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310 MEMS and Microstructures in Aerospace Applications

14.1 INTRODUCTION

Microelectromechanical systems (MEMS) were born out of the integrated circuit
revolution of the 1960s, 1970s, and 1980s, and as such share much of the same
fabrication technology and materials. While many of these materials are space
qualified, and have been used in space electronics for decades, new issues must
be accounted for when using them in sensors and actuators. When choosing which
materials to use in a MEMS device, it is important to look at the device as a system
of materials. The fabrication processes used to make the device, as well as their
intended use will control the material selection. These selections will in turn affect
the final performance of the MEMS device.
This chapter discusses some of the issues involved in selecting MEMS materials
for space applications. Myriads materials available for MEMS fabrication prevent
inclusion of all of them in this chapter; however, key materials and their properties
are reviewed here. The reader is referred to the ever growing literature for materials
that are not discussed.

14.2 SCALING LAWS
When dealing with objects on the microscale it is first useful to understand how
physical phenomena here can differ from the macro world we are all accustomed to
living in everyday. If we let l represent a linear dimension of an object, and then make
the object 1000 times smaller, all of the linear dimensions decrease by a factor of
1000. We say this object has been scaled by l/1000. As the size of an object shrinks
2
3
the surface area shrinks as a function of l while volume decreases as l . So our
2
object’s surface area has decreased by (1/1000) ¼ 1/1,000,000 and volume has
3
decreased by (1/1000) ¼ 1/1,000,000,000. Table 14.1 summarizes how physical
phenomenon behaves and changes as dimensions scale. While these scaling laws are
important when designing MEMS devices, they also play a role in material selection.
One of the important outcomes is a general increase in material strength. For
2
1
example, single-crystal silicon whiskers and SiC fibers may be an order of magni-
tude stronger than their bulk counterparts. 3,4 The thin film materials commonly used
5
in MEMS devices are typically stronger than their bulk values. As size is further and
further decreased down to the nanoscale, material properties approach their ideal
values. Macroscale and microscale materials have defects and dislocations that can
severely compromise their mechanical performance. Nanoscale materials are capable
of becoming perfect with no defects or dislocations and consequently tend to have
extraordinary properties. Carbon nanotubes for instance have a Young’s modulus of
1.28 TPa and are capable of strains exceeding 15%. 6
Since the surface area to volume ratio increases as dimensions shrink, surface
effects become dominant. Van der Waals, surface tension, and frictional forces
increase. Heat dissipation increases as thermal isolation becomes difficult, and
cooling improves. Note that MEMS devices exposed to the vacuum of space have
much lower heat dissipation due to the lack of convective heat transfer in space. The
thin films typically used in MEMS devices behave differently from their bulk

© 2006 by Taylor & Francis Group, LLC