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MEMS Packaging for Space Applications 271

printed circuit board (PCB) or a hybrid-like ceramic substrate and have nothing but
a ‘‘housing’’ to protect it from mechanical damage such as dropping or something as
simple as damage from the operator’s thumb.

12.1.2.2 Traditional — Hermetic and Non-Hermetic
Many elements in the environment can cause corrosion or physical damage to the
metal lines of the MEMS as well as other components in the package. Although
there is little to no moisture in space, moisture remains a concern for MEMS in
space applications since it may be introduced into the package during fabrication
and before sealing. The susceptibility of the MEMS to moisture damage is depen-
dent on the materials used in its manufacture. For example, aluminum lines can
corrode quickly in the presence of moisture, whereas gold lines degrade slowly, if at
all, in moisture. Also, junctions of dissimilar metals can corrode in the presence of
moisture. Moisture is readily absorbed by some materials used in MEMS fabrica-
tion, die attachment, or within the package; this absorption causes swelling, stress,
and possibly delamination.
To minimize these failure mechanisms, MEMS packages for high-reliability
applications may need to be hermetic with the base, sidewalls, and lid constructed
from materials that are good barriers to liquids and gases and do not trap gases
releasing them later.

12.1.2.3 Custom — Vacuum Sealed Hermetic
Some MEMS chips, such as a MEMS accelerometer or MEMS magnetometer,
require vacuum conditions to operate properly. For dual use devices (Earth-ambient
conditions and space conditions), the chips need to be sealed in a package that
contains vacuum conditions. As opposed to a typical hermetic microcircuit package
that contains an inert atmosphere such as nitrogen, gases within the package can
inhibit the movement of the extremely small moveable structures by causing
aerodynamic drag. Selection of packaging materials becomes even more critical
(than standard hermetic packages as described earlier) because even a small amount
of trapped gas can raise the package internal pressure to levels which degrade the
performance of the MEMS chip. In space applications, such devices could be
packaged in a housing that contains a small hole to allow the atmosphere to escape
after launch.

12.1.3 ELECTRICAL CONNECTION TO OTHER SYSTEM COMPONENTS
Because the package is the primary interface between the MEMS and the system, it
must be capable of transferring DC power and in some designs, RF signals. In
addition, the package may be required to distribute the DC and RF power to other
components inside the package. The drive to reduce costs and system size by
integrating more MEMS and other components into a single package increases
the electrical distribution problems as the number of interconnects within the
package increases.

© 2006 by Taylor & Francis Group, LLC