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MEMS Sensor Packaging
Doug Sparks
Hanking Electronics LLCwww.hkmems.com
Author contact information: [email protected]
MEMS Sensors Packaging
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
TSVBumps
Shipping packages
Industrial‐Chemical Exposure, High Pressure
CSP
Wafer Thinning
Hermeticity
Reliability Testing
FEM
FDA
EMC
ESD
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
Package Material Constraints
TCE, Modulus, Fracture Toughness, Melting Points/Eutectics, Creep, Corrosion, Embrittlement, Adhesion Loss, Acoustic Loss, Cost
FEM‐MEMS PackagingTCE‐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
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
Burst Pressure‐ Si Low Fracture ToughnessThe 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
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
MEMS Packaging Development Stages
Lab Cerdip
TO
Plastic
CSP
& Historical
Vacuum Sensor(Pirani Gauge)
Au-Sn (Eutectic) Solder
Kovar Cap
Solder Sealed Kovar/Ceramic Vacuum Package
Ceramic Package
DenseGlass
Vacuum Sensor(Pirani Gauge)
CapTO Can
ProjectionWeld
FeedthroughPins
Welded Kovar Lid under Vacuum – IR Filter Lids
Welded Metal Lids
Glass SealedFeedthroughs
Wafer Level Packaging
Top Cap Wafergetter
MEMS Wafer
MEMS/Micromachined Element Metal Cross Under
Bond Pad Opening
Bond Pad
Seal Ring
Vacuum Cavitywire
Wafer to Wafer Alignmentnot just bonding
Types of Wafer Bonding & TemperaturesSi‐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 1100Bond Temperature ( ⁰C)
WLP
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 RoughnessProblems: particles, scratches, flatness
Goselle & Tong 1999
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.
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.
Glass VoidsCarbon Rich RegionsC + 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
Metallic Sealing‐ Eutectics for LowerTemperature Sealing
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)
Zoomed Image (Quarter Wafer P)
Page 15 of 19Job#: 3073‐IL15
Metal‐ Solder/Eutectic Seals can also have microvoids at the bonding interface
UltrasoundImage
IntermetallicWetting problemsOxidationParticles
Kirkendall Voiding of Metallic AlloysEven in a particle free fab with planar sealing surfaces, voids can appear in metal alloy interfaces.
Intermetallics can also lead to bond failures
CVD MEMS Vacuum SealingCVD 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.
Vacuum Packaging – GettersAtmosphere Control
Gas Desorption of Microcavity SurfacesReversible changes in Q and zero offset in absolute pressure sensors have been observed
110011501200125013001350
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
02000400060008000
1000012000
0 50 100
Pressure (mTorr)
Q
Chip‐Scale Vacuum PackageSilicon on Glass Chip Stack
Improved Vacuum Performance – High Q ResonatorsUsing 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)
Gai
n (d
b)
Q=61,700
D.Sparks et al, Hilton Head, SC p.75, 2004
Getter and Wafer Bonding Process
Sparks et al, IEEE Transactions on Advanced Packaging, Vol. 26,No. 3, August 2003
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 getteron a Kovarlid
Getter on Steel LidsMay need an activationstep with welded lids.
Backend Process‐MEMS Wafer Saw Options
Diamond Blade Si slurry – wirebondproblems, 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
Backend Process: Wire Bonding & Passivation
Wire Material
Aluminum, Gold, Trend‐ Copper, AlloysForming 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
Major MEMS Package Applications
in order of the environmental challenge
• Industrial• Automotive• Medical• Consumer
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
Metal Diaphragms‐Oil Fill Pressure Sensor
Welded CorrugatedSteel Diaphragms
MEMS Frontside sense
CVD PolySi Bridge on Stainless Steel Pressure Sensor
Nagano Pressure Sensor
CVD Fabrication Process
Media Contacts Steel‐ unique MEMS on steel process and fab
Zhang et al, JMM 201
Si Strain Gauge On Stainless Steel‐Glass Reflow Attachment
Low Sensitivity, Media contact with stainless steel
MEMS Pressure Media Compatible
Higher SensitivityLower hysteresisSmaller die size‐ cheaper that NKS polySiprocess
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
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
Can we shrink all industrial MEMS‐based sensor?
SMT
CSP
Through Hole
Cable Connectors
Metal Sealed
?
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
MAP, MAP/T, TPMS, TURBO PackagesThere 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
MetalInserts
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
Medical Pressure Sensor Packaging
Merit Sensors
Applications drive the packaging‐ Blood pressure, respiration, catheters, implants
Implant‐ blood contacttissue 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
Packaging for Consumer MEMS Sensors
Thin, low power, bumps, CHEAP!
Glass
Si CIS
GlassSi to Si Si to Si
Si
1.5‐3mm
Industrial & Gen 1 Automotive
Current Automotive & Consumer
Pressure Sensors ‐ CIS
Temporary Bond & Debond, Thinning ProcessesDevice Wafer
Carrier WaferCoat with release agent
Temporary AdhesiveBond
Thin Device Wafer&
Other Processing
Debond Process
Clean Tape Frame Mounting
Singulate‐Dice Chips
• Solvent Release• Thermal Slide• Mechanical Peel• Laser Assist
CMP
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
Integrating Solder Bumps into MEMS
TSVs & DRIE are most often needed as well
KLA /Tencor 2015Also Au Bumps & Cu Pillars
Motion Sensors 3D ICs
Automotive MEMS Motion Sensors
Pads
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
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
Consumer Packaging Convergence
MEMS ‐ CIS ‐ 3D IC
DRIE, Wafer Bonding, Thinning, TSV, Bumps
WL‐CSP
Conclusion‐MEMS PackagingMEMS 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
Contact Hanking Electronics LLC for MEMS packaging services
Author; Doug SparksEmail: [email protected]