David E. Franz

4H-SiC UV photo detectors with large area and very high specific detectivity

Pt/4H-SiC Schottky photodiodes have been fabricated with the device areas up to 1 cm/sup 2/. The I-V characteristics and photoresponse spectra have been measured and analyzed. For a 5 mm/spl times/5 mm area device leakage current lower than 10/sup -15/ A at zero bias and 1.2/spl times/10/sup -14/ A at -1 V have been established. The quantum efficiency is over 30% from 240 to 320 nm. The specific detectivity, D/sup */, has been calculated from the directly measured leakage current and quantum efficiency are shown to be higher than 10/sup 15/ cmHz/sup 1/2//W from 210 to 350 nm with a peak D/sup */ of 3.6/spl times/10/sup 15/ cmHz/sup 1/2//W at 300 nm.

Magnetically actuated microshutter arrays

Two-dimensional microshutter arrays are being developed at NASA Goddard Space Flight Center (GSFC) for the Next Generation Space Telescope (NGST) for use in the near-infrared region. Functioning as focal plane object selection devices, the microshutter arrays are 2-D programmable masks with high efficiency and high contrast. The NGST environment requires cryogenic operation at 45 K. Arrays are close-packed silicon nitride membranes with a unit cell size of 100x100 micrometer. Individual shutters are patterned with a torsion flexure permitting shutters to open 90 degrees with 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 the shutters from the silicon substrate. A layer of magnetic material is deposited onto each shutter. Onto the side-wall of the support structure a metal layer is deposited that acts as a vertical hold electrode. Shutters are rotated into the support structure by means of an external magnet that is swept across the shutter array for opening. Addressing is performed through a scheme using row and column address lines on each chip and external addressing electronics.

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.

Microshutter arrays for near-infrared applications on the James Webb Space Telescope

Magnetically actuated MEMS microshutter arrays are being developed at the NASA Goddard Space Flight Center for use in a multi-object spectrometer on the James Webb Space Telescope (JWST), formerly Next Generation Space Telescope (NGST). The microshutter arrays are designed for the selective transmission of light with high efficiency and high contrast. The JWST environment requires cryogenic operation at 45K. Microshutter arrays are fabricated out of silicon-on-insulator (SOI) wafers. Arrays consist of close-packed shutters made on silicon nitride (nitride) membranes with a pixel size of 100 × 100 m. Individual shutters are patterned with a torsion flexure permitting shutters to open 90°, with a minimized mechanical stress concentration. Shutters operated this way have survived fatigue life test. The mechanical shutter arrays are fabricated using MEMS technologies. The processing includes a multi-layer metal deposition, patterning of shutter electrodes and magnetic pads, reactive ion etching (RIE) of the front side to form shutters in a nitride film, an anisotropic back-etch for wafer thinning, and a deep RIE (DRIE) back-etch, down to the nitride shutter layer, to form support frames and relieve shutters from the silicon substrate. An additional metal deposition and patterning has recently been developed to form electrodes on the vertical walls of the frame. Shutters are actuated using a magnetic force, and latched electrostatically. One-dimensional addressing has been demonstrated.

Microshutter Array Development For The James Webb Space Telescope

Micro Electromechanical System (MEMS) microshutter arrays are being developed at NASA Goddard Space Flight Center for use as a field selector of the Near Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope (JWST). The microshutter arrays are designed for the spontaneous selection of a large number of objects in the sky and the transmission of light to the NIRSpec detector with high contrast. The JWST environment requires cryogenic operation at 35 K. Microshutter arrays are fabricated out of silicon-on-insulator (SOI) silicon wafers. Arrays are close-packed silicon nitride membranes with a pixel size of 100 x 200 μm. Individual shutters are patterned with a torsion flexure permitting shutters to open 90 degrees with a minimized mechanical stress concentration. Light shields are processed for blocking light from gaps between shutters and frames. The mechanical shutter arrays are fabricated using MEMS technologies. The processing includes multi-layer metal depositions, the patterning of magnetic stripes and shutter electrodes, a reactive ion etching (RIE) to form shutters out of the nitride membrane, an anisotropic back-etch for wafer thinning, followed by a deep RIE (DRIE) back-etch to form mechanical supporting grids and release shutters from the silicon substrate. An additional metal deposition is used to form back electrodes. Shutters are actuated by a magnetic force and latched using an electrostatic force. Optical tests, addressing tests, and life tests are conducted to evaluate the performance and the reliability of microshutter arrays.

A microshutter-based field selector for JWST's multi-object near infrared spectrograph

One of the James Webb Space Telescopes (JWST) primary science goals is to characterize the epoch of galaxy formation in the universe and observe the first galaxies and clusters of galaxies. This goal requires multi-band imaging and spectroscopic data in the near infrared portion of the spectrum for large numbers of very faint galaxies. Because such objects are sparse on the sky at the JWST resolution, a multi-object spectrograph is necessary to efficiently carry out the required observations. We have developed a fully programmable array of microshutters that will be used as the field selector for the multi-object Near Infrared Spectrograph (NIRSpec) on JWST. This device allows apertures to be opened at the locations of selected galaxies in the field of view while blocking other unwanted light from the sky background and bright sources. In practice, greater than 100 objects within the field of view can be observed simultaneously. This field selection capability greatly improves the sensitivity and efficiency of NIRSpec. In this paper, we describe the microshutter arrays, their development, characteristics, fabrication, testing, and progress toward delivery of a flight-qualified field selection subsystem to the NIRSpec instrument team.