Ari-David Brown

Close-packed arrays of transition-edge x-ray microcalorimeters with high spectral resolution at 5.9 keV

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.

Development of Embedded Heatsinking Layers for Compact Arrays of X-Ray TES Microcalorimeters

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.

Fabrication of MKIDS for the MicroSpec Spectrometer

Microspec is a new class of submillimeter and millimeter (250-700 μm wavelength) spectrometer, in which the wavelength separation and detection of incident light is done on a single substrate. The instrument is designed for space exploration by offering high spectral resolving power over a broad band, while being orders of magnitude smaller in mass and volume than the present state-of-the-art. The key enabling components for Microspec are background-limited microwave kinetic inductance detectors, which operate over the full bandwidth of the spectrometer. Here we present our fabrication strategy for making these sensitive detectors. A microstrip architecture utilizing a 0.45-μm crystalline silicon dielectric with a molybdenum nitride kinetic inductor material has been adopted. We have optimized wafer-scale lithographic patterning, and have developed processes that allow us to minimize surface roughness that may contribute to detector noise. Additionally, we have optimized the low-temperature wafer bonding process; this process allows us to build superconductors on both sides of the silicon dielectric layer. We present a final fabricated device and resonator operation at cryogenic temperatures.

Fabrication of an absorber-coupled MKID detector and readout for sub-millimeter and far-infrared astronomy

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.

Large‐Absorber TES X‐ray Microcalorimeters and the Micro‐X Detector Array

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.

Heat Sinking, Crosstalk, and Temperature Uniformity for Large Close-Packed Microcalorimeter Arrays

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.

Fabrication of Compact Superconducting Lowpass Filters for Ultrasensitive Detectors

Optimal performance of background limited thermal detectors requires adequate control over all relevant sources of incident electromagnetic radiation. In addition to the radiant power incident from the scene of interest, undesired or spurious power can potentially couple to the sensor via its bias and readout circuitry employed to operate the device. One means of limiting the contribution of this stray radiation is to filter or block leakage associated with electrical connections in the detector environment. Here we discuss a fabrication methodology for realizing compact planar filters embedded in the wall of the detector enclosure whose tailored response controls the propagation of light through the far infrared. This approach consists of fabricating an array of boxed-stripline transmission line blocking filters to control thermal radiation incident via this path. Topologically, each superconducting center conductor is encased by a silicon dioxide dielectric insulator and surrounded by a metallic shield to form a single mode transmission line structure. We report on achieved attenuation and return loss and find that it replicates simulated data to a high degree.

Development of superconducting transition edge sensors based on electron-phonon decoupling

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.

Transition Measurements of a Micron‐Sized Transition‐Edge Hot‐Electron Microbolometer

We are developing a Transition‐Edge Hot‐electron Microbolometer (THM) for use in large detector array applications in millimeter‐wave astronomy. This bolometer detector consists of a superconducting bilayer TES with an overlapping thin‐film semi‐metal absorber. The detector is deposited directly on the substrate and thermal isolation of the bolometer is controlled by electron‐phonon scattering within the small volume of the detector. We present measurements characterizing the transition behavior of several micron‐sized THM test devices which are optimized for photon‐noise‐limited observations of the Cosmic Microwave Background (CMB). We interpret these measurements in terms of a lateral proximity effect between the TES and the superconducting Nb TES leads. We discuss possible modifications to the THM design to compensate for this effect while retaining the small detector volume necessary to obtain the desired value of electron‐phonon thermal conductivity.

Electrostatic microshutter arrays

Based on the Microshutter Array (MSA) subsystems developed at NASA Goddard Space Flight Center (GSFC) for the James Webb Space Telescope (JWST), Next Generation Microshutter Array (NGMSA) has been developed to be used as multi-object selectors for future telescopes in space applications. Microshutter arrays function as transmission devices. Selected shutters fully open 90 degrees permitting incoming light to go through, while the rest of shutters remain closed. The programmable microshutters open and close making the device perform as a multi object selector that can be used on space telescopes. Utilizing a multi object selector, the telescope efficiency can be increased to 100 times or more. Like JWST MSAs, NGMSA features torsion hinges, light shields, front and back electrodes for shutter actuation, latch, and closing. The difference is that JWST MSA utilized magnetic actuation while NGMSA uses electrostatic actuation.