolayers within a given phase space of several important process parameters. The monolayers are sputtered or electron-beam deposited exclusively on LPCVD silicon nitride coated silicon wafers. In our current bilayer process, molybdenum is electron-beam deposited at high wafer temperatures in excess of 500 degrees C. Identifying process parameters that yield high quality bilayers at a significantly lower temperature will increase options for incorporating process-sensitive auxiliary array components (AAC) such as array heat sinking and electrical interconnects into our overall device process. We are currently developing two competing technical approaches for heat sinking large compact TES microcalorimeter arrays. Our efforts to improve array heat sinking and mitigate thermal cross-talk between pixels include copper backside deposition on completed device chips and copper-filled micro-trenches surface-machined into wafers. In addition, we fabricated prototypes of copper through-wafer microvias as a potential way to read out the arrays. We present an overview on the results of our molybdenum monolayer study and its implications concerning our device fabrication. We discuss the design, fabrication process, and recent test results of our AAC development.

Arrays of superconducting transition-edge sensors (TES) can provide high spatial and energy resolution necessary for x-ray astronomy. High quantum efficiency and uniformity of response can be achieved with a suitable absorber material, in which absorber x-ray stopping power, heat capacity, and thermal conductivity are relevant parameters. Here we compare these parameters for bismuth and gold. We have fabricated electroplated gold, electroplated gold/electroplated bismuth, and evaporated gold/evaporated bismuth 8x8 absorber arrays and find that a correlation exists between the residual resistance ratio (RRR) and thin film microstructure. This finding indicates that we can tailor absorber material conductivity via microstructure alteration, so as to permit absorber thermalization on timescales suitable for high energy resolution x-ray microcalorimetry. We show that by incorporating absorbers possessing large grain size, including electroplated gold and electroplated gold/electroplated bismuth, into our current Mo/Au TES, devices with tunable heat capacity and energy resolution of 2.3 eV (gold) and 2.1 eV (gold/bismuth) FWHM at 6 keV have been fabricated.

Two challenges facing the use of large area (2 mm ×2 mm ) bismuth absorbers for microcalorimetry are uncertainties in the heat capacity of bismuth and the effects of lateral heat conduction and position dependence due to the absorber's large size. We have measured the heat capacity of three Bi samples to be 0.3−0.6 J K −1 m −3 at 100 mK . These absorbers also exhibit response variations as phonons created by an X-ray event at an absorber edge will take longer to propagate to the thermometer attachment point than those at the absorber center. This effect may degrade the detector's energy resolution if the propagation time is not very short compared to the thermometer time constant. We show that the response of the largest absorber varies by ∼4% across its area.

We have developed a model for transition edge sensors to optimize performance under a variety of different conditions. There are three design trade-offs when engineering a microcalorimeter for a particular application: energy resolution, energy range and maximum count rate. All three are interdependent and are determined by various design parameters such as the detector heat capacity, the sharpness of the transition, and the thermal conductance of the detector to the frame. Our model includes all known sources of intrinsic noise in our calorimeters including the observed broad band excess noise. We will present the results of this model, and its predictions for optimally designed microcalorimeters.

We have developed an improved microcalorimeter array that will be used on the AstroE-2 satellite mission. The 6×6 array consists of a grid of 36 suspended pixels. Each 1.5 μm thick pixel has an ion-implanted thermometer, four thermal links (support beams), and four X-ray absorber support tabs. Improvements in Silicon micro-machining capabilities and the availability of custom Silicon-on-Insulator (SOI) wafers has enabled us to precisely control pixel geometry, lead widths, and develop a more compact array. Knowing the silicon thickness, we can calculate a precise implant dose for the thermometer. Using a high-temperature anneal, we can uniformly diffuse the implant throughout the depth of the top layer of the SOI wafer. Defining the length, width, and thickness of the support beams, we can control the thermal conductance of the pixel. Advancements in polymer-photo resists have enabled us to develop a new absorber support tab attachment scheme resulting in more controlled heat dissipation from the absorber to the thermometer on the pixel. An overview of fabrication improvements focusing on these topics will be discussed.

