06 hsu.pdf
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Wafer Level Vacuum Packaging
for Sensors
Wan-Thai Hsu, PhD.
Chief Technology Officer
Discera Inc.
IEEE Sensors 2012
Oct. 28, 2012
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Outline
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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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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
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
Thermal Compression Bonding
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Thermal Compression Bonding
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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Interconnection/Bonding Matrix 1
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
Interconnection/Bonding Matrix 2
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Interconnection/Bonding Matrix 2
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
Interconnection from the side
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
Interconnection from the side
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
Interconnection from the side
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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-
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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?