Evaluation Engineering - 19

MEMS

MEMS TECHNOLOGY
IS TRANSFORMING
HIGH-DENSITY
SWITCH MATRICES

Figure 1:
A Menlo Micro
6-channel
switch in a
surface mount
BGA package.
Menlo Micro

by Chris Giovanniello
Electromechanical relays (EMR) and
reed relays have served automated
test equipment (ATE) applications well,
but they're not likely to satisfy their needs
in the future, as ATE manufacturers strive
to increase channel densities in a smaller
system footprint and as more tests need
to be performed, faster, on devices under
test. The answer lies in MEMS switch
technology that, after years of development, has transitioned to commercialization thanks to advances in materials,
fabrication, and packaging.
There are many reasons why MEMS
technology is so appealing for switching
applications. Consider for a moment that
hundreds of MEMS switch contacts can
be fit in a space smaller than a single EMR
or reed relay, and a MEMS switch matrix
housed in a small surface-mount package
can replace multiple EMRs (Figure 1).
This alone would represent a dramatic
breakthrough for high-density switching applications, but as this article will
demonstrate, size alone is only one major
benefit of this technology.

What took so long?
So, why is it that MEMS switches are just
now appearing? It's not for lack of trying, as many companies worked feverishly
to develop MEMS switches for decades.
Most fell by the wayside for lack of funding, acceptable fabrication processes,
access to advanced materials, and the
inability to solve crucial problems such
as contact reliability, actuator deformation, and effective hermetic packaging.

The solution to these and other obstacles came from General Electric, which-
beginning in 2004-took on the challenge
of finding an alternative to the mechanical relays it relied on for use as high-power
circuit breakers. Of the two basic types of
MEMS switch technology, capacitive and
ohmic, GE focused on ohmic. Other companies, principally Analog Devices, also
use ohmic technology, while companies
like Cavendish Kinetics and Wispry use
the capacitive approach.
The primary failure mechanisms in
ohmic MEMS switches are metal fatigue
of the MEMS actuator (cantilever) and
contact wear. Even though metals are
excellent conductors, when used as cantilevers they deform over time and exhibit
variations in temperature. After researching many materials combinations, GE realized and patented a fabrication process
and electrodeposited alloys
whose mechanical properties are very close to silicon
and have the conductivity
of a metal. The packaging
problem was solved in collaboration with Corning,
which had been developing
Through Glass Via (TGV)
technology (Figure 2).

By eliminating wire bonds and replacing them with short metallized vias, TGV
reduced package parasitics by more than
75%, allowing the switches to be housed
in wafer-scale packages 60% smaller
than wire bond packages. In addition to
increasing channel density and reducing
cost, TGV also dramatically decreased
electrical losses while increasing linearity-key metrics for ATE applications
from low-power signal switching to RF
and microwave testing.
The advances proved successful in its
first applications: GE's MRI and FRMI
medical imaging systems and hundreds of
thousands of these switches have been delivered into those systems as of today. In
2016, GE spun off its MEMS capabilities,
creating Menlo Microsystems, Inc. (Menlo
Micro), which has continued to advance
this work. The result today is what the

Figure 2: Through-glass-via
packaging has significant
advantages over other techniques
including size reduction and
performance gains.
Menlo Micro
JULY 2019 EVALUATIONENGINEERING.COM

http://www.EVALUATIONENGINEERING.COM

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