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Microtechnologies for Science Instrumentation Applications 135

Space Agency (ESA) will be providing the NIRSpec instrument, and NASA will
provide the detectors and the MEMS aperture mask as part of their contribution to
JWST. Two approaches were initially proposed in 1996 to NASA for the NIRSpec,
one using a MEMS micromirror array 36,37 and one using a MEMS shutter array. 31–
34
The requirements for both mirror and shutter arrays are very strict: The size of
each pixel was to be 100 100 mm, with a fill factor better than 80% and a contrast
better than 2000:1, expandable to an array size of 1800 1800 square elements,
operating at 40K. Two mirror microarray technologies were considered, one devel-
oped at NASA GSFC 38 and one at Sandia National Laboratories (SNL). 39
A scanning electron microscope (SEM) photograph of the SNL micromirrors is
shown in Figure 7.6. The mirrors are made using SNL’s SUMMiT V process as seen
in Chapter 3. An advantage of the mirror design is that the drive and selection
electronics can be hidden under the mirrors. However, in order to improve the
image quality and the contrast, the mirror needs to be fabricated with different
materials, gold on silicon in this case, which causes stress when cooled down to
cryogenic temperatures. This results in required distortion of the image quality and
may cause de-lamination of the gold coating itself.
The shutter approach was selected because of its better contrast and image
quality, since no reflective surface is involved, and scattered light from the edges is
predominantly reflected back away from the spectrograph. A major challenge for
this approach is to integrate the actuation mechanism as well as the single shutter
control — each shutter needs to be uniquely addressed — within less than 20% of
the entire area. The JWST ‘‘microshutter array’’ is a programmable aperture which

FIGURE 7.6 A micromirror developed by SNL, pictured in the tilted position. Each mirror is
1
about 100 mm in width. (Courtesy of Sandia National Laboratories, http://www.sandia.gov/
media/NewsRel/NR1999/space.htm.)

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