1. MEMS Sensor Packaging
Doug Sparks
Hanking Electronics LLC
www.hkmems.com
Author contact information: sparskdr@hanking.com

2. MEMS Sensors Packaging

3. 50 Shades of MEMS Packaging
Wafer to wafer bond sealing
– vacuum and inert gasesKovar, Ge, Si soldered lids
CVD cavity sealing
Getters
Cavity packages – plastic, ceramic, Kovar
Lead frames, panels
Wirebonding: Al, Au, Cu
Chip coating‐ silicone, parylene, silicone oil
Plastic overmolding
Welded Kovar lids, stainless steel diaphragms
Test & Calibration Integration
Medical ApplicationsAutomotiveConsumer Electronics
Sterilization
Intrinsically Safe
TSV
Bumps
Shipping packages
Industrial‐Chemical Exposure, High Pressure
CSP
Wafer Thinning
Hermeticity
Reliability Testing
FEM
FDA
EMC
ESD

4. Sensor Applications & Specification…
and how this affects the package design
Temperature Operating Range
Chemical or Fluid Exposure?
Pressure or Flow Rate Range
Mechanical‐ Shock & Vibration
Electrical‐ EMC, ESD, Power Requirement
Package Size

5. Package Material Constraints
TCE, Modulus, Fracture Toughness,
Melting Points/Eutectics, Creep, Corrosion,
Embrittlement, Adhesion Loss, Acoustic Loss,
Cost

6. FEM‐ MEMS Packaging
TCE‐Young’s Modulus tradeoff, operating temperature range &
materials, fluid interactions, turbulence, EMC, pressure, load,
resonant frequencies, damping
COMSOL
FEM‐Feedback from real
world great for improving
existing products via package
design changes

7. Fluid Exposure & Corrosion
‐ Corrosive gases and liquids/condensates can attack metal runners (Al). Silicone gels
and parylene offer some protection.
‐ Rapid decompression of air saturated gels can lift wires due to gas bubble expansion
Backside sense silicon packages (Freescale, Merit, First Sensor…) have been
employed to reduce these problems, however epoxy and solder used for die
attach can also be chemically attacked and detach under high pressure.
Corrosion of Metallization

8. Burst Pressure‐ Si Low Fracture Toughness
The diaphragms should be able to handle 3X to 10X the operating pressure
without bursting
Pressure snubbers are used with silicon pressure sensors to prevent the fracture of
diaphragms due to water hammer and other gas/liquid mixtures like steam and natural gas.
Seal Diaphragms also built into industrial pressure meters
Drop test, mechanical shock also affected by low fracture toughness

9. Levels of MEMS Packaging
Wafer Level – metallization and passivation selection, bond pads, TSV
Chip Scale Packaging – Wafer bonding, CVD sealing, TSV, interposer, ASIC
Subpackage‐ plastic, ceramic, metal, panel, overmolding, gel, parylene
System Housing – electrical, fluid interconnects
Shipping Container – avoid damage

10. MEMS Packaging Development Stages
Lab Cerdip
TO
Plastic
CSP
& Historical

11. Vacuum Sensor
(Pirani Gauge)
Au-Sn (Eutectic)
Solder
Kovar Cap
Solder Sealed Kovar/Ceramic Vacuum Package
Ceramic Package

12. Dense
Glass
Vacuum Sensor
(Pirani Gauge)
Cap
TO Can
Projection
Weld
Feedthrough
Pins
Welded Kovar Lid under Vacuum – IR Filter Lids
Welded Metal Lids
Glass Sealed
Feedthroughs

13. Wafer Level Packaging
Top Cap Wafer
getter
MEMS Wafer
MEMS/Micromachined Element Metal Cross Under
Bond Pad Opening
Bond Pad
Seal Ring
Vacuum Cavity
wire
Wafer to Wafer Alignment
not just bonding

14. Types of Wafer Bonding & Temperatures
Si‐Si Fusion
Glass Reflow/Frit
Eutectic Ge‐Al
Cu‐Cu & Au‐Au Thermocompression
Anodic
Eutectic Au‐Si
Solder & TLP Au‐Sn, Cu‐Sn
Si‐Si Direct plasma act.
Adhesive/SU‐8
0 100 200 300 400 500 600 700 800 900 1000 1100
Bond Temperature ( ⁰C)
WLP

15. Even a well bonded Si
to Si or Glass to Si
interface may have
tiny areas that are not
bonded‐
microscratches, pits.
Reflowed glass or solder should do a better job of sealing over surface micro roughness
Si‐Si or Si‐Glass Wafer Surface Roughness
Problems: particles, scratches, flatness
Goselle & Tong 1999

16. Voids Due to Particles
Si‐Si Voids after fusion bonding
Ultrasonic and IR void mapping can
find voids in Si‐Si and Si‐Glass
interfaces due to particles. If dense
enough they could provide a path
for gas movement at the bonding
interface.
Wafer cleaning, chemical purity improvements and modern bonding cluster tools
can greatly reduce this class of interface defects and potential source of
hermeticity loss.

