Robust, high sensitivity kilopixel format arrays with large focal plane filling factors and low cosmic ray cross sections that operate over the entire far-IR regime are required for future NASA missions, such as Origins and a future far-IR Probe. Our kilopixel Backshort Under Grid (BUG) detectors are designed to meet all those requirements: By bump-bonding two-dimensional detector arrays to readout multiplexers are gaplessly tileable in one spatial direction with the integration of the multiplexer scalable beyond wafer sizes. The detector arrays provide high filling factors (<90% at 1mm pixel pitch) and are designed for low Cosmic ray cross sections. The major missing technology is a detector array architecture that can be gaplessly tiled to deliver the desired pixel counts of npixel ~105, while being providing a robust process to produce these detector arrays. We introduce a new array architecture that is very flexible allowing for a variety of tileable solutions and describe its individual components and the tests of those. Our results demonstrate that this architecture allows for flexible designs with high yields and reliable superconducting bump-bond connections of detectors and the cold readout SQUID multiplexers directly under the detector array, or on a different board that can be connected with e.g. flex lines for compact tiling.
We present aluminum (Al) mirrors protected with a flash lithium fluoride (LiF) overcoat. Each of these Al and LiF layers are produced with a novel room-temperature reactive Physical Vapor Deposition (rPVD) process that consists of exposing these films growth to a fluorine-containing xenon di-fluoride (XeF2) gas. We report two sets of Al/LiF mirrors produced with this rPVD process. The first set is optimized at a wavelength of 121.6 nm and presents an unprecedented reflectance of 92.6% at this wavelength. The second set is optimized at shorter wavelengths by reducing the thickness of the LiF overcoat to have a more balance reflectance performance in the far-ultraviolet (FUV) spectral range from 90-200 nm. This new process is observed to produce more durable and less hygroscopic mirrors than those fabricated with standard PVD process, and has utility in realizing an intrinsic high reflectance of aluminum in the critical FUV spectral range.
This paper describes a cryogenic optical testbed developed to characterize µ-Spec spectrometers in a dedicated dilution refrigerator (DR) system. μ-Spec is a far-infrared integrated spectrometer that is an analog to a Rowland-type grating spectrometer. It employs a single-crystal silicon substrate with niobium microstrip lines and aluminum kinetic inductance detectors (KIDs). Current designs with a resolution of R = λ/Δλ = 512 are in fabrication for the EXCLAIM (Experiment for Cryogenic Large Aperture Intensity Mapping) balloon mission. The primary spectrometer performance and design parameters are efficiency, NEP, inter-channel isolation, spectral resolution, and frequency response for each channel. Here we present the development and design of an optical characterization facility and preliminary validation of that facility with earlier prototype R=64 devices. We have conducted and describe initial optical measurements of R = 64 devices using a swept photomixer line source. We also discuss the test plan for optical characterization of the EXCLAIM R = 512 μ-Spec devices in this new testbed.
The Origins Survey Spectrometer (OSS) is a multi-purpose far-IR spectrograph for Origins. Operating at the photon background limit, OSS covers the 25- to 588-μm wavelength range instantaneously at a resolving power (R) of 300 using six logarithmically spaced grating modules. Each module couples at least 30 and up to 100 spatial beams simultaneously, enabling true [three-dimensional (3D)] spectral mapping. In addition, OSS provides two high-resolution modes. The first inserts a long-path Fourier-transform spectrometer (FTS) into a portion of the incoming light in advance of the grating backends, enabling R up to 43 , 000 × [ λ / 112 μm ] , while preserving the grating-based sensitivity for line detection. The second incorporates a scanning etalon in series with the FTS to provide R up to 300,000 for the 100-to 200-μm range.
Sustained and enhanced land imaging is crucial for providing high-quality science data on change in land use, forest health, environment, and climate. Future thermal land imaging instruments operating in the 10-12 micron band will provide essential information for furthering our hydrologic understanding at scales of human influence, and producing field-scale moisture information through accurate retrievals of evapotranspiration (ET). To address the need for cost-effective future thermal land imaging missions we are developing novel uncooled doped-silicon thermopile detectors, an extension of a detector design concept originally developed at NASA-Goddard for planetary science applications. These doped-Si thermopile detectors have the potential to offer superior performance in terms of sensitivity, speed, and customization, when compared to current commercial-off-the-shelf uncooled detector technologies. Because cryocooler technology does not need to be fielded on the instrument, these and other uncooled detectors offer the benefit of greatly reduced instrument cost, mass, and power at the expense of some acceptable loss in detector sensitivity. We present the motivation for an uncooled thermal imaging instrument, our doped-Si thermopile detector concept, and performance expectations and comparisons. We also provide an update on the current status of this detector technology development.
