Making Sense of MEMS and ASIC
Monolithic Integration
> Iain Rutherford X-FAB MEMS Foundry
What is Monolithic Integration?
> Monolithic integration of CMOS and MEMS means make both types of
devices on the same piece of silicon
X-FAB 0.35um Integrated Pressure Sensor
Open-Platform Technology
> Can be MEMS built on top of CMOS
> Can be CMOS built on top of MEMS
> Can be MEMS build in the middle of the CMOS process
CMOS / MEMS Integration
Integration of CMOS & MEMS has key advantages
Discrete Sensor and ASIC
> Performance of end device can be improved
Lower parasitic resistance / capacitance
Lower noise, lower power
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Area of silicon required can be reduced
Size of packaging can be reduced
Complexity of packaging is reduced
Subsequent manufacturing processes can be
simplified
Pick & place
Wire-bonding
SMT / soldering
> But there are implications
Integrated Sensor and ASIC
Implications of Monolithic Integration
> Complexity of process / development
> Yield aggregation
> Cost of silicon
Implications of Monolithic Integration
> Complexity of process / development
If MEMS layers are added to CMOS layers
MEMS process must not affect CMOS parameters
Thermal budget, mobile ion contamination
Stress of MEMS layers needs to be controlled in CMOS environment
Wafer bow, delamination
> Yield aggregation
Every layer added has a potential yield loss
MEMS process associated with lower yield than CMOS
With integrated process, yield loss affects both MEMS and CMOS
> CMOS friendly techniques exist and MEMS yield can be brought
manufactured with CMOS-like yields at higher volumes
> The cost of silicon is harder to deal with
Cost and Size Implications of
CMOS/MEMS Integration
> Integrating ASIC + MEMS can result in higher cost per die
> Even if the integrated wafer price is less than the total of the separate
CMOS and MEMS wafer prices
WaferSize(mm)
DieSize(mm)
DieArea(sqmm)
DieperWafer
PriceperWafer
PriceperDie
TotalPriceofSilicon/Unit
DiscreteASIC
200
2.1mmx2.1mm
4.4
6520
700
0.11
DiscreteMEMS
150
1.8mmx1.8mm
3.3
4450
400
0.09
0.20
IntegratedASICandMEMS
200
2.5mmx2.5mm
6.25
4300
1000
0.23
0.23
> This is because making two small die on two separate wafers is
cheaper than making one large die on a single wafer.
Effect of Die Size on Price
> Die size has huge impact on die
per wafer and so cost per die
> Better to half the die size than
half the wafer price
Price
Size
Die/Wafer Price/Die
$1,000
3x3mm
3149
$0.32
$500
3x3mm
3149
$0.16
$1,000
1.5x1.5mm
12893
$0.08
So Why Integrate?
> Price per die is critical, right? Especially in consumer applications
> But price per silicon die is only part of the story
> The real critical parameter is price per device, packaged and placed in
the final product
If the silicon is cheap but the packaging is expensive then no gain
If the device costs more to place on the board then no gain
> Small form factor of the finished device and standard, automated
assembly is increasingly important
> Higher performance can add value higher price point
Cost and Size Implications of
CMOS/MEMS Integration
> For MEMS & ASICs of similar size, total area of silicon used is smaller
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when monolithically integrated
Where the MEMS or the ASIC is much smaller compared to the other
device, the total area of silicon used actually slightly increases.
Where the MEMS or the ASIC is very, very small compared to the
other device, the total area of silicon used is about the same.
This assumes the simple case
that no area of the CMOS
overlaps the area of the
MEMS except for the pads
> But this does not take into account the packaging
Packaging of CMOS / MEMS
Integrated Devices
> Packaging adds more area than silicon
+
> Packaged area is at least 4 or 5 times larger
> Two chip solution will always add area due
to bond pads
> e.g. silicon area reduced by 18% with
>
monolithic integration but packaged space
would be reduced by 25%
Cheaper & simpler
Performance Improvement
of Integrated CMOS and MEMS
> Reduction of parasitic resistors and capacitors in amplification circuits
> Avoid parasitic capacitance and inductance from bond pads and bond
wires
> Lower noise - Improved detection sensitivity
> Lower power consumption
> Important parameters for consumer devices
Future of Monolithic Integration
> Using existing silicon real estate
CMOS on top of buried MEMS
CMOS on the normally bare silicon
hinterland around MEMS membranes
& structures
> Using existing CMOS layers to make
MEMS
Especially if the MEMS structure is very
small
Stress
> Perhaps MEMS integrated into CMOS
devices will be essential
> Consider the following
X-FAB CMOS Integrated Gyroscope
Adapted from Open-Platform Technology
The Trillion Sensors Roadmap
> There is a lot of interesting discussion about a future market of a trillion
sensor units a year or more.
> 1 Trillion = 142 Sensors for every man, woman and childevery year
> S.I.M.S.
> To make this happen sensors need to be cheap
Make MEMS/sensors smaller
Make MEMS/sensors on larger wafers
The Trillion Sensors Roadmap
Imagine a sensor
0.25mm x 0.25mm
on a 300mm wafer
Some numbers
1 Million die per wafer
7,000 Wafersper year
50,000 Wafersper month
0.25mm x 0.25mm
4.2 Days
1 Trillion
300mm Diameter
2 x 300mm Fabs
Integration in X-FAB MEMS Foundry
Increasing demand for integrated CMOS/MEMS
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Up to 2008 demand mainly driven by automotive
Now driven by automotive AND consumer
Expect rapid growth in CMOS/MEMS integration
~30% of all X-FAB MEMS current opportunities are integrated
May relate to X-FABs expertise in analog/mixed signal
High yielding MEMS parts maintains cost effectiveness
Interesting Debate
> Monolithic integration is not for all
> Cons:
Cost - Larger die size
But possibly cheaper at packaging
Cost - Yield aggregation - ACIS Yield x MEMS Yield
MEMS Yield isnt always low
Complexity Development and manufacturing
Using CMOS layers for MEMS can help
> Pros:
Better performance
Smaller form-factor
Simplified subsequent packaging
> As a result monolithic integration is definitely for some
Thank you for your attention.
www.xfab.com
iain.rutherford@xfab.com
