Fan-in WLCSP matures, what's next? IMT on the role ... - I-Micronews

F E B R U A R Y 2 0 1 0 i s s u e n ° 1 4
N e w s l e t t e r o n 3 D I C , T S V , W L P & E m b e d d e d T e c h n o l o g i e s
C O M P A N Y V I S I O N
<strong>IMT</strong> on the role of Wafer-Level Packag<strong>in</strong>g <strong>in</strong> MEMS
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In the future, WLP will play a greater role <strong>in</strong> the
<strong>in</strong>tegration of heterogeneous technologies. Right
now, we’re see<strong>in</strong>g die-level MEMS and CMOS
devices wire bonded and encapsulated. We also see
build<strong>in</strong>g MEMS directly on CMOS wafers. Not too
far off, we expect to see completed MEMS wafers
or wafer stacks bonded to completed CMOS wafers.
IR emitter/gas sensor is hermetically packaged at wafer
level with sub-mTorr vacuum. As a technology platform
<strong>IMT</strong> <strong>in</strong>tegrates wafer-level packag<strong>in</strong>g to improve
performance and reliability, and reduce backend cost.
Image courtesy of ICx Precision Photonics.
YD: What key materials, equipment, and
technologies still need to developed for MEMS
wafer-level packag<strong>in</strong>g?
CT: There isn’t a simple answer to this question.
<strong>IMT</strong> has been <strong>in</strong>corporat<strong>in</strong>g WLP technology
<strong>in</strong>to our customers’ products for a long time. Our
standard solution already fulfills and exceeds the
requirements of the applications it serves. We
believe all of the build<strong>in</strong>g blocks are already out
there. There are, however, specific applications that
require additional R&D to better understand how
those blocks fit together. For those applications,
WLP has to be customized to meet a number of
variables, such as vacuum level, thermal budget,
and cost.
YD: What is <strong>IMT</strong>’s approach to MEMS waferlevel
packag<strong>in</strong>g?
CT: <strong>IMT</strong> began WLP development <strong>in</strong> 2002, and
manufactured its first product us<strong>in</strong>g WLP <strong>in</strong> 2003.
We traveled some roads that led to dead ends,
but my po<strong>in</strong>t is that WLP is not new to <strong>IMT</strong>. We’ve
learned what works and, just as important, what
doesn’t. Through the years, we’ve developed proven
approaches and technologies used specifically for
WLP applications.
<strong>IMT</strong> now makes hermetic WLP us<strong>in</strong>g glass frit,
eutectic, anodic, Au-Au thermocompression, and
silicon fusion bond<strong>in</strong>g. We tailor the use of these
depend<strong>in</strong>g upon the application.
As device and wafer <strong>in</strong>tegration <strong>in</strong>creases, we’re
keen to recognize temperature sensitivity to
mechanical structures or material stresses that can
occur from the results of additional high-temperature
bond<strong>in</strong>g. For this reason, <strong>IMT</strong> developed a
proprietary metal alloy bond that can be used to
create a hermetic seal at a temperature less than
200°C. The bond l<strong>in</strong>e width is reduced compared to
other techniques, provid<strong>in</strong>g more usable space for
the MEMS. This can be as big a benefit as the lowtemperature
feature of the bond. This bond is well
proven and has been used <strong>in</strong> many programs with
product yields exceed<strong>in</strong>g 99%.
Along with our diverse list of bond<strong>in</strong>g options, we
believe that our low-temperature bond sets us apart
<strong>in</strong> the market today.
YD: How is <strong>IMT</strong> ‘WLP-specific’ compared to
other MEMS foundries?
CT: Like any pure-play foundry, <strong>IMT</strong> is very
fortunate to get great perspective on the market
because we’re like the ‘Switzerland’ of the MEMS
space—we’re exposed to and service multitudes
of customers across many diverse markets. In the
past, technology requirements for creat<strong>in</strong>g products
were as separate as the markets themselves.
This isn’t the case anymore. We now see that
technology requirements are converg<strong>in</strong>g and <strong>IMT</strong>
f<strong>in</strong>ds it commonplace to <strong>in</strong>tegrate what were once
disparate process technologies and modules <strong>in</strong>to
s<strong>in</strong>gle devices at wafer level.
