Electronic Design

Micromirrors May Ride With Next-Generation Space Telescope

Micromirrors constructed from silicon may one day be part of the Next Generation Space Telescope (NGST). Tentatively scheduled for launch in 2008, the NGST is the successor to the Hubble telescope. It will peruse the universe, looking for remnants from the period in which the first stars and galaxies formed. The prototype microelectromechanical-systems (MEMS) mirrors are being developed by the Department of Energy's Sandia National Laboratories, Albuquerque, N.M.

The mirrors, each slightly larger than a cross-section of a human hair, will be sensitive to infrared radiation. As a result, they will be able to detect faint signals that date back to the first billion years after the Big Bang. This should help scientists develop a better understanding of the origins surrounding the universe.

"We are designing mirrors that will be very, very small, move independently, and be able to withstand the very cold temperatures and extreme conditions of space," said Ernie Garcia, the Sandia engineer leading the mirror-development effort.

Last September, Garcia demonstrated an array of working mirrors at NASA's Goddard Space Flight Center, Greenbelt, Md. Each mirror measured 100 by 100 µm. There were 1-µm gaps between adjacent mirrors, which were lined up in rows of three. Each row tilted 10° in unison—a large angle for this design.

"Getting these miniaturized mirrors to rotate to such a large angle was a real milestone in the research," says Garcia. "It's something that NASA wanted, and we did it."

The mirrors are built by depositing thin films of polycrystalline silicon on a silicon wafer. The first layer, called poly0, contains connection wires. The others, poly1, poly2, and poly3, are mechanical layers that allow the MEMS device to move. Garcia plans to soon add a final thin layer of gold on top of the poly3 to reflect infrared light.

The micromirrors will work in conjunction with a very large mirror—possibly 8 meters in diameter—that will collect light from a broad area in space. When the mirrors encounter an object that appears interesting, the smaller micromirrors will tilt to reflect the image from only that area, beaming the information to an infrared detector.

Garcia said he still faces challenges in developing moving mirrors for the NGST. One is enabling the mirrors to function in extremely cold temperatures. "Instrument operating temperatures in space can be 30K (−405°F) or lower," Garcia said. "That means we have to build these mirrors a special way so that they won't break at such extremes."

Different materials shrink at different rates when subjected to temperature changes, Garcia noted. As the temperature is reduced, the gold layer shrinks faster than the polysilicon. This causes stress. If the stress gets too high, the mirror could break or deform, or the gold could peel away. The researchers must develop the smallest thickness of gold so it doesn't cause excessive stress, but is thick enough to reflect the infrareds.

Another challenge is making each of the mirrors move independently. The newest design, which will soon be fabricated at Sandia's Microelectronics Development Laboratory (MDL), has each row tilting in unison while one mirror in the middle tilts and moves on its own. The goal is to have 4 million of these independently moving mirrors in the NGST. Each mirror could be tilted in different directions to redirect optical signals to an infrared detector.

NASA is pursuing the NGST as the successor to the Hubble Space Telescope in an effort to observe material from the "Dark Zone." This era occurred 100 million to 1 billion years after the Big Bang, when primordial seeds began to evolve into galaxies and stars . It also will examine subjects that aren't nearly as old. The Hubble has provided data about more recent formations, but has been unable to detect the earlier stars that fall in the infrared range because it was designed as an optical telescope.

The NGST, on the other hand, will be extremely sensitive to infrared radiation. With its large light-gathering mirror and superb resolution, it will be able to detect the earlier signals. The new telescope will be placed in orbit well beyond the Earth's moon to reduce stray light and achieve the cold temperatures needed to observe in infrared.

To find out more about MEMS mirrors, visit Sandia's web site at www.sandia.gov. For more information on the NGST, visit www.ngst.gsfc.nasa.gov.

See associated figure.

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