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84 MEMS and Microstructures in Aerospace Applications

(UV, x-ray and gamma rays) that degrade the electrical, optical, thermal, and mech-
anical properties of materials and coatings are present. Charged particles also pose a
danger to the spacecraft. For instance, plasmas are a significant hazard because they
alter the spacecraft’s electrical ‘‘ground’’ potential through the buildup of charge.
After sufficient exposure, dielectrics may suddenly discharge, damaging sensitive
electronic components in the process. Individual energetic ionized particles such as
electrons, protons, alpha particles, and heavier ions are another hazard. They are able
to penetrate the spacecraft’s superstructure as well as the electronic, opto-electronic,
and microelectromechanical systems (MEMS) devices contained on board. As they
travel through matter, the particles collide with the atoms of the MEMS devices
materials, which in turn, liberate charge and disrupt the lattice structure. Both of these
effects contribute to the performance degradation of devices.
Radiation damage is sometimes gradual and at other times sudden. Gradual
degradation is the result of cumulative radiation exposure, and is caused by total
ionizing dose (TID) or displacement damage dose (DDD) or both. At some dose
level the device may no longer function. For radiation sensitive parts, that dose may
be quite small, whereas for radiation-hardened parts the level may be orders of
higher magnitude. Sudden degradation sometimes occurs following the passage of a
single particle through the device, and usually takes the form of a loss of data,
disruption of normal operation, or even destructive failure. These effects are
collectively known as single-event effects (SEEs) and have been the object of
many investigations over the past two decades.
Over the years, these hazards have been responsible for numerous space
mission failures. From reduced capability to total loss of the spacecraft, the asso-
1
ciated financial losses have been significant. Now that MEMS are being consid-
ered for space applications, particularly in microsatellites, which provide relatively
little shielding against the harsh space environment, it is necessary to study how
MEMS respond to all of the above-mentioned hazards.

5.1.1 THE SPACE RADIATION ENVIRONMENT
The radiation environment in space varies with both time and location, and
models of particle flux include effects of the Sun, local magnetic fields, and galactic
cosmic rays.
Radiation emitted by the Sun dominates the environment throughout the entire
solar system. Solar emissions include both negatively charged electrons and posi-
tively charged ions that span the periodic table from hydrogen to uranium. These
2
particles travel with velocities up to 800 km/sec. Complicated processes that will
not be discussed here are believed to be responsible for the electron emission from
the Sun. To maintain electrical neutrality, the electrons ‘‘drag’’ positively charged
particles with them as they speed away from the Sun. Positively charged solar
emissions consist primarily of protons (85%) and alpha particles (14%), with the
remaining 1% consisting of ions with atomic numbers greater than two. Both the
relative and total numbers of solar particle emissions vary with time, exhibiting
large increases during solar-particle events.

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