fys4260/fys9260: microsystems and electronics packaging ... · fys4260/fys9260: microsystems and...

FYS4260/FYS9260: Microsystems and Electronics Packaging and Interconnect

MEMS Packaging

Lecture topics

• Introduction to MEMS packaging concerns: Why MEMS packaging are more challenging than IC packaging

• Case example: MEMS for petroleum flow measurements

FYS4260/FYS9260 2

From another lecture on MEMS packaging:

How realistic is this simplified picture on MEMS packaging? DISCUSS!

FYS4260/FYS9260 Frode Strisland 3

Realistic MEMS packages usuallyinclude:• A sensor or actuator interface to the world

outside• Readout electronics (dedicated ASIC)• Careful mechanical design to avoid package

stress affecting MEMS elements• Must combine MEMS specific features with

footprint compatible with circuit boards


MEMS are not ICs! Parallels to IC production are misleading and canbe dangerously naive

• IC development times ~ 12-18 months– Well established IC processes, design rules and

simulation tools– Competitive wafer costs– Standardized packaging

• MEMS design to product time line 3-5 years– Processes less standardized– Packaging challenges are hughe, and solutions are

considered a proprietary advantage• Packaging cost can exceed 50% or unit cost


Example: Generic pressure sensor package


MEMS specific packaging issues• Some systems must be open towards the

environment– Pressure sensors, microphones, loudspeaker– Chemical and fluidic elements

• Others need to be hermetically sealed, sometimeswith a reference gas and/or an anti stiction agent– Accelerometers/gyroscopes– Absolute pressure sensors (reference cavity)– Oscillators

• Delicate structures• MEMS sensors can sense package induced

stresses, which can cause zero offset, drift and non-linear behaviour


MEMS packaging: No single solution possible


Main package technologies for MEMS

• Plastic• Metal• Ceramic


Metal package• Metal package is robust and easy to

assemble – good choice for prototypes• Standard TO-type (transistor outline)• Can only accommodate fewer than 10 pins.• TO-type package remain in use in a few

applications• Metal packages are attractive to MEMS,

especially for microfluidic devices• A metal hermetic package is often made of

ASTM F-15 (Kovar® ), stainless steel is also common

• Holes are punched, either through the bottom for plug-in packages, or sides for flat packages

• Metal leads are placed through the holes and beads of borosilicate glass


Sensiron STD 60A series pressure sensors

Molded plastic package


• Not hermetic• Two general approaches

• postmolding• premolding

• – In postmolding, the plastic housing is molded after the die is attached top a “lead frame” (a supporting metal sheet). The process subjects the die and the wire bonds to the harsh molding environment.

• – In premolding, the die is attached to a lead frame over which plastic was previously molded. It is attractive in situations where the risk of damaging the die is high, or openings through the plastic are necessary. More expensive.

Ceramic package• Wire bonding establish

electrical connectivity between the die and the metal traces on the ceramic header

• A Cu-Al brazed Kovar® sealring

• Transparent window consists of a polished Corning 7056 glass fused to a stamped gold-nickel-plated Kovar® frame

• Resistance seam welding of the seal ring on the ceramic base to the Kovar® glass frame

• Zeolite getters to ensure long-term desiccation


MEMS vs IC design process

• IC: Design IC, offer IC in a range ofpackages

• MEMS: Co-design MEMS element, readoutelectronics and package, only offer a dedicated packaged product


Wafer level packaging

• Wafer-level packaging (WLP) is the technology of packaging an integrated device while still part of the wafer.

• From a MEMS perspective, it is a question about which functions can be integrated on a wafer level, and what has to be done on a dice level.

• Can for example a vacuum cavity or smart connector solution for monolithic integration of MEMS and ASIC be realized on a wafer level in order to simplify subsequent packaging?


