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Space Radiation Effects and Microelectromechanical Systems 107

Application of a voltage across the PZT causes it to flex and, in so doing, it deforms
the mirror membrane, which turns the DMD off. Exposure to an ionizing radiation
dose of 1 Mrad(Si) causes unbiased mirrors to deflect by about 5% and biased
mirrors to deflect by about 10%. The degradation is due to charge trapped in the
silicon nitrite on which the PZT is deposited and not to degradation of the PZT
itself.
The results of TID testing of DMDs show once again that the radiation
sensitivity of MEMS may be reduced by eliminating dielectric layers between
electrodes, because any charge generated in the dielectrics will modify the electric
field applied between the two electrodes, thereby causing an erroneous reading.

5.4 MITIGATION OF RADIATION EFFECTS IN MEMS

Reducing the sensitivity of MEMS to radiation effects is possible but may be quite
challenging given that MEMS are manufactured using normal silicon processing
steps, some of which are not necessarily compatible with radiation immunity. Most
commercial-off-the-shelf devices are well known for being radiation intolerant
because they do not use the special processing techniques required for obtaining
radiation hardened devices. For instance, hardening a device to TID requires the
avoidance of both high-temperature anneals and exposure to gases such as hydro-
gen. However, using an approach termed ‘‘hardening by design’’ makes it possible
to harden commercial processes to both TID and SEE. 18
One way to mitigate the effects of the trapped charge produced by the radiation
in MEMS is to modify the design to include a grounded conducting plane over the
13
insulator to shield the mechanical part from the effects of the trapped charge. This
approach proved effective for the accelerometer. Another approach is to place the
insulator in a position where charge trapped in it will have no effect on the device,
as in RF switches.
In summary, it should be possible to use special processing and design ap-
proaches to harden MEMS against radiation damage, but so far the only approach
that has been tried and proved to be effective is the avoidance of dielectric layers
between metal electrodes used for applying electric fields to the MEMS.

5.5 CONCLUSION

The performance of MEMS devices in space depends, critically, on the character-
istics of the radiation environment. The environment, in turn, depends on spacecraft
location and time. Characterizing the environment requires knowledge of such
factors as launch date, mission duration and orbit, as well as the amount of shielding
provided by the spacecraft. Fortunately, there are models for solar activity, cosmic
ray intensity, and fluxes in the radiation belts surrounding planets that simplify
calculations of total radiation exposure. By combining the calculated levels of
radiation exposure with the results of ground testing, it will be possible to assess
whether MEMS will meet mission requirements.

© 2006 by Taylor & Francis Group, LLC