The calorimeter array of the JAXA Astro-H (renamed Hitomi) soft x-ray spectrometer (SXS) was designed to provide unprecedented spectral resolution of spatially extended cosmic x-ray sources and of all cosmic x-ray sources in the Fe-K band around 6 keV, enabling essential plasma diagnostics. The SXS had a square array of 36 x-ray calorimeters at the focal plane. These calorimeters consisted of ion-implanted silicon thermistors and HgTe thermalizing x-ray absorbers. These devices demonstrated a resolution of better than 4.5 eV at 6 keV when operated at a heat-sink temperature of 50 mK. We will discuss the basic physical parameters of this array, including the array layout, thermal conductance of the link to the heat sink, resistance function, absorber details, and means of attaching the absorber to the thermistor-bearing element. We will also present the thermal characterization of the whole array, including thermal conductance and crosstalk measurements and the results of pulsing the frame temperature via alpha particles, heat pulses, and the environmental background. A silicon ionization detector was located behind the calorimeter array and served to reject events due to cosmic rays. We will briefly describe this anticoincidence detector and its performance.

In the course of developing microcalorimeters as detectors for astronomical X-ray spectroscopy, we have undertaken an empirical characterization of non-ideal effects in the doped semiconductor thermometers used with these detectors, which operate at temperatures near 50 mK. We have found three apparently independent categories of such behavior that are apparently intrinsic properties of the variable-range hopping conduction mechanism in these devices: 1/f fluctuations in the resistance, which seems to be a 2D effect; a departure from the ideal coulomb-gap temperature dependence of the resistance at temperatures below T 0 /24; and an electrical nonlinearity that has the time dependence and extra noise that are quantitatively predicted by a simple hot electron model. This work has been done largely with ion-implanted Si:P:B, but similar behaviors have been observed in transmutation doped germanium.

We have demonstrated a kilopixel, filled, infrared bolometer array for the balloon-borne Primordial Inflation Polarization Explorer (PIPER). The array consists of three individual components assembled into a single working unit: 1) a transition-edge-sensor bolometer array with background-limited sensitivity, 2) a quarter–wavelength backshort grid, and 3) an integrated Superconducting Quantum Interference Device (SQUID) multiplexer (MUX) readout. The detector array is a filled, square–grid of suspended, one-micron thick silicon bolometers with superconducting sensors. The Backshort–Under–Grid (BUG) is a separately fabricated component serving as a backshort to each pixel in the array. The backshorts are positioned in the cavities created behind each detector by the back–etched well. The spacing of the backshort beneath the detector grid can be set from ~30-300_microns by independently adjusting process parameters during fabrication. Kilopixel arrays are directly indium–bump–bonded to a 32x40 SQUID multiplexer circuit. The array architecture is suitable for a wide range of wavelengths and applications. Detector design specific to the PIPER instrument, fabrication overview, and assembly technologies will be discussed.

We present fabrication details of our Mo/Au X-ray microcalorimeters, which are being developed as one of the candidate high resolution spectrometers for the Constellation-X mission. We have reproducibly fabricated Mo/Au transition-edge sensors with Tc's of /spl sim/100 mK on etched silicon nitride membranes and connected via superconducting Nb leads. Our single pixel devices have, so far, attained resolution of 3.7 eV at 3.3 keV. We also discuss our plans for fabrication and testing of fully functional multi-pixel array of X-ray microcalorimeters.

We report on progress made so far at NASA Goddard Space Flight Center towards the development of arrays of X-ray microcalorimeters as candidates for the high-resolution X-ray spectrometer on the Constellation-X mission. In the design concept presently under consideration, the microcalorimeter consists of (i) a Bi/Cu multilayer absorber for stopping and thermalizing the incident X-rays, (ii) an e-beam evaporated Mo/Au proximity bilayer with sputtered Nb leads for sensing the resultant temperature rise, and (iii) a silicon nitride membrane to provide a weak thermal link to the sink temperature so that the calorimeter can return to its equilibrium temperature. Fabrication details and preliminary results are reported.

