Audrey J. Ewin

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.

Fabrication of microshutter arrays for space application

Two-dimensional microshutter arrays are being developed at NASA Goddard Space Flight Center for the Next Generation Space Telescope (NGST) for use in the near-infrared region. Functioning as object selection devices, the microshutter arrays are designed for the transmission of light with high efficiency and high contrast. The NGST environment requires cryogenic operation at 45K. Arrays are close-packed silicon nitride membranes with a pixel size of 100 X 100 micrometers . Individual shutters are patterned with a torsion flexure permitting shutters to open 90 degrees with a minimized mechanical stress concentration. The mechanical shutter arrays are fabricated with MEMS technologies. The processing includes a RIE front-etch to form shutters out of the nitride membrane, an anisotropic back-etch for wafer thinning, and a deep RIE (DRIE) back-etch down to the nitride shutter membrane to form frames and to relieve shutters from the silicon substrate. Two approaches for shutter actuation have been developed. Shutters are actuated using either a combined mechanical and electrostatic force or a combined magnetic and electrostatic force. A CMOS circuit embedded in the frame between shutters allows programmable shutter selection for the first approach. A control of row and column electrodes fulfills shutter selection for the second approach.

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.

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.

A programmable CCD driver circuit for multiphase CCD operation

A programmable CCD (charge-coupled device) driver circuit was designed to drive CCDs in multiphased modes. The purpose of the drive electronics is to operate developmental CCD imaging arrays for NASAs tiltable moderate resolution imaging spectrometer (MODIS-T) Four objectives for the driver were considered during its design: the circuit drives CCD electrode voltages between 0 V and +30 V to produce reasonable potential wells; the driving sequence is started with one input signal; the circuit allows programming of frame sequences required by arrays of any size; it produces interfacing signals for the CCD and the DTF (detector test facility). Simulation of the driver verified its function with the master clock running up to 10 MHz. This suggests a maximum rate of 400000 pixels/s. Timing and packaging parameters were verified. The design uses 54 TTL (transistor-transistor logic) chips. Two versions of hardware were fabricated: wirewrap and printed circuit board. Both were verified functionally with a logic analyzer. >

Design and Performance of Hybrid Arrays of Mo/Au Bilayer Transition-Edge Sensors

For future X-ray astrophysics missions, X-ray microcalorimeters can be optimized with different properties in different regions of the focal plane. This approach has the potential to improve microcalorimeter instrument capabilities with efficient use of instrument resources. For example a point-source array optimized for high angular resolution, high count-rate observations could be accompanied by a main array to expand the field of view for diffuse observations. In this approach, it is desirable to be able to simultaneously optimize different transition-edge sensor (TES) geometries on a single wafer design. The key properties of TESs such as transition temperature and shape are a strong function of size and geometry due to the complex interplay between the proximity effect from the superconducting bias electrodes and the normal metal features used for noise suppression and absorber contact. As a result, devices fabricated with the same deposited layer but with different sizes will have different transition temperatures and different response to X-ray events. In this paper, we present measurements of the transition temperature and properties of devices with different sizes and normal metal features, and discuss how by tuning the geometry we can achieve the desired pixel parameters for a given application. We also describe measurements of transition properties from large-format hybrid arrays containing three different pixel types.

Fabrication of X-Ray Microcalorimeter Focal Planes Composed of Two Distinct Pixel Types

We develop superconducting transition-edge sensor (TES) microcalorimeter focal planes for versatility in meeting the specifications of X-ray imaging spectrometers, including high count rate, high energy resolution, and large field of view. In particular, a focal plane composed of two subarrays: one of fine pitch, high count-rate devices and the other of slower, larger pixels with similar energy resolution, offers promise for the next generation of astrophysics instruments, such as the X-ray Integral Field Unit Instrument on the European Space Agencys ATHENA mission. We have based the subarrays of our current design on successful pixel designs that have been demonstrated separately. Pixels with an all-gold X-ray absorber on 50 and 75 μm pitch, where the Mo/Au TES sits atop a thick metal heatsinking layer, have shown high resolution and can accommodate high count rates. The demonstrated larger pixels use a silicon nitride membrane for thermal isolation, thinner Au, and an added bismuth layer in a 250-μm2 absorber. To tune the parameters of each subarray requires merging the fabrication processes of the two detector types. We present the fabrication process for dual production of different X-ray absorbers on the same substrate, thick Au on the small pixels and thinner Au with a Bi capping layer on the larger pixels to tune their heat capacities. The process requires multiple electroplating and etching steps, but the absorbers are defined in a single-ion milling step. We demonstrate methods for integrating the heatsinking of the two types of pixel into the same focal plane consistent with the requirements for each subarray, including the limiting of thermal crosstalk. We also discuss fabrication process modifications for tuning the intrinsic transition temperature (

Development of individually addressable micromirror arrays for space applications

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SAGE III alternative detector design and characterization

) of the bilayers for the different device types through variation of the bilayer thicknesses. The latest results on these “hybrid” arrays will be presented.

Development of TES Microcalorimeter Arrays for the Micro-X Sounding Rocket Experiment

A 2D array of individually addressable micro-mirrors with 100 micrometers by 100 micrometers pixel size, capable of tilting +/- 100 by electrostatic actuation is being developed and fabricated at the Detector Development Laboratory of NASA, GSFC. The development requires integration of CMOS and MEMS fabrication processes. We have competed extensive analytical studies and performed laboratory test to compare model predictions with actual performance of a 3 by 3 array. We are testing the address and driver circuit for a 32 by 32 array and also developing the process integration of the CMOS and MEMS fabrication of the larger arrays. The mirrors are capable of operating at cryogenic temperature for astronomical applications. Our goal is to extend the development to a 25 6by 256 array for a wide variety of space applications including the multi-object-spectrometer in the next generation space telescope.