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E. J. Garcia (Sandia National Laboratories), M. A. Greenhouse, B. Mott, J. L. Kuhn (NASA Goddard)
This poster describes how advanced surface micromachining (SMM) technology is being used to develop prototype cryogenic micromirror arrays for evaluation as an instrument optical component for the NGST. When used as a spectrograph reflective slit mask, these arrays can yield a factor of ~1000 reduction in mass and power over, traditional motor-driven slit wheels used on HST instruments. The advantage of micromirrors as a new approach to instrument aperture control is particularly apparent when it is coupled with new large format focal plane arrays to enable multi-object spectroscopy. In this application, the micromirror-enabled capability goes beyond mass and power reduction to offer increased observing efficiency (targets/hour). In the case of NGST, a factor of 100 improvement in efficiency relative to traditional instrument designs has been estimated.
SMM devices use fabrication processes adapted from integrated circuit manufacturing to build microscopic-sized electromechanical devices from polycrystalline silicon. Because these devices can be batch fabricated thousands or even millions of devices can be fabricated on a single wafer at costs several orders of magnitude less than conventionally fabricated devices.
We describe how the SMM process is used to produce micromirror arrays with 100-micron pixels in formats scalable to 2048 x 2048 assemblies. Each pixel consists of a mirror, which is held in position by an appropriately designed suspension, and electrodes that are used to electrostatically tilt the mirror pixel to the desired orientation. The Sandia National Laboratories SMM process uses four independent layers of polycrystalline silicon to produce the appropriate structures. A special chemical-mechanical polishing process is used to produce a very flat top mirror surface. The design of a typical mirror, and how the process is used to create the desired structures is described and illustrated.
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