The X-Ray Spectrometer (XRS) instrument, scheduled to be launched as part of the Japanese X-ray astronomy satellite Astro-E in February 2000, has an array of 32 microcalorimeters at the focal plane. These calorimeters consist of ion-implanted silicon thermistors and HgTe thermalizing X-ray absorbers. These devices provide spectral resolution of 9 eV at 3 keV and 11 eV at 6 keV. The process of determining the detector parameters for this instrument will be discussed. This will include discussion of the array layout, thermal conductance of the link to the heat sink, operating temperature, thermistor size, absorber choice, and means of attaching the absorber to the thermistor bearing element. We consider the XRS calorimeters to be operating in a local optimum, with the absolute optimization yet to be performed, thus we will discuss directions for future development of this sensor technology.

The X-ray Quantum Calorimeter (XQC) sounding rocket experiment utilizes a microcalorimeter array for observing the diffuse soft X-ray background. Observations of such low surface-brightness targets require a large-area detector. We will be using an array of large absorbers. Good absorbers must rapidly and completely thermalize photons, have small heat capacity for high stopping efficiency and have good lateral thermal transport. For observing the soft X-ray background (energies

We report the latest results from our development of Position-Sensitive Transition-edge sensors (PoSTs), which are one-dimensional imaging spectrometers. In PoSTs with segmented Au absorbers, we obtained 8 eV energy resolution on K K α lines, which is consistent to the baseline energy resolution and the design values, on all of the nine pixels, by choosing the best combination of the thermal conductance in absorbers and in links that connects the absorbers. The pulse decay time of 193 μ s is fast enough for our purpose. In a PoST with a continuous Bi/Cu absorber, by dividing the events into 63 effective pixels, we obtained energy resolutions of 16 eV at the center “pixel”, which is comparable to the baseline energy resolution, and 33 eV at the outer “pixel”. The degradation of the energy resolution in the outer “pixel” is due to position dependence, which we can cancel out by dividing the events into smaller “pixels” when we have sufficient X-ray events.

Bismuth's large atomic number and low carrier density makes it an attractive X-ray absorber material for microcalorimeters. Bismuth's long fermi wavelength and long mean free paths have motivated much interest in the fabrication of high quality bismuth films to study quantum size effects. Despite such incentives, fabrication of high quality bismuth films has proven difficult, and measured properties of such films are highly variable in the literature. Implementing a bismuth deposition process for TES (superconducting Transition Edge Sensor) device fabrication presents additional challenges particularly at interfaces due to the inherent granularity and surface roughness of its films, its low melting point, and its tendency to diffuse and form undesired intermetallic phases. We report observations of Bi–Cu and Bi–Au diffusion in our devices correlating with large shifts in T c (superconducting transition temperature). Using SEM and in situ R vs T annealing experiments we have been able to study these diffusion processes and identify their activation temperatures.

We measured noise in a variety of Mo/Au transition-edge sensor (TES) X-ray calorimeters. We investigated the relationship between the noise, bias, and the superconducting phase transition in the TESs. Our square TES calorimeters have achieved very good energy resolutions (2.4 eV at 1.5 keV) but their resolutions have been limited by broadband white excess noise generated by the TES when it is biased in the phase transition. We have recently fabricated Mo/Cu TESs with interdigitated normal metal bars deposited on top of the bilayer. The new TES calorimeters have demonstrated little or no excess noise in the phase transition. These results point the way to development of TES calorimeters with higher energy resolution.

Concurrent with our microcalorimeter array fabrication for Constellation-X technology development, we are developing ultra-low impedance Through-Wafer Micro-Vias (TWMV) as electrical interconnects for superconducting circuits. The TWMV will enable the electrical contacts of each detector to be routed to contacts on the backside of the array. There, they can be bump-bonded to a wiring fan-out board which interfaces with the front-end Superconducting Quantum Interference Device readout. We are concentrating our developmental efforts on ultra-low impedance copper and superconducting aluminum TWMV in 300–400 micron thick silicon wafers. For both schemes, a periodic pulse-reverse electroplating process is used to fill or coat micron-scale through-wafer holes of aspect ratios up to 20. Here we discuss the design, fabrication process, and recent electro-mechanical test results of Al and Cu TWMV at room and cryogenic temperatures.

NASA's Constellation-X (Con-X) mission is a driver for advancing low-temperature detector technologies, requiring 1000-pixel two-dimensional arrays with >95% filling fraction and quantum efficiency at 6 keV and better than 4 eV energy resolution for 1–10 keV photons. We describe a robust transition edge sensor fabrication process that can produce detector arrays with integrated absorbers approaching the specifications of Con-X. We address issues such as the methods of superconducting bilayer deposition, detector geometry definition, and absorber interface.

