Mary Li

Programmable microshutter arrays for the JWST NIRSpec: optical performance

Two-dimensional microshutter arrays (MSAs) are being developed at the NASA Goddard Space Flight Center for the James Webb Space Telescope (JWST) for use as a programmable aperture mask for object selection for the Near Infrared Multiobject Spectrograph (NIRSpec). The MSAs are designed to provide high transmission efficiency for the selected objects and high on to off contrast ratio at the /spl sim/35 K operating temperature of JWST. The arrays of shutters are produced from silicon nitride membranes on a 100/spl times/200 /spl mu/m pitch. Individual shutters consist of a shutter blade of silicon nitride suspended from the shutter frame by a nitride torsion flexure. The shutters are normally closed. All shutters in the array are opened by the scanning magnetic field, and are held open by an electrostatic potential applied between the open shutters and the shutter support grid electrodes. To close the required shutters for a specific configuration, the potential between the shutter to be deselected and the support frame is set to zero, allowing the shutter to close. In this way, full random access addressing is achieved. We have produced such shutters and have demonstrated mechanical actuation and selection. Optical tests of open and closed shutters have demonstrated the required contrast for the JWST application. The MSA is a pioneering technology that provides the most capable possible multiobject spectrograph for JWST. It provides high contrast selection, high transmission efficiency, and can meet the environmental requirements for JWST.

JWST microshutter array system and beyond

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 Councils 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 JWSTs, and we discuss strategies for fabrication of a proof-of-concept NGMSA which will be modular in design and electrostatically actuated.

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.

Microshutters arrays for the JWST near-infrared spectrometer

The Near Infrared Spectrograph (NIRSpec) for the James Webb Space Telescope (JWST) is a multi-object spectrograph operating in the 0.6-5.0 μm spectral range. One of the primary scientific objectives of this instrument is to measure the number and density evolution of galaxies following the epoch of initial formation. NIRSpec is designed to allow simultaneous observation of a large number of sources, vastly increasing the capability of JWST to carry out its objectives. A critical element of the instrument is the programmable field selector, the Microshutter Array. The system consists of four 175 x 384 close packed arrays of individually operable shutters, each element subtending 0.2” x 0.4”on the sky. This device allows simultaneous selection of over 200 candidates for study over the 3.6’ x 3.6’ field of the NIRSpec, dramatically increasing its efficiency for a wide range of investigations. Here, we describe the development, production, and test of this critical element of the NIRSpec.

Cryogenic Characterization and Testing of Magnetically-Actuated Microshutter Arrays for the James Webb Space Telescope

Two-dimensional MEMS microshutter arrays (MSA) have been fabricated at the NASA Goddard Space Flight Center (GSFC) for the James Webb Space Telescope (JWST) to enable cryogenic (~35 K) spectrographic astronomy measurements at near-infrared wavelengths. Functioning as a focal plane object selection device, the MSA is a 2D programmable aperture mask with fine resolution, high efficiency and high contrast. The MSA are close-packed silicon nitride shutters (cell size of 100 µm × 200 µm) patterned with a torsion flexure to allow their opening to 90°. A layer of magnetic material is deposited onto each shutter to permit magnetic actuation. Two electrodes are deposited, one onto each shutter and another onto the support structure side-wall, permitting electrostatic latching and 2D addressing. New techniques were developed to test MSA under mission-similar conditions (8 K ≤ T < 300 K). The magnetic rotisserie has proven to be an excellent tool for rapid characterization of MSA. Tests conducted with the magnetic rotisserie method include accelerated cryogenic lifetesting of unpackaged 128 × 64 MSA and parallel measurement of the magneto-mechanical stiffness of shutters in pathfinder test samples containing multiple MSA designs. Lifetest results indicate a logarithmic failure rate out to ~106 shutter actuations. These results have increased our understanding of failure mechanisms and provide a means to predict the overall reliability of MSA devices.

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.

Development and Operation of the Microshutter Array System

The microshutter array (MSA) is a key component in the James Webb Space Telescope Near Infrared Spectrometer (NIRSpec) instrument. The James Webb Space Telescope is the next generation of a space-borne astronomy platform that is scheduled to be launched in 2013. However, in order to effectively operate the array and meet the severe operational requirements associated with a space flight mission has placed enormous constraints on the microshutter array subsystem. This paper will present an overview and description of the entire microshutter subsystem including the microshutter array, the hybridized array assembly, the integrated CMOS electronics, mechanical mounting module and the test methodology and performance of the fully assembled microshutter subsystem. The NIRSpec is a European Space Agency (ESA) instrument requiring four fully assembled microshutter arrays, or quads, which are independently addressed to allow for the imaging of selected celestial objects onto the two 4 mega pixel IR detectors. Each microshutter array must have no more than ~8 shutters which are failed in the open mode (depending on how many are failed closed) out of the 62,415 (365x171) total number of shutters per array. The driving science requirement is to be able to select up to 100 objects at a time to be spectrally imaged at the focal plane. The spectrum is dispersed in the direction of the 171 shutters so if there is an unwanted open shutter in that row the light from an object passing through that failed open shutter will corrupt the spectrum from the intended object.

An investigation into graphene exfoliation and potential graphene application in MEMS devices

The design of microelectromecanical systems (MEMS) and micro-opto-electromechanical systems (MOEMS) are often materials-limited with respect to the efficiency and capability of the material. Graphene, a one atom thick honeycomb lattice of carbon, is a highly desired material for MEMS applications. Relevant properties of graphene include the materials optical transparency, mechanical strength, energy efficiency, and electrical and thermal conductivity due to its electron mobility. Aforementioned properties make graphene a strong candidate to supplant existing transparent electrode technology and replace the conventionally used material, indium-tin oxide. In this paper we present preliminary results on work toward integration of graphene with MEMS structures. We are studying mechanical exfoliation of highly ordered pyrolytic graphite (HOPG) crystals by repeatedly applying and separating adhesive materials from the HOPG surface. The resulting graphene sheets are then transferred to silicon oxide substrate using the previously applied adhesive material. We explored different adhesive options, particularly the use of Kapton tape, to improve the yield of graphene isolation along with chemical cross-linking agents which operate on a mechanism of photoinsertion of disassociated nitrene groups. These perfluorophenyl nitrenes participate in C=C addition reactions with graphene monolayers creating a covalent binding between the substrate and graphene. We are focusing on maximizing the size of isolated graphene sheets and comparing to conventional exfoliation. Preliminary results allow isolation of few layer graphene (FLG) sheets (n<3) of approximately 10μm x 44μm. Photolithography could possibly be utilized to tailor designs for microshutter technology to be used in future deep space telescopes.

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.