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74 MEMS and Microstructures in Aerospace Applications

in this chapter. The impact of radiation effects is addressed more fully in a
dedicated chapter. The radiation issues are well worth an in-depth chapter as
MEMS is a relatively new and emerging technology compared to microcircuits.
For microelectronics there is a well-established knowledge base for space-grade
parts. Unfortunately, there are no similar foundations for MEMS. Microelectronics
for space are typically qualified to four standard total dose radiation levels, namely
3, 10, and 100 krads, and 1 megarad. Parts qualified to these levels are identified in
MIL-M-38510 and MIL-PFR-19500 by the symbols M, D, R, and H, respectively.
For the purposes of standardization, programs are encouraged to procure parts
through the mentioned specifications using the designation, which most closely
corresponds to their individual program requirements. The level of radiation hard-
ness of a part must correspond to the expected program requirements. In addition, a
safety margin (i.e., a de-rating factor) of 2 is frequently used. For example, if a
system will be seeing a total dose level of 2 krads per year and the system is
specified to operate for 5 years, then the individual part must either be capable of
tolerating a total of 20 krads (10 krads 2) or must be shielded so that it will not
receive the total dose level of 2 krads per year. Any testing performed on actual
MEMS devices is relatively recent. Commercial MEMS accelerometers such as the
AD XL50 have been tested, and the IC component of the devices was found to be
sensitive. 5,6 The author in one of these studies iterates the requirement that CMOS
circuits in particular are known to degrade when exposed to low doses of ionizing
radiation. Therefore, before MEMS can be used in the radiation environment of
space, it is important to test them for their sensitivity to radiation ion-induced
6
7
radiation damage. In addition MEMS optical mirrors, electrostatic, electrother-
9
8
mal, and bimorph actuators, and RF relays add to the rapidly growing database of
components tested. In all fairness, these tests are performed on commercial grade
MEMS as the concept of radiation-hardened space-qualified MEMS has yet to
mature.
4.3 DESIGN THROUGH MISSION OPERATION ENVIRONMENTS

MEMS devices for space flight use are exposed to two application areas: design-
through-prelaunch and launch-through-mission. The first phase includes the manu-
facture, qualification, integration, and test of the parts to the component level. The
launch or mission environment includes the launch, lift-off, acceleration, vibration,
and mission until the end-of-life (EOL).
The prelaunch period includes planning, procurement, manufacture, test, com-
ponent assembly, and component acceptance testing. The procurement process for
MEMS devices includes the fabrication run time and may well exceed the lengthy
requirements of space grade microcircuits (48 to 70 weeks). Iterative runs must be
considered when scheduling and planning for the incorporation of MEMS devices
in space programs. Although vendors are claiming lead times for manufacturing
consistent with the microcircuit world, the lack of high-volume manufacturing and
the absence of low-cost packaging continue to keep most MEMS in a custom

© 2006 by Taylor & Francis Group, LLC