The square-format 32-pixel microcalorimeter array at the focal plane of the high-resolution X-ray spectrometer on the Astro-E2 X-ray Observatory is the first of a new generation of silicon-based microcalorimeters. This array has numerous advantages over its predecessor, the bilinear array that was launched on Astro-E. Foremost among its benefits are: (1) the energy resolution is improved by a factor of two at 6 keV (now 6 eV FWHM), (2) the thermal time constant is a factor of two faster, and (3) each pixel has a Gaussian line response. We will discuss the design changes that have led to these and other advantages.

We have recently produced and tested two-dimensional arrays of Mo/Au transition-edge-sensor (TES) calorimeters with Bi/Cu absorbers. The arrays represent a significant step towards meeting the specifications of NASA's Constellation-X mission. The calorimeters are compactly spaced within 5×5 arrays of 250 μm square pixels necessary for an angular resolution of 5 arcsec. Lithographically produced absorbers hang over the substrate and wiring between the TESs for high filling fraction and high quantum efficiency. We designed the calorimeters with heat capacities and thermal couplings such that X-rays produce pulses with fall times of approximately 300 μs to allow relatively high count rates with low dead time. We read out up to four of the pixels simultaneously. The arrays demonstrated very good energy resolution (5 eV at 1.5 keV and 7 eV at 6 keV) and little crosstalk between neighboring pixels.

Transition‐edge‐sensor (TES) X‐ray microcalorimeters have mostly been targeted at mid‐band energies from 0.05–10 keV and high energies to above 100 keV. However, many other optimizations are possible. Here we present results from devices optimized for soft X‐ray applications. For spectroscopy below 1 keV, the X‐ray stopping power and heat capacity (C) of the TES itself are high enough that we can omit a separate absorber. The resulting devices have low C and the best‐achievable energy resolution should be under 1 eV. We are interested in pursuing such devices primarily for astrophysical applications and laboratory astrophysics at LLNL’s Electron‐Beam Ion Trap. To this end, we have studied arrays in which ‘bare’ TESs are interspersed with broad‐band pixels that have absorbers. By extending the absorbers to cover the area where the leads contact the low‐energy pixels, we have eliminated a significant source of non‐Gaussian detector response. The bare devices are in a different regime from our typical devices in that C is ten times lower and the conductance to the bath is four times lower. We have explored this regime through simultaneous fitting of noise and impedance data. These data cannot be fit by the simple model we employ to describe our typical broad‐band devices. In this contribution we present X‐ray spectra and the results from modeling.

We present experimental results and designs of large‐absorber transition‐edge‐sensor (TES) X‐ray microcalorimeters. Much of our effort has focused on developing close‐packed arrays of 250–300 μm‐sized pixels suitable for the X‐ray Microcalorimeter Spectrometer (XMS) on the International X‐ray Observatory. These efforts have produced devices with the requisite energy resolution of ≳2.5 eV (FWHM) at 6 keV. There are several upcoming applications, however, that require arrays composed of significantly larger pixels. In this contribution we present experimental results from 490 μm‐sized pixels that have attained 3.5 eV energy resolution at 6 keV. These devices are precursors to the pixels that are being developed for the XMS extended array. In addition, we briefly describe detector designs for the Micro‐X sounding rocket experiment, which also requires an array of large‐area TES microcalorimeters.

We are developing transition‐edge sensor (TES) X‐ray detectors optimized for high count‐rate applications. These devices are fabricated on thick (300 μm) Si substrates, resulting in a 20 times increase in thermal conductance to the heat sink compared to our conventional membrane isolated TES’s. Operating a TES with higher heat sink conductance requires 4.5 times more bias current. This results in a 2.7 times increase in β, the logarithmic derivative of resistance with respect to current. Noise measurements show a lower limit on the TES excess noise scales as (2β)1/2, consistent with the near‐equilibrium, non‐linear expansion of the Ohmic Johnson noise. This is consistent with our membrane devices though the increased β means the theoretical best attainable resolution is degraded by 25–35%. We have tested devices with different contact geometries between the absorber, and the TES and substrate. This allows us to investigate the loss of athermal phonons to the substrate, which can degrade the resolution. Re...