17. Anodic Bonding‐ Particles
Anodic bonding has an
electrostatic force that will bond
a wafer in spite of particles. The
rigged surfaces are sensitive to
particles and scratches, but not
as bad a Si‐Si direct bond which
has a weaker attractive force.

18. Glass Voids
Carbon Rich Regions
C + O2  CO2
C + 2PbO  CO2 + 2Pb
Burn out step must eliminate
all carbon from the acrylic
paste.
If not bubbles can form during
reflow as CO2/CO is formed in
the glass. PbO reduction from
Si and C reactions can produce
gas
Frit Voids
Cavity
Reflow Glass Bonding

19. Metallic Sealing‐ Eutectics for Lower
Temperature Sealing

20. Transient Liquid Phase Eutectic Bonding‐
Higher Operational Temperatures
TLP wafer bonding uses a thin lower melting point wafer to wafer
interface
Metal melting allows for lower bonding force and can cover surface
irregularities
Once the layers intermix and pure metals and the eutectic phase are
consumed the operational temperature that the bonded wafer and
then chips can see is higher than this initial melting temperature
Melt Temperatures 200 ⁰C (180‐300⁰C ) Example Gold‐Indium
Remelt Temperatures > 400⁰C (300‐800 ⁰C)

21. Zoomed Image (Quarter Wafer P)
Page 15 of 19
Job#: 3073‐IL15
Metal‐ Solder/Eutectic Seals can also have
microvoids at the bonding interface
Ultrasound
Image
Intermetallic
Wetting problems
Oxidation
Particles

22. Kirkendall Voiding of Metallic Alloys
Even in a particle free fab
with planar sealing surfaces,
voids can appear in metal
alloy interfaces.
Intermetallics can also lead
to bond failures

23. CVD MEMS Vacuum Sealing
CVD seal MEMS resonators and reference vacuum
cavities with CVD poly‐silicon, oxide and nitride.
Kim et al Stanford Transducers 2008
Thinner vacuum sealed
MEMS chips
Can be CMOS integrated but
this can increase wafer
process complexity.

24. Vacuum Packaging – Getters
Atmosphere Control

25. Gas Desorption of Microcavity Surfaces
Reversible changes in Q and zero offset in absolute pressure sensors have been observed
1100
1150
1200
1250
1300
1350
0 500 1000
Q
Time (hours)
Q Versus Time 100C/5V
Early MEMS gyro resonator reliability testing showed reversible Q changes over temperature.
Sensors and Actuators A, 95 (1), pp.61‐68, (2001)
Q versus Pressure in a Vacuum
Chamber with a Decapped Resonator
0
2000
4000
6000
8000
10000
12000
0 50 100
Pressure (mTorr)
Q

26. Chip‐Scale Vacuum Package
Silicon on Glass Chip Stack
Improved Vacuum Performance – High Q Resonators
Using the Thin‐Film Getter Process/Design
With no getter – the average Q value is 36,
Q-Plot Silicon Tube Resonator with
NanoGetters
-30
-25
-20
-15
-10
-5
0
16354 16355 16356 16357 16358
Frequency (Hz)
Gain(db)
Q=61,700
D.Sparks et al, Hilton Head, SC p.75, 2004

27. Getter and Wafer Bonding Process
Sparks et al, IEEE Transactions on Advanced
Packaging, Vol. 26,No. 3, August 2003

28. Ceramic & Metal Package Integration
Getters can be selectively deposited on Kovar, Si, steel, glass,
sapphire or Ge lids for resonators, pressure sensors, optical and
tunneling devices.
Thin film getter
on a Kovar
lid
Getter on Steel Lids
May need an activation
step with welded lids.

29. Backend Process‐ MEMS Wafer Saw Options
Diamond Blade Si slurry – wirebond
problems, sensing element fouling
Stealth‐ internal voids‐ must break apart
chips
Plasma‐ rounded corners, must remove
metal films from saw street‐ great for
thinning small chips – pressure
microphones.
Diamond Blade, Stealth Laser, Plasma‐DRIE
Si Slurry on Al

30. Backend Process: Wire Bonding & Passivation
Wire Material
Aluminum, Gold, Trend‐ Copper, Alloys
Forming gas – high bond force damage
Wire Gel Coating‐ corrosion & shorts prevention
Low loop heights for thin packages
Moving from 1‐2 chips per package to Combos
Combo Packages‐ stacked chips‐ gyros, accelerometers, e‐compass, pressure

31. Major MEMS Package Applications
in order of the environmental challenge
• Industrial
• Automotive
• Medical
• Consumer

32. Media Compatible Options
Industrial Pressure
• Corrugated Steel Diaphragms, Oil Filled Packages
• Silicon Bonded to Steel
• Backside Si Sense
Trade‐offs – Cost , Size, Accuracy, Corrosion,
Over Pressure & Water Hammer Burst