The OSS on the Origins Space Telescope is designed to decode the cosmic history of nucleosynthesis, star formation, and supermassive black hole growth with wide-area spatial-spectral 3-D surveys across the full 25 to 590 micron band. Six wideband grating modules combine to cover the full band at R=300, each couples a long slit with 60-190 beams on the sky. OSS will have a total of 120,000 background-limited detector pixels in the six 2-D arrays which provide spatial and spectral coverage. The suite of grating modules can be used for pointed observations of targets of interest, and are particularly powerful for 3-D spectral spectral surveys. To chart the transition from interstellar material, particularly water, to planetary systems, two high-spectral-resolution modes are included. The first incorporates a Fourier-transform spectrometer (FTS) in front of the gratings providing resolving power of 25,000 (δv = 12 km/s) at 179 µm to resolve water emission in protoplanetary disk spectra. The second boosts the FTS capability with an additional etalon (Fabry-Perot interferometer) to provide 2 km/s resolution in this line to enable detailed structural studies of disks in the various water and HD lines. Optical, thermal, and mechanical designs are presented, and the system approach to the detector readout enabling the large formats is described.
Photon-counting detectors address the single most difficult technology challenge for the Origins Space Telescope (OST) and are highly desirable for reaching the ~ 10^-20 W/√Hz sensitivity permitted by the observatory. One objective of this facility is rapid spectroscopic surveys of the high redshift universe at 420 – 800 μm, using arrays of integrated spectrometers with moderate resolutions (R = λ/Δλ ~1000), to explore galaxy evolution and growth of structure in the universe. A second objective is to perform higher resolution (R > 100,000) spectroscopic surveys at 20–300 μm for exploring the distribution of the ingredients for life in protoplanetary disks. Lastly, the OST aims to do sensitive mid-infrared (5–30 μm) spectroscopy of rocky planet atmospheres in the habitable zone using the transit method. These objectives represent a well-organized community agreement, but they are impossible to reach without a significant leap forward in detector technology, and the OST is likely not to be recommended if a path to suitable detectors does not exist.
Our team is developing photon-counting Kinetic Inductance Detectors (KIDs) for the OST. Since KIDs are highly multiplexable in nature their scalability will be a major improvement over current technologies that are severely limited in observing speed due to small numbers of pixels. Moreover, KIDs are an established strong competitor to TESs and have achieved NEP ~ 1.5—3x10^-19 W/√Hz in a fully operational 1000-pixel science grade array made by SRON under the SpaceKID program. To reach the sensitivities for OST we are developing KIDs made from very thin aluminum films on single-crystal silicon substrates. Under the right conditions, small-volume inductors made from these films can become ultra-sensitive to single photons >90 GHz. Understanding the material physics and electrodynamics of excitations in these superconductor-dielectric systems is critical to performance. We have achieved world-record material properties, which are within requirements for photon-counting: microwave quality factor of 0.5 x 10^6 for a 10-nm aluminum resonator at single microwave photon drive power, residual dark electron density of < 5 /µm^3 and extremely long excitation lifetime of ~ 6.0 ms. Using a detailed model we simulated our detector when illuminated with randomly arriving single photon events and show that photon counting with >95% efficiency at 0.5 - 1.0 THz is achievable. Combined with µ-Spec - our Goddard-based on-chip far-IR spectrometer - these detectors will enable the first OST science objective mentioned above, and provide a clear path for the shorter wavelength objectives as well.