Following our development of a superconducting transition-edge-sensor (TES) microcalorimeter design that enables reproducible, high performance (routinely better than 3 eV FWHM energy resolution at 6 keV) and is compatible with high-fill-factor arrays, we have directed our efforts towards demonstrating arrays of identical pixels using the multiplexed read-out concept needed for instrumenting the Constellation-X X-ray Microcalorimeter Spectrometer (XMS) focal plane array. We have used a state-of-the-art, time-division SQUID multiplexer system to demonstrate 2×8 multiplexing (16 pixels read out with two signal channels) with an acceptably modest level of degradation in the energy resolution. The average resolution for the 16 multiplexed pixels was 2.9 eV, and the distribution of resolution values had a relative standard deviation of 5%. The performance of the array while multiplexed is well understood. The technical path to realizing multiplexing for the XMS instrument on the scale of 32 pixels per signal channel includes increasing the system bandwidth by a factor of four and reducing the non-multiplexed SQUID noise by a factor of two. In this paper we discuss the characteristics of a uniform 8×8 array and its performance when read out nonmultiplexed and with various degrees of multiplexing. We present data acquired through the readout chain from the multiplexer electronics, through the real-time demultiplexer software, to storage for later signal processing. We also report on a demonstration of real-time data processing. Finally, because the multiplexer provides unprecedented simultaneous access to the pixels of the array, we were able to measure the array-scale uniformity of TES calorimeter parameters such as the individual thermal conductances and superconducting transition temperatures of the pixels. Detector uniformity is essential for optimal operation of a multiplexed array, and we found that the distributions of thermal conductances, transition temperatures, and transition slopes were sufficiently tight to avoid significant compromises in the operation of any pixel.

We are developing arrays of position-sensitive transition-edge sensor (POST) X-ray detectors for future astronomy missions such as NASA's Constellation-X. The POST consists of multiple absorbers thermally coupled to one or more transition-edge sensor (TES). Each absorber element has a different thermal coupling to the TES. This results in a distribution of different pulse shapes and enables position discrimination between the absorber elements. POST'S are motivated by the desire to achieve the largest possible focal plane area with the fewest number of readout channels and are ideally suited to increasing the Constellation-X focal plane area, without comprising on spatial sampling. Optimizing the performance of POST'S requires careful design of key parameters such as the thermal conductances between the absorbers, TES and the heat sink. as well as the absorber heat capacities. Using recently developed signal processing algorithms we have investigated the trade-off between position-sensitivity, energy resolution and pulse decay time. based on different device design parameters for PoST's. Our new generation of PoST's utilize technology successfully developed on high resolution (approximately 2.5eV) single pixels arrays of Mo/Au TESs. also under development for Constellation-X. This includes noise mitigation features on the TES and low resistivity electroplated absorbers. We report on the first experimental results from these new one and two-channel PoST"s, consisting of all Au and composite Au/Bi absorbers, which are designed to achieve an energy resolution of < 10 eV. coupled with count-rates of 100's per pixel per second and position sensitivity over the energy range 0.3-10 keV.

We report on our studies of possible configurations for the focal plane of the Constellation-X mission. Taking advantage of new developments in both SQUID multiplexing technology and position-sensitive detectors, we present a viable focal plane intrument design that would greatly enhance the reference Constellation-X configuration of a 32 × 32 array. An order of magnitude increase in the number of pixels of the focal plane array from the current 1024-pixel reference design is achievable.

We present recent measurements obtained using a new method for characterizing transition edge sensor (TES) calorimeters: We measured the electrical impedance of a TES calorimeter throughout the superconducting to normal metal phase transition. The impedance method enables us to previously measure how the resistance and heat capacity of the TES varied throughout the phase transition. These measurements probe the internal state of oru Mo/Au TES. We also present recent results from measurements of noise in our TESs. Our measurements are instrumental toward understanding and optimizing our TES calorimeters.