Wafer bonding: Joining wafers

• Anodic Bonding• Direct Wafer Bonding

– Fusion Bonding– Plasma activated bonding

• Metallurgical methods for bonding

Anodic bonding

Anodic bonding is a wafer bonding process to hermetically seal glass to either silicon or metal without introducing an intermediate layer. This bonding technique is also known as field assisted bonding or electrostatic sealing


Example of chip made from glass-silicon-glass anodicbonding

Anodic bonding geometry

In anodic bonding, an electrical field is applied to cause ion drift in the(borosilicate) glass which in turn causesbonding


Scheme of anodic bonding procedure. The top tool works as a cathode and the chunk as anode. The process parameter are bond voltage UB, current limitation IB and bond temperature TB.

Source: http://en.wikipedia.org/wiki/Anodic_bonding

Anodic bonding process


Ion drifting in bond glass influenced by electrostatic field.[8] (1) Formation of depletion zone (gray) through Na+ drifting. (2) Drift of O− ions in the depletion zone.

Source: http://en.wikipedia.org/wiki/Anodic_bonding

Direct wafer bonding:(Fusion bonding)• Direct bonding describes a wafer bonding

process without any additional intermediate layers. The bonding process is based on chemical bonds between two surfaces.

• Important that joined surfaces have low roughness level (at atomical levels e.g. 3-5 Å) and are planar


Metallurgical approaches to waferbondingSoldering/bracing approaches can be used to join wafers. The most common example is theuse of deposited gold on wafers, following by an annealing step.The minimum in the liquidus line in the Au-Si phase diagram is at 363 C for 19 at % Si.


Advanced packaging:3D packaging with thru silicon vias


TSVs used by stacked DRAM-dice in combination with a High Bandwidth Memory interface.Courtesy Shmuel Csaba Otto Traian under CC-BY-SA4.0

Advanced packaging:Highly integrated MEMS components

• 9 axis Inertial MeasurementUnit from InvenSense withdigital output– 3 axis accelerometer– 3 axis gyroscope– 3 axis magnetometer– Signal processing

• Control of package stresses very challenging in smalldimensions


Advanced packaging:Monolithic integrationMonolithic 3D ICs are built in layers on a single semiconductor wafer, which is then diced into 3D ICs. There is only one substrate, hence no need for aligning, thinning, bonding, or through-silicon vias.


Cross section of a pixel consisting of CMOS backplane and MEMS mirror (16 µm pitch).From: http://www.ipms.fraunhofer.de/en/mems-foundry/micromechanics/technology-modules/monolithic_integration.html

http://www.ipms.fraunhofer.de/en/mems-foundry/micromechanics/technology-modules/monolithic_integration.html

Intro to case: Downhole petroleum flow measurements

Motivation: Knowand controlpetroleum flowfrom individualzones in branchedoil wells


Application specific constraints

• High temperature (175 deg C)• High pressure (up to 1000 bar)• Hostile environment (highly corrosive)• Inaccessible• Relatively high cost acceptable


How can we measure fluid flow in a pipe?

For example:• Rotary sensors (measure

propeller spin frequency) • Heat transfer sensors (thermal

mass flow sensor)• Velocity measurements, e.g. by

using Dopper-based methods. • Pressure change through a flow

constriction (venturi)


Bernoulli flow sensorBernoulli equation for incompressible flow:

In constrictions, the speed goes up, and thepressure goes down.


p1 p2

22222

21111 2

121 vVVpvVVp ρρ +=+

2211 vAvA =

)1(21

2

2

12112

−=−=∆

AAvppp ρ

Measurement geometry


Pressure inlets

dP

Sensing principle for MEMS element?

• Piezoresitive?• Capacitive?


30

Recall: What is piezoresistivity?• Some materials change

electrical conductivigy whenstretched due to change in shape (metals) and/or deformations in their crystallattice

• Silicon has a large resistancechange as a function ofmechanical tension

• The resistance change is oftenmeasured using a Wheastonebridge configuration

Piezoresistive appraoch


Torsion sensitive axis

Silicon element principle used – WHY?