33. Metal Diaphragms‐Oil Fill Pressure Sensor
Welded Corrugated
Steel Diaphragms
MEMS Frontside sense

34. CVD PolySi Bridge on Stainless Steel
Pressure Sensor
Nagano Pressure Sensor
CVD Fabrication Process
Media Contacts Steel‐ unique MEMS on steel process and fab

35. Zhang et al, JMM 201
Si Strain Gauge On Stainless Steel‐
Glass Reflow AttachmentLow Sensitivity, Media contact
with stainless steel

36. MEMS Pressure Media Compatible
Higher Sensitivity
Lower hysteresis
Smaller die size‐ cheaper that NKS polySi
process
Do coat the bottom of the Si diaphragm with
gold‐ for chemicals that etch Si (hot caustic)
Au‐Sn eutectic die attach‐ to ceramic (17‐4?)
Top side motion stop
Can still have a burst pressure problems that
steel diaphragms do not have
Merit Sensors, SMI, Freescale
Fluid

37. Merit’s ‐ Media Compatible MEMS
• Integrated temperature sensing capability
• Back-side media monitoring protects
electronic circuitry from harsh media
• <1% total accuracy
• Wide operating temp. range (-40° to 150°C)
• Pressure range to 500 psi (34.5 bar)
ceramic

38. Can we shrink all industrial MEMS‐based sensor?
SMT
CSP
Through Hole
Cable Connectors
Metal Sealed
?

39. Automotive Operating Environment
The wide operating temperature range, cost targets and EMC goals heavily influence
package design for the automotive market
• Pressure
• Flow
• Angular Rate
• Acceleration
• Microphones
• Tilt
• Gas

40. MAP, MAP/T, TPMS, TURBO Packages
There are more than 100 different MAP, TMAP, MAPT‐ MEMS
pressure sensor packaged for the automotive market. Virtually
every car manufacturer specifies a different housing, connector
and attachment fixture.
Glass Filled PBT
Metal
Inserts

41. Automotive MEMS Flow Sensor Package
Flow rate range, coating of elements, particle
impingement, resonance, bypass, turbulence
Automotive temperature range ‐40⁰C to 150⁰C
MAF – 2 part package
MEMS Chip
2 Part Package

42. Medical Pressure Sensor Packaging
Merit Sensors
Applications drive the packaging‐ Blood pressure, respiration, catheters, implants
Implant‐ blood contact
tissue growth
Sterilization method‐ gamma ray, ETO, autoclave – can damage the MEMS sensor
Narrow temperature range often eliminates need for an ASIC
FDA Approval process – Class II or Class III‐ implants
Corrosive –blood salt water, biologic, tissue growth
BP‐ low cost application

43. Packaging for Consumer MEMS Sensors
Thin, low power, bumps, CHEAP!

44. Glass
Si CIS
Glass
Si to Si Si to Si
Si
1.5‐3mm
Industrial & Gen 1 Automotive
Current Automotive & Consumer
Pressure Sensors ‐ CIS

45. Temporary Bond & Debond, Thinning
ProcessesDevice Wafer
Carrier Wafer
Coat with release agent
Temporary Adhesive
Bond
Thin Device Wafer
&
Other Processing
Debond Process
Clean Tape Frame Mounting
Singulate‐Dice Chips
• Solvent Release
• Thermal Slide
• Mechanical Peel
• Laser Assist
CMP

46. Through Silicon Vias‐ TSV
ChipScale Review Dec 2014
TSVs are being integrated into MEMS, such as capping wafers and
Silicon Interposers‐ Need Wafer thinning, DRIE, trench fill.
A Key Backend Process
TSV
&
TGV

47. Integrating Solder Bumps into MEMS
TSVs & DRIE are most often needed as well
KLA /Tencor 2015Also Au Bumps & Cu Pillars

48. Motion Sensors
3D ICs
Automotive MEMS Motion Sensors
Pad
s
Consumer MEMS 2015+
Cap Wafer
Wirebond
Bumps
TSV
Cap
CMOS
100um
100‐300 um
500‐750um
500‐750um
3D IC Stack MEMS Metallic Bond & Bumps

49. What is a MEMS Process?
DRIE Si Etch
HF Undercut
Wafer to Wafer Bonding
Wafer Thinning
A‐HF Tube Cleaning
HF Contact Etch
TSV Etch
Thin Power ICs
Thin CIS Stack
CIS Carrier Wafer
Plasma Poly‐Si Etch CMOS
3D IC Stack

50. Consumer Packaging Convergence
MEMS ‐ CIS ‐ 3D IC
DRIE, Wafer Bonding, Thinning, TSV, Bumps
WL‐CSP

51. Conclusion‐ MEMS Packaging
MEMS sensors packaging is quite diverse, depending on application
New packaging methods are being developed to shrink even
industrial sensor product
Package design depends on the science, cost, form factor and
application
Consumer MEMS packages are converging with CIS and 3‐D ICs
Hanking Electronics MEMS Packaging Fab

52. Contact Hanking Electronics LLC for
MEMS packaging services
Author; Doug Sparks
Email: sparksdr@hanking.com