After the design of the calorimeter array for the high-resolution x-ray spectrometer (XRS) on the original Astro-E was frozen, new fabrication techniques became available and our understanding of these devices continually increased. We are now able to complete the optimization of this technology and, potentially, to increase the capability of new XRS instrument for Astro-E2, our on-going sounding recket experiments, and possible further applications. The most significant improvement comes from greatly reducing the excess noise of the ion-implanted thermistors by increasing the thickness of the implanted region.

We are optimizing Mo/Au transition-edge-sensor (TES) calorimeters to meet the specifications of NASA’s Constellation-X mission. Our calorimeters have already demonstrated very good energy resolution of X rays (2.4 eV at 1.5 keV). We wish to further improve the energy resolution by reducing excess noise in the calorimeters. Development of a detailed model and understanding of the noise is instrumental to reaching this goal. Towards that end, we employ a linear model that describes the response of a calorimeter to signal and various sources of noise. The model is based on detailed measurements of the parameters that affect the calorimeter’s performance, such as current-voltage characteristics of the TES, thermal conductance of our silicon-nitride membranes, and inductance in the electronic circuit used to bias the TES. We determine the sharpness of the superconducting phase transition by fitting the model to the measured responsivity of the calorimeter. The model relates sources of noise, such as phonon noi...

We are developing superconducting transition-edge sensor (TES) microcalorimeters for astronomical x-ray spectroscopy. We have obtained very high energy resolution (2.4 eV at 1.5 keV and 3.7 eV at 3.3 keV) in large, isolated TES pixels using Mo/Au proximity-effect bilayers on silicon-nitride membranes several mm wide. In order to be truly suitable for use behind an x-ray telescope, however, such devices need to be arrayed with a pixel size and focal-plane coverage matched to the telescope focal length and spatial resolution. For the Constellation-X mission, this requires fitting the TES, its thermal link, and contact wiring into a 0.25 mm square, a far more compact geometry than has previously been investigated. We have demonstrated that the weak thermal link can be restricted to a narrow (∼10 micron) perimeter of membrane around the TES and still provide a thermal conductance in the acceptable range. Varying the size and placement of slits in that nitride perimeter, we can tune that value.

We present an overview of our efforts in fabricating Transition-Edge Sensor (TES) microcalorimeter arrays for use in astronomical x-ray spectroscopy. Two distinct types of array schemes are currently pursued: 5×5 single pixel TES array where each pixel is a TES microcalorimeter, and Position-Sensing TES (PoST) array. In the latter, a row of 7 or 15 thermally-linked absorber pixels is read out by two TES at its ends. Both schemes employ superconducting Mo/Au bilayers as the TES. The TES are placed on silicon nitride membranes for thermal isolation from the structural frame. The silicon nitride membranes are prepared by a Deep Reactive Ion Etch (DRIE) process into a silicon wafer. In order to achieve the concept of closely packed arrays without decreasing its structural and functional integrity, we have already developed the technology to fabricate arrays of cantilevered pixel-sized absorbers and slit membranes in silicon nitride films. Furthermore, we have started to investigate ultra-low resistance throug...

The XRS instrument on Astro-E is a fully self-contained microcalorimeter x-ray instrument capable of acquiring optimally filtering, and characterizing events for 32 independent pixels. With the launch of the Astro-E spacecraft, a full flight spare detector system has been integrated into a laboratory cryostat for use on the electron beam ion trap (EBIT) at Lawrence Livermore National Laboratory. The detector system contains a microcalorimeter array with 32 instrumented pixels heat sunk to 60 mK using an adiabatic demagnetization refrio,erator. The instrument has a composite resolution of 8eV at 1 keV and 12eV at 6 keV with a minimum of 95% quantum efficiency. This will allow high spectral resolution, broadband observations of collisionally excited plasmas which are produced in the EBIT experiment. Unique to our instrument are exceptionally well characterized 1000 Angstrom thick aluminum on polyimide infrared blocking filters. The detailed transmission function including the edc,e fine structure of these filters has been measured in our laboratory using an erect field grating spectrometer. This will allow the instrument to perform the first broadband absolute flux measurements with the EBIT instrument. The instrument performance as well as the results of preliminary measurements will be discussed. Work performed under the auspices of the U.S. D.o.E. by Lawrence Livermore National Laboratory under contract W-7405-ENG-48 and was supported by the NASA High Energy Astrophysics Supporting Research and Technology Program.

