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Wafer Level Vacuum Packaging

for Sensors

Wan-Thai Hsu, PhD.

Chief Technology Officer

Discera Inc.

[email protected]

IEEE Sensors 2012

Oct. 28, 2012

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IEEE Sensors Oct 28, 2012 Wan-Thai Hsu, Ph.D. p. 2

Outline

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-

- - -

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MEMS Sensors

The past few decades, micro electro-mechanicalsystems (MEMS) becomes the trend of sensor

designs

MEMS shows great advantages over traditionalsensors in terms of

- Size & cost- Sensitivity- Batch fabrication on wafers- Circuit integration

One of the key factors for successful MEMSproducts is wafer level packaging

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4/61IEEE Sensors Oct 28, 2012 Wan-Thai Hsu, Ph.D. p. 4

Why Is Packaging Important?

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Why Is Packaging Important?

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Bonding Technologies

Bonding

Technologies

Vacuum

Level

Sealing

Width

Temp.

range Note

Thermal compression without intermediate layers

Anodic Bonding 1 Torr Yes 200-400oC

High Na+ content glass wafer is needed, not CMOSfriendly, limited material selections

Si Fusion bonding 0.01~1 Torr Yes > 1000

o

C

Need low TTV, low roughness, clean surface, the

resonator may be affected at bonding temp

Plasma enhanced Sibonding

0.1~1 Torr Yes 200-250oC Need smooth clean surface

Thermal compression with intermediate layers

Epoxy Poor > 500um 150-250oC Out-gassing

Polymers Poor > 500um 150-250oC Out-gassing

Glass frit > 1 Torr ~200um 350-500oC Out-gassing

Au 0.1~1 Torr ~200um 400-450oC Clean surface needed

Eutectic bonding

Au-Sn (80/20) ~1 Torr ~100um 280-310oC

Plated materials need pre-bonding treatment, Au isalso not CMOS friendly

Au-Si ~1 Torr ~100um ~370oC

Plated materials need pre-bonding treatment, Au isalso not CMOS friendly

Al-Ge ~1 Torr ~100um 420~440oC

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Vacuum Level by Applications

Applications Vacuum Level Packaging Technology

High Q

Resonators 0.01~0.1 Torr

Silicon fusion bonding

Epi-Si encapsulation

Metal eutectic with getterGlass frit with getter

Gyroscopes 0.1~1 TorrMetal eutectic

Metal thermal compression

Accelerometers 1~10 TorrMetal eutectic

Metal thermal compression

Pressure

Sensors0.5~1 Torr

Glass frit

Anodic bonding

Silicon fusion

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Outline

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Anodic Bonding 1

Principle: one polished glass wafer and onepolished Si wafer are sandwiched at 400C

A DC voltage of 1000V is applied to move the mobileions to the electrode, leaving O2on Si-glass surface

The oxygen bond the silicon surface, form a verystrong bond than Si-Si.

-+

Vs

Top Electrode

Hot Plate

Glass

Si

200C

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Anodic Bonding 2

Bonding starts on the electrical contacts. The bondfront gradually move outwards

Bond front is sensitive to surface roughness andwill move around the particulates on the surface

Si Wafer

Glass WaferElectricalContacts

Hot Plate

Bond frontSi

Glass Bond Interface

Courtesy: Neil Welch

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Anodic Bonding with Ti Getter

Anodic bonding with Tifilm as getter Use Q of resonators for

vacuum measurement Without getter: 2 Torr With small getter: 0.7 Torr With larger getter: 3 mTorr

Packaging & Wire Bonding

Ti

1E-4

1E-3

0.01

0.1

1

10

With Ti (Area X5)With Ti (Area X1)Anodic bonding Anodic bonding Anodic bonding

Without Ti

Pressu

re

[Torr]

10 100 1000

100

1000

10000

100000

LR type I

LR type II

VR type I

VR type II

Qf

actor

Pressure [mTorr]

Lee et al, J. Micromech. Microeng. 13 (2003) 663669

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Anodic Bonding 3

Pros:- Form a strong bond at low temperature (so metal

can be pattern on the glass or sil icon wafer)

