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Space Radiation Effects and Microelectromechanical Systems 97

bipolar transistors and LEDs, to suffer from degraded performance. Although
nuclear interactions also occur in metals and insulators, their effects are typically
not detectable. Thus, MEMS that contain bipolar devices or LEDs may be expected
to degrade via displacement damage.
At extremely high levels of displacement damage, bulk material properties,
such as stiffness, could be affected. This will be evident in MEMS devices that rely
on the values of these bulk properties for proper operation. For example, changes in
a bulk material property such as stiffness would modify the degree of flexibility of
silicon layers used in comb drives that form part of MEMS engine. 12 Levels of
radiation exposure for most space missions, except perhaps those to Jupiter, are
several orders of magnitude lower than what would be necessary to have a notice-
able effect on the bulk material properties and may largely be ignored.

5.2.3 RADIATION TESTING OF MEMS
Radiation testing of MEMS can be accomplished by following well-established
procedures developed for radiation testing electronic and photonic devices. SEE
testing is usually accomplished with heavy ions and protons at accelerators. TID
60
susceptibility is most conveniently measured with gamma rays in a Co cell or with
x-rays. DD is typically produced with protons at accelerators, as well as with neutrons
in reactors or at accelerators. Parts are exercised either during (for SEE) or following
(for TID and DD) irradiation to ascertain how they respond to the radiation.
One issue relevant for MEMS is that of ion range. Heavy ions available at most
accelerators have relatively short ranges in material — at the most a few hundred
microns. In some MEMS the radiation sensitive parts are covered by material, such as
in the case of digital mirror devices, where a transparent glass covers the mechanical
part. Removal of the glass destroys the mirror so that testing must be performed at
those accelerators with sufficient energy for the ions to penetrate the overlying
material. Particle range is not a problem for protons or gamma ray exposures.

5.3 EXAMPLES OF RADIATION EFFECTS IN MEMS

MEMS are unique from a radiation-effects point of view because they contain
electronic control circuits coupled with mechanical structures, both of which are
potentially sensitive to radiation damage. The electronic circuits in MEMS are
either CMOS or bipolar technologies that are known potentially to exhibit great
sensitivity to radiation damage. It is not at all obvious that radiation doses that
produce measurable changes in performance in electronic circuits will have any
effect on mechanical structures; however, they can.
The first commercial MEMS tested for radiation sensitivity was an accelerom-
eter exposed to an ion beam. 13 By using a small aperture it was possible to confine
the beam to the area of the chip containing only the mechanical structure. Signifi-
cant changes in performance were noted following moderate particle fluences. The
radiation damage was attributed to charge generated in an insulating layer that was
part of the mechanical structure. The charge altered the magnitude of the applied

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