The X-ray spectrometer (XRS) on the Japanese Astro-E Spacecraft is the first ultra low temperature space borne instrument. The system utilizes a 900g Ferric Ammonium Alum (FAA) Adiabatic Demagnetization Refrigerator (ADR) with a helium-3 gas gap heat switch to cool the detector assembly to 0.060K. The system operates in a "single shot" configuration allowing the system to remain at its operating temperature for about 40 hours in the lab. The on-orbit performance is expected to be about 35 hours with a 97% duty cycle. The detector assembly for XRS consists of a 32 channel microcalorimeter array bias electronics, thermometry, and an anti-coincidence detector that are attached to the cold stage of the ADR. To thermally Isolate the detector system from the superfluid helium reservoir, the detector system is suspended by Kevlar cords and electrical connection in made by L30, 17-micron diameter, tensioned NbTi leads. The detectors are read out in a source-follower arrangement using FET amplifiers operating at 130K mounted in multiply-thermally-isolated assemblies that also use Kevlar and stainless steel wiring. The design and thermal performance of this system will be discussed and compared to the theoretical limits.

The XRS instrument has an array of 32 micro-calorimeters at the focal plane. These calorimeters consist of ion-implanted silicon thermistors and HgTe thermalizing x-ray absorbers. These devices have demonstrated a resolution of 9 eV at 3 keV and 11 eV at 6 keV. We will discuss the basic physical parameters of this array, including the array layout, thermal conductance of the link to the heat sink, operating temperature, thermistor size, absorber choice, and means of attaching the absorber to the thermistor bearing element. We will present representative performance data, though a more detailed presentation of the results of the instrument calibration is presented elsewhere in these proceedings. A silicon ionization detector is located behind the calorimeter array and serves to reject events due to cosmic rays. We will briefly describe this anti-coincidence detector and its performance in conjunction with the array.

COnstellation-X is a cluster of identical observatories that together constitute a promising concept for a next- generation, high-throughput, high-resolution, astrophysical x-ray spectroscopy mission. The heart of the Constellation-X mission concept is a high-quantum-efficiency imaging spectrometer with 2 eV resolution at 6 keV. Collectively across the cluster, this imaging spectrometer will have twenty times the collecting efficiency of XRS on Astro-E and better than 0.25 arc minute imaging resolution. The spectrometer on each satellite will be able to handle count rates of up to 1000 counts per second per imaging pixel for a point source and 30 counts per second per pixel for an extended source filling the array. Focal plane coverage of at least 2.5 arc minutes X arc minutes, comparable to XRS but with a factor of thirty more pixels, is required. This paper will present the technologies that have the potential to meet al these requirements. It will identify the ones chosen for development for Constellation-X and explain why those were considered closer to realization, and it will summarize the results of the development work thus far.

We present measurements of high fill-factor arrays of superconducting transition-edge x-ray microcalorimeters designed to provide rapid thermalization of the x-ray energy. We designed an x-ray absorber that is cantilevered over the sensitive part of the thermometer itself, making contact only at normal metal-features. With absorbers made of electroplated gold, we have demonstrated an energy resolution between 2.4 and 3.1 eV at 5.9 keV on 13 separate pixels. We have determined the thermal and electrical parameters of the devices throughout the superconducting transition, and, using these parameters, have modeled all aspects of the detector performance.

Semiconductor thermistors have been used for several years and their ideal behavior is well known both experimentally and theoretically. Their current performance is limited by non-ideal behaviors. These include 1/f noise and non-ohmic effects. We find that the 1/f noise appears to be a 2-D effect, and can be greatly reduced by fabricating thicker thermistors. Eliminating this noise could improve the intrinsic detector resolution as much as 40%. It also allows us to study other sources of excess noise in the thermometer. The non-ohmic behavior can be empirically explained using a hot-electron model. Although this model does not seem suitable for semiconductors in the variable range-hopping regime, where the electrons are localized, it fits the experimental data quite well. We measured an excess white noise at low frequencies consistent with the predicted thermodynamic fluctuations between electrons and phonons. We also measured a characteristic time of the non-ohmic behavior that is consistent with a C/G ...