- Form a very good hermetic sealing Cons:

- High voltage may damage CMOS devices- High sodium content from the glass could

contaminate CMOS wafers

-Although hermetic, its hard to get high vacuumdue to glass outgas need getter for goodvacuum

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Silicon Fusion Bonding

The Van der Wall force on silicon surface bring twowafers together

Bonding criteria:- Very clean and smooth surface (

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Surface Activation

Hydrophilic

OH groups formed on the surfaceafter RCA clean

OH attract water moisture eventhe wafer is dried

Bond could init iate at low temp.Si-O-Si is formed after high temp

post-bond anneal

Hydrophobic

After HF release, surface ishydrophobic

H and F are hanging on thesurface free of oxygen

Init ial bond is weaker even init ialbond temp is higher

Si-Si bond is formed after anneal

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IR Images of Silicon Fusion Bonding

Left: Bond wave propagation through the wafer Right: Bond wave is sensitive to surface roughness and

particulates on the wafer. Voids will be seen under IR after bonding

Post-bond annealing is needed for bond strength. Voids may beeliminated or reduced after annealing (vacuum may still be kept)

As bonded

After annealHarendt et. Al, Sensors and Actuators, A21-23,p. 927, 1990

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Plasma Assisted Fusion Bond

Plasma (O2, N2, Ar, NH3etc) is used to create roughsurface as well OH bonds on the surface

Activate the bonding surface like hydrophilic activation Due to plasma activation, the post bond annealing can

be reduced from 1000C to 400C major advantage

Accep

tedMan

Kowal et al Sensors and Actuators A: Physical, 155(1), pp. 145151.

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Fusion Bonding

Pros:- Very strong bond- No intermediate layer is needed- Various surface activation for different

processes both clean and release

- Cap and device wafers have same CTE Cons:

-High temperature post-bond anneal isneeded to ensure bonding quality (mitigated

by plasma activated surface)

-Require very clean and very smooth surface

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Glass Frit Bonding 1 Frit Printing

Glass fri t consists of- glass base: with lead content to reduce

melting point

- Filler: provide CTE match- Solvent: provide fuididity

Glass fr it is applied on the screen andstencil. The fri t is swiped through the wafer

with coating blade. The frit is then patterned

on the wafer.

Wafer

Drying (120C)glazing (300C) sealing (440C) bonding (450C)

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Glass Frit Bonding Process

Drying: remove solvent @ 120C Glazing: further drive out of organic bond @ 340C Sealing: melt the glass to continuous form @ 440C Bonding: align the wafer, make the bond @ 450C

Courtesy of R. Knechtel from XFAB foundries

O f Gl F i

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Outgas of Glass Frit

1.0E-06 1.0E-05 1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01 1.0E+02

Vacuum (Torr)

9000

8000

7000

6000

5000

4000

3000

2000

1000

0

Resonator Qwithout getter

Roll-off Region~1 Torr

Desired operationregion 0.01~0.1 Torr

Q of free-free beam resonator:- Glass frit package with getter: 7500~8000- Silicon fusion bonding: 7500~8000- Glass frit without getter: 3000 (Q degrades due to trapped outgas)

Th N d f G tt

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The Need of Getter

To order to achieve good vacuum, getter is neededfor absorbing outgas from glass frit

Non-evaporable getters: normally alloys of Ti, Zr, V,and Fe

Getter has pil lar type of crystall ine (increase surfacearea) and is usually deposited by sputtering

Getter is usually activated about 300~450C

Ref: M. Maroja et al, Proceedings ofSPIE Vol. 4980 (2003) Patterned defined by shadow mask

R d ti f S li Ri Width

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Reduction of Sealing Ring Width

400um wide 200um wide

Size and Cost reduction is achieved by simply reducingthe seal ring width from 400um to 200um

