Nick Costen

Microshutter array system for James Webb Space Telescope

We have developed microshutter array systems at NASA Goddard Space Flight Center for use as multi-object aperture arrays for a Near-Infrared Spectrometer (NIRSpec) instrument. The instrument will be carried on the James Webb Space Telescope (JWST), the next generation of space telescope, after the Hubble Space Telescope retires. The microshutter arrays (MSAs) are designed for the selective transmission of light from objected galaxies in space with high efficiency and high contrast. Arrays are close-packed silicon nitride membranes with a pixel size close to 100x200 μm. Individual shutters are patterned with a torsion flexure permitting shutters to open 90 degrees with minimized stress concentration. In order to enhance optical contrast, light shields are made on each shutter to prevent light leak. Shutters are actuated magnetically, latched and addressed electrostatically. The shutter arrays are fabricated using MEMS bulk-micromachining and packaged utilizing a novel single-sided indium flip-chip bonding technology. The MSA flight system consists of a mosaic of 2 x 2 format of four fully addressable 365 x 171 arrays. The system will be placed in the JWST optical path at the focal plane of NIRSpec detectors. MSAs that we fabricated passed a series of qualification tests for flight capabilities. We are in the process of making final flight-qualified MSA systems for the JWST mission.

Detector Architecture of the Cosmology Large Angular Scale Surveyor

The cosmic microwave background (CMB) provides a powerful tool for testing modern cosmology. In particular, if inflation has occurred, the associated gravitational waves would have imprinted a specific polarized pattern on the CMB. Measurement of this faint polarized signature requires large arrays of polarization-sensitive, background- limited detectors, and an unprecedented control over systematic effects associated with instrument design. To this end, the ground-based Cosmology Large Angular Scale Surveyor (CLASS) employs large-format, feedhorn- coupled, background-limited Transition-Edge Sensor (TES) bolometer arrays operating at 40, 90, and 150 GHz bands. The detector architecture has several enabling technologies. An on-chip symmetric planar orthomode transducer (OMT) is employed that allows for highly symmetric beams and low cross-polarization over a wide bandwidth. Furthermore, the quarter-wave backshort of the OMT is integrated using an innovative indium bump bonding process at the chip level that ensures minimum loss, maximum repeatability and performance uniformity across an array. Care has been taken to reduce stray light and on-chip leakage. In this paper, we report on the architecture and performance of the first prototype detectors for the 40 GHz focal plane.

Kilopixel backshort-under-grid arrays for the Primordial Inflation Polarization Explorer

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.

Integration of a carbon nanotube field emission electron gun for a miniaturized time-of-flight mass spectrometer

A carbon nanotube (CNT) field emission electron gun has been fabricated and assembled as an electron impact ionization source for a miniaturized time-of-flight mass spectrometer (TOF-MS). The cathode consists of a patterned array of CNT towers grown by catalyst-assisted thermal chemical vapor deposition. An extraction grid is precisely integrated in close proximity to the emitter tips (20-35 μm spacing), and an anode is located at the output to monitor the ionization beam current. Ultra-clean MEMS integration techniques were employed in an effort to achieve three improvements, relative to previous embodiments: reduced extraction voltage during operation to be resonant with gas ionization energies, enhanced current transmission through the grid, and a greater understanding of the fundamental current fluctuations due to adsorbate-assisted tunneling. Performance of the CNT electron gun will be reported, and implications for in situ mass spectrometry in planetary science will be discussed.

Complex MEMS device: microshutter array system for space applications

A complex MEMS device, microshutter array system, is being developed at NASA Goddard Space Flight Center for use as an aperture array for a Near-Infrared Spectrometer (NirSpec). The instrument will be carried on the James Webb Space Telescope (JWST), the next generation of space telescope after Hubble Space Telescope retires. The microshutter arrays (MSAs) are designed for the selective transmission of light with high efficiency and high contrast. Arrays are close-packed silicon nitride membranes with a pixel size close to 100x200 &mgr;m. Individual shutters are patterned with a torsion flexure permitting shutters to open 90 degrees with a minimized mechanical stress concentration. Light shields are made on to each shutter for light leak prevention so to enhance optical contrast. Shutters are actuated magnetically, latched and addressed electrostatically. The shutter arrays are fabricated using MEMS bulk-micromachining technologies and packaged using single-sided indium flip-chip bonding technology. The MSA flight concept consists of a mosaic of 2 x 2 format of four fully addressable 365 x 171 arrays placed in the JWST optical path at the focal plane.

Silicon-based antenna-coupled polarization-sensitive millimeter-wave bolometer arrays for cosmic microwave background instruments

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.

Enabling large focal plane arrays through mosaic hybridization

We have demonstrated advances in mosaic hybridization that will enable very large format far-infrared detectors. Specifically we have produced electrical detector models via mosaic hybridization yielding superconducting circuit paths by hybridizing separately fabricated sub-units onto a single detector unit. The detector model was made on a 100mm diameter wafer while four model readout quadrant chips were made from a separate 100mm wafer. The individually fabricated parts were hybridized using a flip-chip bonder to assemble the detector-readout stack. Once all of the hybridized readouts were in place, a single, large and thick silicon substrate was placed on the stack and attached with permanent epoxy to provide strength and a Coefficient of Thermal Expansion match to the silicon components underneath. Wirebond pads on the readout chips connect circuits to warm readout electronics; and were used to validate the successful superconducting electrical interconnection of the model mosaic-hybrid detector. This demonstration is directly scalable to 150 mm diameter wafers, enabling pixel areas over ten times the area currently available.

Understanding the effect of electron gun geometry on emission characteristics for the purpose of optimizing the VAPoR time-of-flight mass spectrometer

Simulations of field emission of electrons from a custom built electron gun are used to determine the effect that offsets in alignment of the cathode to the grid and the cathode to grid gap have on the trajectories of electrons transmitted through the grid. The simulations are a first step towards understanding how to optimize the electron gun and determine the magnitude of optical aberrations that limit focusing of the emitted electron beam. A finite element method is employed to simulate field emission from the cathode, an array of carbon nanotube pillars. This is done using a three dimensional model that attempts to account for field enhancement due to both the pillar geometry and individual carbon nanotubes. Qualitative comparisons of the configurations tested indicate larger transmissivity through the grid can be achieved by offsetting the pillar array but only at the expense of a skewed elevation angle distribution.

MEMS Microshutter Arrays for James Webb Space Telescope

MEMS microshutter arrays (MSAs) are being developed at NASA Goddard Space Flight Center for use as an aperture array for the Near-Infrared Spectrometer (NirSpec). The instruments will be carried on the James Webb Space Telescope (JWST), the next generation of space telescope after Hubble Space Telescope retires. The microshutter arrays are designed for the selective transmission of light with high efficiency and high contrast. Arrays are close-packed silicon nitride membranes with a pixel size of 105x204 μm. Individual shutters are patterned with a torsion flexure permitting shutters to open 90 degrees with a minimized mechanical stress concentration. Light shields are made on each shutter for light leak prevention to enhance optical contrast. Shutters are actuated magnetically, latched and addressed electrostatically. The shutter arrays are fabricated using MEMS technologies. Single-side indium flip chip bonding is performed to attach microshutter arrays to substrates.