The far-IR band is uniquely suited to study the physical conditions in the interstellar medium from nearby sources out to the highest redshifts. FIR imaging and spectroscopy instrumentation using incoherent superconducting bolometers represents a high sensitivity technology for many future suborbital and space missions, including the Origins Space Telescope. Robust, high sensitivity detector arrays with several 104 pixels, large focal plane filling factors, and low cosmic ray cross sections that operate over the entire far-IR regime are required for such missions. These arrays could consist of smaller sub-arrays, in case they are tileable. The TES based Backshort Under Grid array architecture which our group has fielded in a number of FIR cameras, is a good candidate to meet these requirements: BUGs are tileable, and with the integration of the SQUID multiplexer scaleable beyond wafer sizes; they provide high filling factors, low cosmic cross section and have been demonstrated successfully in far-infrared astronomical instrumentation. However, the production of BUGs with integrated readout multiplexers has many time and resource consuming process steps. In order to meet the requirement of robustness and efficiency on the production of future arrays, we have developed a new method to provide the superconducting connection of BUG detectors to the readout multiplexers or general readout boards behind the detectors. This approach should allow us to reach the goal to produce reliable, very large detector arrays for future FIR missions.
KEYWORDS: Camera shutters, James Webb Space Telescope, Space telescopes, Optical fabrication, Magnetism, Telescopes, Microelectromechanical systems, Silicon, Aerospace engineering, Astronomical imaging
Microshutter array (MSA) subsystems were developed at NASA Goddard Space Flight Center as multiobject selectors for the Near-Infrared Spectrograph (NIRSpec) instrument on the James Webb Space Telescope (JWST). The subsystem will enable NIRSpec to simultaneously obtain spectra from >100 targets, which, in turn, increases instrument efficiency 100-fold. This system represents one of the three major innovations on the JWST that is scheduled to be launched in 2018 as the successor to the Hubble Space Telescope. Featuring torsion hinges, light shields, magnetic actuation, and electrostatic latching and addressing, microshutters are designed for the selective transmission of light with high efficiency and contrast. Complete MSA assemblies consisting of 365×171 microshutters were successfully fabricated and tested, and passed a series of critical reviews for programmable 2-D addressing, life tests, and optical contrast tests. At the final stage of the JWST MSA fabrication, we began to develop the next generation microshutter arrays (NGMSA) for future telescopes. These telescopes will require a much larger field of view than JWSTs. We discussed strategies for fabrication of a proof-of-concept NGMSA that will be modular in design and electrostatically actuated. The details of NGMSA development will be discussed in a follow-up paper.
We describe feedhorn-coupled polarization-sensitive detector arrays that utilize monocrystalline silicon as the dielectric substrate material. Monocrystalline silicon has a low-loss tangent and repeatable dielectric constant, characteristics that are critical for realizing efficient and uniform superconducting microwave circuits. An additional advantage of this material is its low specific heat. In a detector pixel, two Transition-Edge Sensor (TES) bolometers are antenna-coupled to in-band radiation via a symmetric planar orthomode transducer (OMT). Each orthogonal linear polarization is coupled to a separate superconducting microstrip transmission line circuit. On-chip filtering is employed to both reject out-of-band radiation from the upper band edge to the gap frequency of the niobium superconductor, and to flexibly define the bandwidth for each TES to meet the requirements of the application. The microwave circuit is compatible with multi-chroic operation. Metalized silicon platelets are used to define the backshort for the waveguide probes. This micro-machined structure is also used to mitigate the coupling of out-of-band radiation to the microwave circuit. At 40 GHz, the detectors have a measured efficiency of ∼90%. In this paper, we describe the development of the 90 GHz detector arrays that will be demonstrated using the Cosmology Large Angular Scale Surveyor (CLASS) ground-based telescope.
μ-Spec is a compact submillimeter (~ 100 GHz - 1:1 THz) spectrometer which uses low loss superconducting microstrip transmission lines and a single-crystal silicon dielectric to integrate all of the components of a diffraction grating spectrometer onto a single chip. We have already successfully evaluated the performance of a prototype μ-Spec, with spectral resolving power, R=64. Here we present our progress towards developing a higher resolution μ-Spec, which would enable the first science returns in a balloon flight version of this instrument. We describe modifications to the design in scaling from a R=64 to a R=256 instrument, as well as the ultimate performance limits and design concerns when scaling this instrument to higher resolutions.
Kinetic inductance detectors (KIDs) are a promising technology for low-noise, highly-multiplexible mm- and submm-wave detection. KIDs have a number of advantages over other detector technologies, which make them an appealing option in the cosmic microwave background B-mode anisotropy search, including passive frequency domain multiplexing and relatively simple fabrication, but have suffered from challenges associated with noise control. Here we describe design and fabrication of a 20-pixel prototype array of lumped element molybdenum KIDs. We show Q, frequency and temperature measurements from the array under dark conditions. We also present evidence for a double superconducting gap in molybdenum.