Printed fri t is about 1/2 or 2/3 of the sealing ring 150um sealing ring was doable but suffer from lower

printed yield very tough to print 75um side line with

>90% yield

2003 2005

Gl F it B di

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Glass Frit Bonding

Pros:-Strong bond strength-Can adopt larger surface topography-Good hermeticity but need getter for

vacuum

Cons:-Pb content is being banned by EU and

Japan-Relatively dirty process-Large material squeeze out

Thermal Compression Bonding

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Thermal compression bonding islike a welding process. Au, Cu

and Al are suitable materials

Welding temp is 300C~400Cwith 20kN to 80kN force

100um sealing ring can be done


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Pros:-Easy preparation of materials-Strong bond strength-OK CMOS compatible-Good hermeticity

Cons:-Requires very clean metal surface-De-oxidation treatment may be needed for

some materials

-Large CTE mismatch

Eutectic Bonding

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Eutectic Bonding

At eutectic temperature, certain composition ofmetals can transfer from solid to liquid

This temperature is much lower than the meltingtemperature of two individual metals

The material mix is solidified when the temperaturedecreases below eutectic point or the concentration

ratio changes (for Si-Au: T < 370 C)

Alloy Wt% EutecticTemp

Au-Si 2.9-97.1 370C

Au-Sn 80-20 280C

Al-Ge 49-51 419C

Eutectic Bonding

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Eutectic Bonding

An intermetallic compounds may be formedbetween solder alloy and substrate

Intermetallic compound may reduce (or enhance)the bond strength

Prevent solder alloy consume substrate material barrier material (typically SiO2) is needed

Eutectic Bonding

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Eutectic Bonding

Pros:-Strong bond strength-

CMOS compatible-Can adopt some surface topography-Great hermeticity

Cons:-Requires very good control of material

composition and temperature

-Large CTE mismatch

Outline

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Outline

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-

-

- - -

Interconnection/Bonding Matrix 1

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Conductive Sealing

Connect

from

Top

Connect

from

Bottom

Connect

from

Side

Need isolation layer betweeninterconnection and sealing

Additional or existing layers (Si ormetal) for interconnection

Step topography make assembly moredifficult

Allow different material for sealingand interconnection processes can

be optimized Vias on MEMS wafer: Limited MEMSprocess and bonding temperature

Allow same material for sealing andinterconnection

Interconnection process is separatedfrom MEMS

Allow higher MEMS process temp Cap can be CMOS wafers

Plated Metal Vias

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Plated Metal Vias

Plated metal (Cu, Au) along thewall of via hole

Top of the via has smalleropening for better hemeticity

Hermeticity after thermal cycleis yet to proven (TCE mismatch)

Conductive seal

Interconnection

from the top Sealing ringConnectionVacuum Cavity

Different shape of through cap viaCourtesy: Silex Microsystems and AAC Microtec

MEMS

ePak Package

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ePak Package

Use carrier wafer of SOI or Si ofbonded Si-glass stack for

interconnection

-Adv: Great electrical isolation (nodielectric loss)

-Adv: flexible to many MEMS devices- Disadv: Higher resistivity than metal

Conductive seal

Interconnection

from the top

Metal Layers

Device Wafer

Connection to outside

Courtesy: ePak

VFT

ViaCap

Isolation Suspension

SOI or bonded glass

RF Switch Package

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RF Switch Package

Frit Ring

Ref: 2001 EUROPEAN MICROWAVECONFERENCE: MARGOMENOS ET AL

Conductive seal

Interconnection at bottom

Au-Au Compression Bonding

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0

1000

2000

3000

4000

5000

6000

7000

1.E-05 1.E-03 1.E-01 1.E+01 1.E+03

Pressure [Torr]

Q

Au Au Compression Bonding

Source: Discera, Inc.

Conductive seal

Interconnection from the side


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Non-conductive Sealing

Connect

from

Top

Connectfrom

Bottom

Connect

from

Side

Isolation may not be needed betweeninterconnection and sealing Sealing ring needs to overcome the

topography of interconnection

Step topography make assembly moredifficult

Separate process for interconnectionand sealing bonding can be done

with or without intermediate layer Vias on MEMS wafer: Limit MEMSprocess and bonding temperature

Vias can be formed before or after thebonding

Hard to do same bonding process forboth interconnection and sealing

Can do bonding first and drill/fill via Via holes on cap: via material needs to

form a good connection to MEMS

Package with Pinari Gauge

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ac age t a Gauge

Frit Ring

CHAEet al.:

FABRICATION AND CHARACTERIZATIONOF A WAFER-LEVEL MEMS VACUUM PACKAGE,JMEMS VOL. 17, NO. 1, FEB 2008

Non-conductive seal

Interconnection from top

Silicon Fusion Bonding with Via

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g

Non-conductive seal

Interconnection from bottom

Glass-Frit Bonding

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Source: Discera Inc

g

Getter

Frit Ring

Vp=2V, OSC=1mVrms, Q=54,270

-30

-25

-20

-15

-10

-5

0

5

1033998 34000 34002 34004 34006 34008 34010

Amp

litude(dB)

-120

-100

-80

-60

-40

-20

0

20

40

60

80

100

Frequency (Hz)

P

hase()

Amp

Phase

Non-conductive seal


Deposition/Interconnection Matrix

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p

Deposited Cap and Sealing

Connect

from

Top

Connect

from

Bottom

Connect

from

Side

Build sensors on top of ASIC or buildsensors with buried interconnections

Deposit sacrificial layer Deposit shell material Etch holes on shell and Release Deposit dielectric to seal the holes

Build MEMS Deposit sacrificial layer on MEMS Deposit thick shell material Etch holes on shell and Release Deposit dielectric to seal the holes Take signal from top of the shell

Build resonator with connections Deposit sacrificial layer Deposit shell material Etch holes on shell and Release Deposit dielectric to seal the holes

Epi-Si Encapsulation

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p p

Double-Clamped Beam:Before cap, Q=14,000

Ref: Candler et al.,IEEE TRAN. ON ADV.PACKAGING, VOL. 26,

NO. 3, Aug. 2003

Deposition seal

Interconnection

from the top

Dielectric Encapsulation

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Ref: Lund et al, Solid-State Sensor, Actuator and

Microsystems Workshop Hilton Head Island, SouthCarolina, p.38-41, June 2-6, 2002

Deposition seal

Interconnection from the bottom (CMOS)

Nitride Shell for RF Switch

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Ref: Ebel et al., GOMAC 05

Deposition seal


Silicon Nitride Shell with Resonators

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Very difficult stress controland release process

Relatively poor vacuum -comb-drive did not function

L. Lin, Selective channel of MEMS: micro-channels, needles,resonators, and electromechanical filters, Ph.D. Thesis, UC Berkeley

Deposition seal


Vacuum Seal with Polymer/Metal Overcoat

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Use thermally released polymerto form a cavity

Polymer overcoat and metalcovered the devices

Resonator showed Qabout 5000,indicating the pressure is about0.75 Torr

tChan

(a)

(b)

(c)

(d)

(e)

Unity

Polymer overcoat

etal tC

tCh

Insulator

1m gap beam

vacuum channel

polymer overcoat

gold coating(d)

0

1200

2400

3600

4800

6000

7200

8E-07 5E-05 0.005 0 .14 0.38 0.5 0.75 2 5.5 6.5

Pressure, Torr

Q

1.5 M Hz 2 .6 M Hz

2 .6 M Hz with island 6 .5 M Hz

Onsetof

resonance

Monajemi et al, J. Micromech.Microeng. 16 (2006) 742750

Measured Q~5000

New: Deep Vacuum Gap (DVG)

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Thickness of density changingmaterial (metal oxides) changes

after anneal at high temperature

the gap is formed due to thicknesschange

CuO + Cu: thickness reduced 46% CuO + Si: thickness reduced 24% CuO + Al: thickness reduced 19%

Nanogap:From a few to 100nm

Courtesy: Scannano

Outline

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- - -

-

- - -

Pressure Measurement

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Center deflection of a clamped square diaphragm ofdimension aand thickness h

