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So what is this MEMS based solution?
I have proposed a “MEMS-based system solutions” approach as
a viable vehicle for current MEMS device producers and future
MEMS device producers and to encourage them to “think
outside the chip”. I am not attempting to diminish the
importance of the MEMS device but rather discuss the need of
adopting a systems approach to solving the application which
includes as a core the MEMS device(s) in addition to multiple
electronic circuit devices, interconnects, packaging and
assembly. All of these elements plus testing must be a part of
the system architecture addressed in the initial solution design.
In the annual METRIC workshop in March 2008 sponsored by
the MEMS Industry Group (MIG), this year’s topic was
packaging. The outcome of the panel discussions was that they Image courtsey of University of Michigan, WIMS
considered the inability of the early consideration of packaging Figure 3: Multiple MEMS sensors and fluidic devices work
issues and tradeoffs to pose a major problem in the final with signal conditioning circuitry, associated
MEMS-based solution. Our independent research has also interconnects and hermetic packaging to create an
corroborated this MIG finding. integrated MEMS-based systems solution
A key element in the MEMS-based solution is the signal
conditioning circuit. The output signal of a MEMS device needs percentage of the total cost of the solution. Karen Lightman,
a great deal of enhancement and modification to be useable in a Managing Director of MIG said, “In our industry report, Focus
system. The inclusion of this device has been widely adopted and on MEMS Packaging
[2]
, that was created as a result of our
in different forms. Some companies have decided to create a recent annual METRIC conference, we reported that the cost of
MEMS device with an integrated/monolithic signal conditioning the package for a MEMS device can be 30-40% of the total
circuit e.g. Analog Devices ADXL accelerometer product line cost of the entire device. When testing and packaging are both
and Akustica’s MEMS microphone whereas most companies considered, the consensus of the METRIC’s working groups is
have take a heterogeneous/multi chip approach e.g. Freescale, that the cost may reach 70% of the total device cost. By
Kionix, STMicroelectronics, VTI , accelerometers and Analog implementing and conforming to standard CMOS processes,
Devices, Pulse/Sonion, and Wolfson microphones. companies can reduce cost, as packaging and testing comprise
The decision on which approach to take is a complex process only 10-15% of the total device cost”. In the general
that is driven by non recurring engineering costs, unit costs, microelectronic area, packaging of ICs is more of an issue of
volumes, time to market, packaging/interconnectivity issues to using standard packages, MEMS packaging tends to be more
name a few. The decision as to how to source the signal application specific. The old mantra of “one product/one
conditioning IC is dependent on the internal resources of the process” has been added to by “one product/one package” for
MEMS device developers. Analog Devices and Freescale are MEMS asserting MEMS application specific nature.
major IC suppliers and it is a natural for them to design and “Packaging is critical for product success” says Nancy
manufacture these signal conditioning IC’s. Some companies Stoffel, Director of the Microsystems Packaging at Infotonics.
like Kionix have in-house design capability to design the ICs and “Not only is it a main cost element but it dictates time to
use a foundry to manufacture them. Many companies who lack market and system performance. The package provides
the expertise and resources to have their own in-house signal interconnections for electrical, fluidic, optical and thermal
conditioning team use companies including Austria inputs. It protects the MEMS fragile structure against particles,
Microsystems, Si Ware Systems design the signal conditioning humidity, vibration and stress while providing controlled
ICs and in the case of Austria Microsystems, to also
manufacture the devices in- house. Functions including analogue
to digital conversion, amplification, filtering, comparators,
temperature sensors and E2PROM are popular functions.
It is important to note that embedded software is a very
critical part of the signal conditioning strategy. The judicious
use of this function can create high performance solutions using
additional temperature sensor inputs and E2PROM to optimally
compensate the MEMS device over temperature. In addition,
algorithms can be used to compensate for device manufacturing
echnologies
variances as well as to introduce highly functional solutions that
can deliver functions including self-test, power management,
control logic and ESD protection.
Packaging, interconnects and assembly
Packaging, interconnecting and assembly of the solution Figure 4: MEMS-based system for telecom apps with a
elements is perhaps the most important and critical part of the lower die which supports an array of MEMS switches and
system solution concept. It is well known and accepted the cost has a wafer-level cap. On top of this is a CMOS controller
Image courtsey of Aspen T
of packaging/interconnects, test and assembly are a major die with wire bonding connecting the CMOS to MEMS die.
December 2008 / January 2009
www.micronanosystems.info
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