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Material Selection for Applications of MEMS 315

temperature. The temperature range in which a device will operate properly will
partially be determined by the coefficient of (linear) thermal expansion (CTE)
or temperature coefficient of expansion (TCE). Typical values are shown in
Table 14.4. MEMS devices with poorly matched coefficients of thermal expansion
will be more sensitive to temperature fluctuations as mismatches will cause bend-
ing. Bending or curling can reduce a sensor’s sensitivity and lessen the strength of
electrostatic actuators. In addition, it can potentially cause stiction, delamination, or
fatigue failures. Since future space missions anticipate temperatures in the range of
100 to 1508C, thermal changes are a growing concern to MEMS designers.
Temperature not only affects the MEMS device, but also how the MEMS device
is packaged. The whole MEMS system must be modeled.
High temperatures also can change the properties of organic materials. Poly-
mers tend to outgas more at high temperatures. The structure of the materials may
also change with elevated temperature. For example, Teflon samples were removed
from the Hubble Space Telescope after astronauts noticed cracking. Upon analysis
it was determined that excessive heating caused an increase in crystallinity, density,
and embrittlement. 20

14.5.4 ATOMIC OXYGEN
The degradation of spacecraft surfaces due to erosion by atomic oxygen (AO) was
discovered during the early Shuttle flights. Surface erosion was seen on ram or
forward-facing surfaces of several types of materials. AO is formed by solar
ultraviolet (UV) radiation, dissociating oxygen molecules (O 2 ) into free oxygen
atoms. Oxygen atoms are highly corrosive to organic materials. In addition, a
spacecraft’s orbital velocity of 7.8 km/sec (17,500 mph) causes the oxygen mol-
ecules to impact the spacecraft with energy of approximately 5 eV, which is high
enough to react with many materials. The reaction can further be enhanced by solar
UV radiation, which energizes molecular bonds and makes the reaction easier. The

TABLE 14.4
Thermal Coefficients of Expansion
Material TCE Microns/8C
Si 2.6, 4.2
Parylene 0.35
7
SiO 2
SiN 0.3
Al 23.6
Au 14.2
Cu 16.6
Polyimide 6
SU-8 52
CMOS dielectric 0.4

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