PaEhw

=

4

3

2)1(

0151.0

Worse resolution while < 1 Torr

Cavity pressure (Torr) 100 10 1 0.1 0.01

a (m) 1.00E-03 1.00E-03 1.00E-03 1.00E-03 1.00E-03

nu 0.2 0.2 0.2 0.2 0.2

E (Pa) 1.50E+11 1.50E+11 1.50E+11 1.50E+11 1.50E+11

h (m) 1.00E-05 1.00E-05 1.00E-05 1.00E-05 1.00E-05

P (Pa) 87978 99975 101174.7 101294.67 101306.667

w (nm) 8502 9662 9778 9789 9790

w

1 atm = 760 Torr 1 Torr = 133.3 Pa

1mm square, 10um thick diaphragm

Membrane Vacuum Sensor

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Pros:- Very simple no device is needed- Fit almost all the processes

Cons:- Limited vacuum range > 1 Torr- Poor resolution between 1 Torr

Pinari Gauge

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Pinari gauge measures filaments impedance changecaused by heat loss of fi lament to the ambient It can measure vacuum between 1e-4 Torr to

atmosphere, depending on the design, material, andits thermal boundary condition

It can be implemented with MEMS process forvacuum measurement for selected WLP process

Theory of Pinari Gauges

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w, l, t: width, length, thickness of heater

: Ohmic power generation

: Heat loss through the gas

: TCR of the heater

gap(P) and b: thermal conductivity

through the gas and through the beam

: coefficient of fringing heat flux

Akin et al, IEEE SENSORS JOURNAL, VOL. 9, NO. 3, MARCH 2009

Temperature change due to heating

Plot Taveversus power dissipated on the bridge at

various pressure

The slope of the plot is thermal impedanceplot is

versus pressure

Example of Pinari Gauges

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Different designs and materials give differentcharacteristics of thermal impedance curves

For better sensitivity in high vacuum- Longer and thinner heaters- Minimize uncontrolled substrate loss- Closer to controlled heat sinks

Pirani Gauge

Courtesy of : Warren (Neil) Welch

Poly-silicon Pinari gauge Metal Pinari gauge on dielectric

platform for better thermal isolation

Pinari Gauge

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Pros:- Simple design- Fit almost all the processes- Testing is not difficult- Great resolution if the design is right within

interested vacuum range

Cons:- Careful design is needed for characterize

correct range of vacuum- Different thermal boundary condition before

and after WLP may cause calibration error

MEMS Resonator Operation

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2/1

3

0

0

2

2 103.1

=

m

P

r kdAV

LhEf

-75

-70

-65

-60

-55

-50

-45

19.39 19.4 19.41 19.42 19.43 19.44 19.45

Amplitude[dB]

-40

-20

0

20

40

60

80

100

120

Frequency [MHz]

Pha

se[]

VP=2.5V

Pin=25dBmQ=8000

Rx=25k

Vacuum Package Resonator Example

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Contamination fluctuations foand Qfluctuations Typical MEMS resonator mass: 10-13 kg Resonators with lower mechanical stiffness shows

larger frequency fluctuations lower Q

Various Resonators

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Comb-drive Cantilever Double Clamped Beam

Free-Free Beam Disk Resonator

Resonator Sensitivity to Vacuum

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Torr

NormalizedQ

Vacuum Requirements for Resonators

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32K to 19M Comparison

10,000

100,000

1E-06 1E-05 1E-04 0.001 0.01 0.1 1 10 100 1000P (Torr)

Q(

32kHz)

100

1,000

10,000

Q(

19MHz)

From B. Wissman, Discera, Inc.

Resonator Vacuum Sensor

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Pros:- Relatively simple design that is compatible

with many vibrating sensors

- Fit almost all silicon/polysilicon processes- Good coverage for wide range of vacuum,

suitable for process development

Cons:- Different mechanical boundary condition

before and after WLP may cause Q difference- [not for me] Resonator testing can be tricky

need correct set up for resonator testing

Outline

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- - -

-

- - -

Conclusions

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Wafer level vacuum package is the key forcommercial success of MEMS

Among all the processes presented, selectthe one that fit the MEMS devices based on- Design compatibility- Material compatibility- Knowledge of testing

Implement different vacuum sensors in orderto characterize vacuum level for the selected

wafer level vacuum package

Calibrate vacuum sensor carefully anddetermine the success of WLP process

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Thank You !

Questions?

[email protected]

06 Hsu.pdf

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