Hybrid Bonding Methods forLower Temperature
3D Integration
James HermanowskiOctober 2010
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Overview
Overview of primary 3D bonding processesMechanics of metal bonding optionsMechanics for hybrid bond materialsProcess requirement comparisonsEquipment requirements for hybrid bond processesSummary
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Expanding CE (consumer electronics) market drives the Semiconductorinnovation
Push for integrationReduction in power consumptionSmaller form factor
Image sensors and memory stacking (for mobile applications) are two massvolume applications for TSVs with close time-to-market
1980‘s1950‘s TodayEnabling new devices
3D Integration: Stacking for Higher Capacity
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Fusion / Adhesive Bonding
Lithography, Adhesive Bonding
CM
OS
Imag
e Se
nsor
CMOS Image Sensor Integration (BSI)
CMOS Image Sensor Packaging
Wafer Level Optics Assembly Imprinting, UV Bonding
Kodak / Intel / Samsung
Mem
ory
Stac
king
DRAM
FLASH
NAND
Metal to Metal Bonding
Fusion bonding
Adhesive Bonding
SUSS Equipment for 3D Packaging
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3D IC Process Sequence VariationsA&C
B&D
E
F
G
H
I
Lithography Temp. Bonding
Aligning and Bonding (Permanent)
Source: Phil Garrou, MCNC 2008
Test
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3D IC Process Sequence Variations
"face-up" Bond (metal bonding)TSV from back (vias first)Wafer Thinning (temp. handle)No TSVI
TSV from front (vias last)"face-up" Bond (all methods)Wafer Thinning (temp. handle)No TSVH
TSV from back (vias last)Wafer Thinning (on 3D stack)"face-down" Bond (all methods)No TSVG
"face-up" Bond (metal bonding)Wafer Thinning (temp. handle)TSV from front (vias first)No TSVF
Wafer Thinning (on 3D stack)"face-down" Bond (metal bonding)TSV from front (vias first)No TSVE
Wafer Thinning (on 3D stack)"face-down" Bond (metal bonding)BEOL TSV (vias first)D
"face-up" Bond (metal bonding)Wafer Thinning (temp. handle)BEOL TSV (vias first)C
Wafer Thinning (on 3D stack)"face-down" Bond (metal bonding)FEOL TSV (vias first)B
"face-up" Bond (metal bonding)Wafer Thinning (temp. handle)FEOL TSV (vias first)A
Step #3Step #2Step #1IC WaferProcess
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Logic to Logic Stacking using Cu-Cu Metal to Metal 3D Technology at the 300mm Wafer to Wafer Level
SOURCE: Intel Developers ForumSOURCE: Intel Developers Forum
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Stacked Memory Modules using Cu-Cu Metal to Metal 3D Method
SOURCE: Intel Developers Forum
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3D Structure using Wafer Level Cu-Cu Bonding
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Silicon Direct Bonded 3D Chip to Wafer Example
DBI employs a chemo-mechanical polish to expose metal patterns embedded in the silicon-oxide surface of each chip. When the metal connection points of each chip are placed in contact using the company's room-temperature die-to-wafer bonding technology, the alignment is preserved, as opposed to other bonding techniques that apply heat or pressure that can result in misalignment. The oxide bonds create high bond energy between the surfaces, which brings the metal contact points close to each other to form effective electrical connections between chips after a 350°C anneal.
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RPI/Albany Nanotech and IBM, Freescale Approach using BCB
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Preparation for Cu-BCB Hybrid Bonding
1. Cu on Device Layer on Si 2. Pattern Cu (this gives larger vias)
3. Coat w/ BCB4. Planarize/Expose Cu Nails
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Preparation for Cu-SiO2 Hybrid Bonding
1. Oxide on Device Layer on Si
4. Planarize to Cu Nails
2. DRIE Etch Via holes
3. Fill Via holes w/ Cu
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Requirements for Diffusion BondingProper materials system: Rapid Diffusion at Low Temperature
Same crystal structure bestMinimal size differenceHigh SolubilityHigh mobility and small activation energy
Diffusion Barriers to protected regionsHigh Quality films - No contamination or oxide layer for metalsIntimate Contact between surfaces
Process VariablesHeatPressureGas AmbientProcess Vacuum levels
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Complete Solid Solubility
• Both Cu and Ni are FCC crystals• ρ(Cu)=8.93 gm/cm3
• ρ(Ni)=8.91 gm/cm3
• Lattice Spacing a0(Cu)=3.6148Å• Lattice Spacing a0(Ni)=3.5239Å
Copper (Cu) - Nickel (Ni)
αα
liqliq
CuCu NiNi
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Microstructure DevelopmentInterface Properties