Dicsussion points:

• Can you see what type of microstructuringtechnology has been used?

• Where are the pressure ports• HTASIC• Is a reference capacitor actually needed?• Can you see what type of wafer bonding

technique is used?


Silicon element principleCould we drop a silicon reference capacitorand do as follows?


• Yes! Would give a smaller sensor element. However, would need:• External reference to compare with (which would then not be

affected by changes in the same way as te measurementmembrane)

• Package stress more likely to affect package

Sensor terms

• Accuracy (nøyaktighet): How close a reading is to a standardized reference

• Sensitivity (følsomhet): What are the smallest changespossible to detect?

• Zero-point offset: What is the reading when it wassupposed to be zero signal

• Drift: Change with time or other parameter• Hysteresis: Change in reading between + - and - +• Linearity: To what extent the sensor output is a constant

times the measured parameter


How do we connect pressureconnectors to the silicon element?


A tube! Whatmaterial, and howcan we attach it?

"Plumbing": Attach a tube to onepressure inlet


Adhesion layer, for example NiCr

High temperature solder, for example 95Pb5Sn

Adhesion layer

Kovar (low CTE nickel–iron alloy) tube

The realized element connected to a Kovar tube

And now – how can we connect to this electrically?


Electrical connection: Wirebonding

What wire metal should we use? (Pads are in aluminium)


Wirebonding wire: Aluminium. But decision alsodepends on terminals outside the element

The sensor element as we see it will be exposed to several hundredbars. Electronics should not be exposed to more than a few bar. How can we work around this?


We need an electrical feed-thru!

The sensor element will be in an environment of ~700 bar, hot petroleum and saline water. Is that ok?


SteelGlass

Plated Kovar

Low pressureelectronics

compartment

No! We need to protect the sensor element from the hostile environments

The sensor element can be exposed to silicone oil, which is relativelyincompressible and does not affect electronics. Still, we needseparation membranes


The solution

Can you figure out why the two membranes areof different size?FYS4260/FYS9260 Frode Strisland 43

Feedthru

Sensor element

Kovar tube

Separation membranes

Why membranes need to be different size: Thermal expansion effects• CTE of steel ~ 10 ppm/K• CTE of silicone oil ~ 100

ppm/K• Want to allow

membranes to be slightlybent outwards all thetime.

• Why should themembranes be bent outward and not inward?


"Large" volume

"Small" volume

Measurement results

• Sensititity better than 1 mbar

• Temperature drift less than<2.5% FSO

• Common mode pressuredrift < 2.5 % FSO

• Can withstand more than15 bar static differentialpressure

-5

-2,5

0

2,5

5

0 200 400 600 800 1000

Absolute pressure, [Bar]Rat

io C

mem

/Cre

f [%

FSO

]

-5

-2,5

0

2,5

5

0 50 100 150 200

Temperature [C]Ratio

C m

em /

Cref

[FSO

]

Test of differentialpressure


y = 1,1027x - 79,811

0

50

100

150

200

250

300

350

400

450

0 50 100 150 200 250 300 350 400 450 500

Dp

[m

Bar

]

Mea

sure

ddi

ffere

ntia

lpre

ssur

e

Estimated differential pressure

Final implementation• Several primary sensor collected in a

commen housing:– Absolute pressure– Differential pressure– Temperature– Capacitance– Acoustic sensor

• All signals transferred on a common"local bus".

Learning points• MEMS packaging often more demanding

than IC packaging:– IC packaging standardized vs MEMS packaging

solution customised and can be a competitiveadvantage

• Development of MEMS element, readoutelectronics and packaging cannot be done independently– Numerous examples of fancy MEMS elements

that turned out to be impossible to package• Knowledge on MEMS packaging is valuable:

– The cost of packaging often much more than the sensor element cost


END OF LECTURE

Any questions?

This presentation is made for FYS4260/FYS9260 teaching purposes, and is not intended for publication elsewhere.

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