<strong>IMT</strong> has developed some of the most advanced
processes that have resulted <strong>in</strong> manufactur<strong>in</strong>g
some of the most lead<strong>in</strong>g-edge MEMS devices
on the market today. We’ve taken the <strong>next</strong> step to
<strong>in</strong>tegrate these processes and materials <strong>in</strong>to s<strong>in</strong>gle,
unique devices. Today, <strong>IMT</strong> is manufactur<strong>in</strong>g a
device that <strong>in</strong>tegrates reflexive and refractive optics,
3D microfluidics, and high-speed electromagnetic
actuation us<strong>in</strong>g numerous materials <strong>in</strong> a fourwafer
stack, mak<strong>in</strong>g what we believe is the most
sophisticated device ever built us<strong>in</strong>g WLP. We like
to th<strong>in</strong>k that <strong>IMT</strong> separates itself by blaz<strong>in</strong>g the trail
<strong>in</strong> complex <strong>in</strong>tegration of process and technologies
through WLP.
YD: What’s <strong>IMT</strong>’s stance on us<strong>in</strong>g 3D TSV <strong>in</strong> MEMS?
CT: Of course we’re ‘bullish.’ We’re already build<strong>in</strong>g
products with nearly 140,000 hermetic copper
TSVs per wafer. There’s no argument that further
m<strong>in</strong>iaturization of products will require TSVs.
Convenient consequences of us<strong>in</strong>g TSVs are that
they reduce rout<strong>in</strong>g complexity and device footpr<strong>in</strong>t,
which can ultimately lead to lower-cost products.
Copper TSVs can dramatically improve electrical
performance-specifically required <strong>in</strong> the RF world. As
an example, <strong>IMT</strong>’s copper TSVs offer a DC resistance
of less than 0.1 Ohms per via, with an <strong>in</strong>sertion loss of
-0.1dB at 6GHz.
Fill<strong>in</strong>g metal TSVs with higher aspect ratios and
achiev<strong>in</strong>g good yields has been a challenge for our
<strong>in</strong>dustry. <strong>IMT</strong> has solved the problem and shipped
hundreds of wafers with a fully characterized 15µm x
60µm hermetic copper TSVs. We plan to announce a
larger TSV soon, which we’re currently characteriz<strong>in</strong>g.
YD: Where do you see wafer-level packag<strong>in</strong>g for
IR sensors head<strong>in</strong>g?
CT: IR sensors are used across commercial
applications for gases <strong>in</strong> biomedical and <strong>in</strong>dustrial
safety, but are also used by the military. It’s an
application <strong>in</strong> which size and weight of the emitter
or sensor are critical. S<strong>in</strong>ce grams and millimeters
count, we feel that WLP represents a great
opportunity for product enhancement. But there are
two considerations: The product must be packaged
<strong>in</strong> high vacuum to m<strong>in</strong>imize heat loss; and due to
the characteristics of VOx, which is the prevalent
material for microbolometers, the bond must be
made at low temperature. While either requirement
on its own isn’t an issue, comb<strong>in</strong><strong>in</strong>g the two for WLP
creates a challenge.
This is where <strong>IMT</strong> can help. We’ve been mass
produc<strong>in</strong>g an IR emitter and gas sensor for
several years, which is a three-wafer-bonded
stack packaged <strong>in</strong> a vacuum of less than 1mTorr.
Leverag<strong>in</strong>g this process and comb<strong>in</strong><strong>in</strong>g it with
our low-temperature hermetic alloy bond, <strong>IMT</strong> is
positioned to help move microbolometer-based
IR sensors <strong>in</strong>to WLP, reduc<strong>in</strong>g weight, size, and
ultimately, cost. This is simply an evolution of
<strong>in</strong>ternal development that we will be talk<strong>in</strong>g about
soon. Stay tuned!
X-ray tomography of a copper TSV wafer, show<strong>in</strong>g
plated metal vias without voids.
Craig Trautman, Vice President of Bus<strong>in</strong>ess
Development at Innovative Micro Technology
www.imtmems.com
Craig Trautman, Vice President
of Bus<strong>in</strong>ess Development at
Innovative Micro Technology
Trautman has more than 25
years’ experience work<strong>in</strong>g <strong>in</strong>
eng<strong>in</strong>eer<strong>in</strong>g, market<strong>in</strong>g, and
bus<strong>in</strong>ess development <strong>in</strong> the semiconductor
and MEMS <strong>in</strong>dustries. He holds B.S. degrees <strong>in</strong>
Electrical Eng<strong>in</strong>eer<strong>in</strong>g and Computer Eng<strong>in</strong>eer<strong>in</strong>g
from the University of Missouri.
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