The Johns Hopkins University sounding rocket group is building the Far-ultraviolet Off Rowland-circle Telescope for
Imaging and Spectroscopy (FORTIS), which is a Gregorian telescope with rulings on the secondary mirror. FORTIS will
be launched on a sounding rocket from White Sand Missile Range to study the relationship between Lyman alpha escape
and the local gas-to-dust ratio in star forming galaxies with non-zero redshifts. It is designed to acquire images of a 30'
x 30' field and provide fully redundant "on-the-fly" spectral acquisition of 43 separate targets in the field with a bandpass
of 900 - 1800 Angstroms. FORTIS is an enabling scientific and technical activity for future cutting edge far- and near-uv
survey missions seeking to: search for Lyman continuum radiation leaking from star forming galaxies, determine the
epoch of He II reionization and characterize baryon acoustic oscillations using the Lyman forest. In addition to the high
efficiency "two bounce" dual-order spectro-telescope design, FORTIS incorporates a number of innovative technologies
including: an image dissecting microshutter array developed by GSFC; a large area (~ 45 mm x 170 mm) microchannel
plate detector with central imaging and "outrigger" spectral channels provided by Sensor Sciences; and an autonomous
targeting microprocessor incorporating commercially available field programable gate arrays. We discuss progress to date
in developing our pathfinder instrument.
We have successfully fabricated a superconducting transition edge sensor (TES), bolometer that centers on the use of
electron-phonon decoupling (EPD) for thermal isolation. We have selected a design approach that separates the two
functions of far-infrared and THz radiative power absorption and temperature measurement, allowing separate
optimization of the performance of each element. We have integrated molybdenum/gold (Mo/Au) bilayer TES and ion
assisted thermally evaporated (IAE) bismuth (Bi) films as radiation absorber coupled to a low-loss microstripline from
niobium (Nb) ground plane to a twin-slot antenna structure. The thermal conductance (G) and the time constant for the
different geometry device have been measured. For one such device, the measured G is 1.16×10-10 W/K (± 0.61×10-
10 W/K) at 60 mK, which corresponds to noise equivalent power (NEP) = 1.65×10-18W/ √Hz and time constant of ~5 μs.
We have designed, fabricated, and tested compact radiative control structures, including antireflection coatings and
resonant absorbers, for millimeter through submillimeter wave astronomy. The antireflection coatings consist of micromachined
single crystal silicon dielectric sub-wavelength honeycombs. The effective dielectric constant of the structures
is set by the honeycomb cell geometry. The resonant absorbers consist of pieces of solid single crystal silicon substrate
and thin phosphorus implanted regions whose sheet resistance is tailored to maximize absorption by the structure. We
present an implantation model that can be used to predict the ion energy and dose required for obtaining a target implant
layer sheet resistance. A neutral density filter, a hybrid of a silicon dielectric honeycomb with an implanted region, has
also been fabricated with this basic approach. These radiative control structures are scalable and compatible for use
large focal plane detector arrays.
We have fabricated absorber-coupled microwave kinetic inductance detector (MKID) arrays for sub-millimeter and farinfrared
astronomy. Each detector array is comprised of λ/2 stepped impedance resonators, a 1.5μm thick silicon
membrane, and 380μm thick silicon walls. The resonators consist of parallel plate aluminum transmission lines coupled
to low impedance Nb microstrip traces of variable length, which set the resonant frequency of each resonator. This
allows for multiplexed microwave readout and, consequently, good spatial discrimination between pixels in the array.
The Al transmission lines simultaneously act to absorb optical power and are designed to have a surface impedance and
filling fraction so as to match the impedance of free space. Our novel fabrication techniques demonstrate high
fabrication yield of MKID arrays on large single crystal membranes and sub-micron front-to-back alignment of the
microstrip circuit.
