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304 MEMS and Microstructures in Aerospace Applications

MEMS shutter array (MSA) was the obvious solution, but even the protective
window must meet the NASA GSFC material requirements. In this application
the external surface of the window must be electrically conductive, and if made of
an organic material, must be resistant to the attack by atomic oxygen in space. In
addition, for the shutter application, high infrared transparency was required.
The protective windows used are a fluorinated polyimide material developed by
NASA Langley Research Center (LaRC) located in Newport News, Virginia.
LaRC-CP1 1 polyimide is a high-performance material with a wide variety of
uses in space structures, thermal insulation, electrical insulators, industrial tapes,
and advanced composites. This polyimide material may be dissolved readily in a
number of solvents for use in various applications such as castings and coatings.
CP1 was selected for the ST5 application for its infrared transparency and space
environment survivability for a 10-year life in geosynchronous earth orbit (GEO).
CP1 is colorless and offers better space UV-radiation resistance than most known
polymer materials (including other polyimides, polyesters, Teflon, Teflon-based
materials, and others). The MEMS dies are fabricated in wafer format using
Sandia’s processing as described in Chapter 3. The wafers go through a standard
backside grind process and then are released, diced, tested, gold coated, and
functionally tested again, in preparation for final attach. The individual dies are
bonded to aluminum nitride (AlN) carriers that are subsequently bonded to the
MSA chassis. This design allows for optimum rework or replacement of each
MEMS shutter die (MSD) as necessary.
Of most significance is the window assembly. As stated previously, the micro-
machined comb drives are sensitive to the abundant contamination in space. The
CP1 fluorinated polyimide material was selected for the fabrication of MEMS
device. A CP1 film, less than 4 mils thick, is sandwiched in tension between two
window frames and bonded in place, as shown in Figure 13.3. CP1 in its relaxed

FIGURE 13.3 MSA radiator assembly. (Source: JHU/APL.)

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