Page 319 - MEMS and Microstructures in Aerospace Applications

Osiander / MEMS and microstructures in Aerospace applications DK3181_c014 Final Proof page 313 1.9.2005 12:47pm

Material Selection for Applications of MEMS 313

with mismatched coefficients of thermal expansion or temperature-induced
stresses. Delaminating materials can cause shorting, stiction, and mechanical im-
pedance failures. Altering the mass and composition of the structures can affect
the designed performance of the devices such as displacement and resonant
frequencies.

14.4.3 FATIGUE
Fatigue is caused by the cyclic loading of a structure below the yield or fracture
stress of the material. This can cause microcracks to form, which, over time can
lead to localized plastic deformations, weakening, and ultimately failure of the
material. Ductile materials like metals tend to exhibit fatigue more than brittle
materials such as silicon.

14.4.4 WEAR
Wear is caused by the motion of one surface over another. This motion causes
material to be removed from the surfaces. There are four types of wear: adhesion,
abrasion, corrosion, and surface fatigue. Adhesive, abrasive, and surface fatigue are
forms of wear that most moving MEMS devices in contact with another surface
encounter. Hard materials such as silicon carbide or diamond are used to reduce this
failure mechanism. Exploratory missions may cause MEMS devices to be exposed
to chemical environments, which can cause chemical interactions at the device
surfaces. The movement of the MEMS device can strip away the reaction products
of surfaces leading to more corrosion.

14.5 ENVIRONMENTAL CONSIDERATIONS

Typical space applications expose devices to harsh environmental conditions.
Radiation, extreme temperatures, pressures, shock, vibrations, thermal cycles,
corrosive atmospheres, dust, and fluid environments are environmental consider-
ations that should be addressed. Table 14.2 shows how some planetary conditions
compare to the Earth’s environment. Packaging may address some of these issues in
part, but may not be sufficient to protect the MEMS devices completely. In addition
some MEMS devices must be exposed to the environment in order to function,
which may require specific materials to be used for device survival.

14.5.1 VIBRATION
Vibrations are typically low acceleration, long duration events. They have been
shown not to be a large reliability concern in MEMS. Long-term vibrations can
contribute to fatigue failures, however. For space applications, initial vibrations
encountered at launch have been shown to be less than 13 g, as shown in Table 14.3.
Dynamic shocks encountered in space flight and surface landings are more import-
ant considerations.

© 2006 by Taylor & Francis Group, LLC