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272 MEMS and Microstructures in Aerospace Applications

When designs also require high-frequency RF signals, the signals can be
introduced into the package along metal lines passing through the package walls,
or they may be electromagnetically coupled into the package through apertures in
the package walls. Ideally, RF energy is coupled between the system and the
MEMS without any loss in power, but in practice, this is not possible since perfect
conductors and insulators are not available. In addition, power may be lost to
radiation, by reflection from components that are not impedance matched, or
from discontinuities in the transmission lines. The final connection between the
MEMS and the DC and RF lines is usually made with wire bonds; although flip-
chip die attachment and multilayer interconnects using thin dielectric may also be
possible.

12.2 TYPES OF MEMS PACKAGES
Each MEMS application usually requires a new package design to optimize its
performance or to meet the needs of the system. It is possible to loosely group
packages into several categories. Four of these categories are: (1) metal packages,
(2) ceramic packages, (3) thin-film multilayer packages, and (4) plastic packages
are presented below.

12.2.1 METAL PACKAGES

Metal packages are often used for microwave multichip modules and hybrid
circuits because they provide excellent thermal dissipation and excellent
electromagnetic shielding. They can have a large internal volume while still main-
taining mechanical reliability. The package can either use an integrated base and
sidewalls with a lid, or it can have a separate base, sidewalls, and lid. Inside the
package, ceramic substrates or chip carriers are required for use with the feed-
throughs.
The selection of the proper metal can be critical. CuW (10/90), Silvar 1 (a Ni–
Fe alloy) (Semiconductor Packaging Materials, Armonk, NY), CuMo (15/85), and
CuW (15/85) all have good thermal conductivity and a higher CTE than silicon,
which makes them good choices. Kovar 1 (ESPI, Ashland, OR), a Fe–Ni–Co
alloy is also commonly used. All of these materials, in addition to Alloy-42, may
be used for the sidewalls and lid. Cu, Ag, or Au plating of the packages is
commonly done.
Before final assembly, a bake is usually performed to drive out any trapped gas
or moisture. This reduces the onset of corrosion-related failures. During assembly,
the highest temperature-curing epoxies or solders should be used first and subse-
quent processing temperatures should decrease until the final lid seal is done at the
lowest temperature to avoid later steps from damaging earlier steps. Au–Sn is a
commonly used solder that works well when the two materials to be bonded have
similar CTEs. Au–Sn solder joints of materials with a large CTE mismatch are
susceptible to fatigue failures after temperature cycling. The Au–Sn intermetallics
that form tend to be brittle and can accommodate only low amounts of stress.

© 2006 by Taylor & Francis Group, LLC