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Space Radiation Effects and Microelectromechanical Systems 93

Where no test data exist, radiation testing of parts identical to those intended for
space is the next step. The parts chosen for testing should have the same date and lot
codes as those selected for the mission because it is well known that performance
degradation during and following exposure to radiation is very device- and process-
dependent. Ground testing involves the use of particle (proton or heavy ion)
accelerators for SEE and displacement damage testing and radioactive sources
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(Co ) or x-rays for total ionizing dose testing. 7,8 The kinds of degradation are
identified and their dependence on particle fluence and deposited energy measured
to quantify the degradation. That information is then used to predict the operation of
the device in the charged particle environment of interest.
Finally, subsystem and system-level analyses must be undertaken to determine
how the specific device degradation affects the overall spacecraft performance.
Some radiation-induced effects may have no adverse effects on the system, whereas
others may cause system failures. In those cases where the effects are pernicious,
one can adopt any one of a host of measures that have been used successfully to
mitigate them. Such measures might include the use of ‘‘cold spares’’ or extra
shielding for devices that are sensitive to TID, or protecting data with ‘‘error-
detecting-and-correcting’’ codes in devices found to be SEE sensitive. When such
measures are not possible, the device should be discarded and an alternate one used
in its place.

5.2.1 SPACE RADIATION INTERACTION WITH MATERIALS AND
DEVICES (IONIZATION)

This section deals primarily with radiation damage by charged particles, including
electrons, protons, and heavy ions (Z > 2). Most of the investigations of radiation
damage have been in electronic, opto-electronic, and optical devices. Those results
will be applied to the case of radiation damage in MEMS.
The first step is to investigate the interactions between incoming charged
particles and the materials (metals, dielectrics, and semiconductors) used in the
manufacture of MEMS. This requires knowledge of the particles’ masses and
energy distributions as well as of the properties, species, and density of the
materials through which they pass.
When radiation interacts with materials it liberates bound charge, breaks chem-
ical bonds, and displaces atoms from their equilibrium positions. These effects have
been investigated for a long time and are quite well understood. Mechanical
properties, such as density, brittleness or stress, are largely unaffected by the
typical particle fluences encountered in space, and are ignored. In contrast, electrical
properties of materials are greatly affected by radiation. Charge generation and
displacement of atoms are known to alter the electrical properties of materials to
such an extent that the performance of devices, such as transistors, may become
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severely degraded. Studies of charged particle interaction with various materials
will be used to draw general conclusions concerning radiation effects in MEMS.
Charged particles traveling through matter scatter off atoms, losing energy and
slowing down in the process. The primary interaction involves Coulomb scattering

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