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MEMS Packaging for Space Applications 273

Welding (using lasers to locally heat the joint between the two parts without
raising the temperature of the entire part) is a commonly used alternative to solders.
Regardless of the seal technology, no voids or misalignments can be tolerated since
they can compromise the package hermeticity. Hermeticity can also be affected by
the feedthroughs that are required in metal packages. These feedthroughs are
generally made of glass or ceramic and each method (glass seal or alumina
feedthrough) has its weakness. Glass can crack during handling and thermal cyc-
ling. The conductor exiting through the ceramic feedthrough may not seal properly
due to metallurgical reasons. Generally, these failures are due to processing prob-
lems as the ceramic must be metallized so that the conductor (generally metal) may
be soldered (or brazed) to it.
The metallization process must allow for complete wetting of the conducting
pin to the ceramic. Incomplete wetting can show up as a failure during thermal
cycle testing.

12.2.2 CERAMIC PACKAGES
Ceramic packages have several features that make them especially useful for
microelectronics as well as MEMS. They provide low mass, are easily mass-
produced, and can be low in cost. They can be made hermetic, and can more easily
integrate signal distribution lines and feedthroughs. They can be machined to
perform many different functions. By incorporating multiple layers of ceramics
and interconnect lines, electrical performance of the package can be tailored to meet
design requirements. These types of packages are generally referred to as co-fired
multilayer ceramic packages. Details of the co-fired process are outlined below.
Multilayer ceramic packages also allow reduced size and cost of the total system by
integrating multiple MEMS and/or other components into a single, hermetic pack-
age. These multilayer packages offer significant size and mass reduction over
metal-walled packages. Most of that advantage is derived by the use of three
dimensions instead of two for interconnect lines.
Co-fired ceramic packages are constructed from individual pieces of ceramic in
the ‘‘green’’ or unfired state. These materials are thin, pliable films. During a typical
process, the films are stretched across a frame in a way similar to that used by an
artist to stretch a canvas across a frame. On each layer, metal lines are deposited
using thick-film processing (usually screen printing), and via holes for interlayer
interconnects are drilled or punched. After all of the layers have been fabricated, the
unfired pieces are stacked and aligned using registration holes and laminated
together. Finally, the part is fired at a high temperature. MEMS and possibly
other components are then attached into place (usually organically [epoxy] or
metallurgically [solders]), and wire bonds are made the same as those used for
metal packages.
Several problems can affect the reliability of this package type. First, the green-
state ceramic shrinks during the firing step. The amount of shrinkage is dependent
on the number and position of via holes and wells cut into each layer. Therefore,
different layers may shrink more than others creating stress in the final package.

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