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324 MEMS and Microstructures in Aerospace Applications

14.7 CONCLUSION

Materials selection is an important consideration when designing and operating
MEMS devices in the space environment. Material properties can greatly affect
device performance. Table 14.9 shows performance indices for various materials.
Specific stiffness is a good metric for high-frequency resonating structures. Specific
strength is a good metric for pressure sensor and valves. Strain tolerance is a good
metric for devices which need to stretch and bend. Table 14.9 also lists thermal and
mechanical properties of various materials used in MEMS; however the reader is
reminded that real world material properties can vary widely. They are useful as a
starting point, but again the material properties of the MEMS materials will vary
based on the fabrication processes used.
The following design features and materials should be avoided:
1. Large temperature coefficient of expansion mismatches, unless designed as a
sense or actuation mechanism
2. Pure tin coatings, except that electrical or electronic device terminals and
leads may be coated with a tin alloy containing not less than 3% lead only
when necessary for solderability
3. Silver
4. Mercury and mercury compounds, cadmium compounds and alloys, zinc and
zinc alloys, magnesium, selenium, tellurium and alloys, and silver which can
sublime unless internal to hermetically sealed devices with leak rates less
than 1 10 4 atm-cm/sec 2
5. Polyvinylchloride
6. Materials subject to reversion
7. Materials that evolve corrosive compounds
8. Materials that sublimate

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2. Yun, H.M. and J.A. DiCarlo, Comparison of the tensile, creep, and rupture strength
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Materials, and Structures, 1999. 20(3): p. 259–272.
3. Ruoff, A.L., On the Ultimate Yield Strength of Solids. 1978. 49(1): p. 197–200.
4. Shackelford, J.F. and W. Alexander, CRC Materials Science and Engineering Hand-
book. 3rd ed. 2001. Boca Raton, FL: CRC Press. 1949 p.
5. Sharpe Jr., W.N., The MEMS handbook. In The Mechanical Engineering Handbook
Series, Gad-el-Hak, M. Editor. 2002. Boca Raton, FL: CRC Press. p. 1v. (various
pagings).
6. Falvo, M.R. and R. Superfine, Mechanics and friction at the nanometer scale. Journal of
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