KEYWORDS: Camera shutters, James Webb Space Telescope, Space telescopes, Optical fabrication, Magnetism, Silicon, Telescopes, Aerospace engineering, Microelectromechanical systems, Indium
We have developed the Microshutter Array (MSA) system at NASA Goddard Space Flight Center (GSFC) as
a multi-object aperture array for the Near Infrared Spectrograph (NIRSpec) instrument on the James Webb
Space Telescope (JWST). The MSA system will enable NIRSpec to simultaneously obtain spectra from more
than 100 targets, which, in turn, increases instrument efficiency one-hundred fold. Consequently, this system
represents one of the three major innovations on the JWST, which has been selected by the National Research
Council's 2001 decadal survey as the top-ranked space-based mission and is scheduled to be the successor to
the Hubble Space Telescope. Furthermore, the MSA system will be one of the first MEMS devices serving
observation missions in space. Microshutters are designed for the selective transmission of light with high
efficiency and contrast and feature torsion hinges, light shields, deep-reactive ion-etched silicon windows,
magnetic actuation, and electrostatic latching and addressing. Complete MSA quadrant assemblies consisting
of 365 x 181 microshutters have been successfully fabricated. The assemblies have passed a series of critical
reviews, which include programmable 2-D addressing, life tests, optical contrast tests, and environmental
tests, required by the design specifications of JWST. Both the MSA and NIRSpec will be delivered to ESA
for final assembly, and JWST is scheduled to launch in 2014. During final assembly and testing of the MSA
system, we have begun to develop the Next Generation Microshutter Arrays (NGMSA) for future telescopes.
These telescopes will require a much larger field of view than JWST's, and we discuss strategies for
fabrication of a proof-of-concept NGMSA which will be modular in design and electrostatically actuated.
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. Our new generation of PoST's utilizes technology successfully developed on high resolution (~ 2.5 eV) 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 new one-channel, four-pixel, PoST's or 'Hydras', consisting of composite Au/Bi absorbers. We have achieved full-width-at-half-maximum energy resolution of between 5-6 eV on all four Hydra pixels with an exponential decay time constant of 620 μs. Straightforward position discrimination by means of rise time is also demonstrated.
Following our development of a superconducting transition-edge-sensor (TES) microcalorimeter design that en-
ables 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 Microcalorime-
ter 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 non-
multiplexed 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.
Individual x-ray calorimeters based on superconducting transition-edge sensors (TES) have already demonstrated
the spectral resolution, speed, and quantum efficiency needed for astrophysical x-ray spectroscopy. We are now
beginning to realize this capability on the array scale for the first time. We have developed a new design for the
x-ray absorber that has connections to the TES engineered to allow contact only in regions that do not serve
as the active thermometer. We have further constrained the design so that a low-resistance absorber will not
electrically short the TES, permitting the use of high-conductivity electroplated gold for the x-ray absorber.
With such a well-behaved material for the absorber, we now achieve energy resolution at 6 keV in the range 2.4
- 3.1 eV FWHM in all the pixels of the same design tested in a close-packed array. We have achieved somewhat
higher resolution and faster response by eliminating some of the gold and electroplating bismuth in its place.
These are important steps towards the high-resolution, high-fill-factor, microcalorimeter arrays needed for x-ray
astrophysics observatories such as Constellation-X.
We have been developing x-ray microcalorimeters for the Constellation-X mission. Devices based on superconducting transition-edge sensors (TES) have demonstrated the potential to meet the Constellation-X requirements for spectral resolution, speed, and array scale (> 1000 pixels) in a close-packed geometry. In our part of the GSFC/NIST collaboration on this technology development, we have been concentrating on the fabrication of arrays of pixels suitable for the Constellation-X reference configuration. We have fabricated 8x8 arrays with 0.25-mm pixels arranged with 92% fill factor. The pixels are based on Mo/Au TES and Bi/Cu or Au/Bi absorbers. We have achieved a resolution of 4.0 eV FWHM at 6 keV in such devices, which meets the Constellation-X resolution requirement at 6 keV. Studies of the thermal transport in our Bi/Cu absorbers have shown that, while there is room for improvement, for 0.25-mm pixels the standard absorber design is adequate to avoid unacceptable line-broadening from position dependence caused by thermal diffusion. In order to improve reproducibility and to push closer to the 2-eV goal at 6 keV, however, we are refining the design of the TES and the interface to the absorber. Recent efforts to introduce a barrier layer between the Bi and the Mo/Au to avoid variable interface chemistry and thus improve the reproducibility of device characteristics have thus far yielded unsatisfactory results. However, we have developed a new set of absorber designs with contacts to the TES engineered to allow contact only in regions that do not serve as the active thermometer. We have further constrained the design so that a low-resistance absorber will not electrically short the TES. It is with such a design that we have achieved 4.0 eV resolution at 6 keV.
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