1. Generally retain elastic properties of noble metals.
2. Resistivity usually obeys Vegard’s rule - linear with % atomic concentration of mix.
3. Full layer diffusion not needed.
4. Adhesion layers may be needed for initial substrate deposition process.
5. Diffusion barrier may be incorporated with adhesion layer to prevent diffusion into substrate.
6. Wetting agents between A & B layers assists in initialization of diffusion.
Silicon
Silicon
Metal A (Ni)
Metal B (Cu)Fully mixed with
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Diffusion Bonding
1. The mechanical force of the bonder establishes intimate contact between the surfaces. Some plastic deformation may occur.
2. During heating the atoms migrate between lattice sites across the interface to establish a void free bond. RMS <2-5 nm required.
3. Vacancies and grain boundaries will exist in final interface area. Hermeticity is nearly identical to a bulk material.
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Diffusion Pathways in Crystals: Poly vs Single
Single Crystalline Fine Grain Poly-Crystalline
Dsurface > Dgrain.boundary > Dbulk
Course Grain Poly-Crystalline
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Type A Kinetics: Rapid Bulk Diffusion Rates
In Type A kinetics the lattice diffusion rates are rapid and diffusion profiles overlap between adjacent grains.
gbgb gbgb gbgbgbgbbulk bulk bulk
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In type B kinetics the grain boundary is isolated between grains. Behavior mimics bulk diffusion. Diffusion is by both grain boundaries and bulk atomic motion. Dominate pathways are related to grain size and density.
Type B Kinetics: Normal Bulk Diffusion w/ GB Effect
gbgb gbgb gbgbgbgbbulk bulk bulk
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In Type C kinetics the lattice diffusion rate is insignificant
and all atomic transport is dominated by grain boundary diffusion only For example room temperature diffusion.
Type C Kinetics: Insignificant Bulk Diffusion
gbgb gbgb gbgbgbgbbulk bulk bulk
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6420
-6-4-2
6420
-6-4-2
2 40 6 8 10 12
6420
-6-4-2
2 40 6 8 10 12
Log
[1/g
.s.(c
m) ]
Log ρd (cm-2) Log ρd (cm-2)
Log
[1/g
.s.(c
m) ]
T/Tm = 0.3T/Tm = 0.4
T/Tm = 0.6 T/Tm = 0.5
gbgb
gbgbgbgb
gbgb
ll ll
ll ll
dddd
dddd
• Regimes of grain size (g.s.) and dislocation density ρdover which (l) lattice diffusion, (gb) grain boundary diffusion of (d) dislocation diffusion is the dominate mechanism for atomic motion.
• All data is normalized to the melting point and applies for a thin film fcc metal at steady state.
• Shaded area is typical of thin film dislocation density 108
to 1012 lines/cm2.
Low Temperature Diffusion Relies on Defects
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Metal Bonding Options
ReactionType
Metal †Bond Temp Oxidizes CMOS Compatible
Cu-Cu >350°C No YesAu-Au >300°C Yes NoAl-Ge >419°C No YesAu-Si >363°C Yes No
Au-Ge >361°C Yes NoAu-Sn >278°C No NoCu-Sn >231°C No Yes
†Eutectic bonds are done ~15°C above the listed eutectic tempereature. Diffusion bonds lower limit expressed.
Diffusion
Eutectic
CMOS compatibility –barrier layers are often used to prevent metal migration to the CMOS structure.
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Key Unique Requirements for Metal Bonds
Surface roughness is important to allow the metal surfaces to come into intimate contact, especially for diffusion bondingMetal oxide formation can prevent strong bond formation
Preventive actions and process controls need to be establishedForce requirements are much tougher
Structural issues with bond chamber will become much more apparent during metal bondingFor example, the chamber shape may change with the application of high heat and force causing unbonded areas to form in the devices
Temperature controls will be pushed harderTo obtain the tighter overlay possible with metal bonding, it isimportant to control both wafers to tight temperature tolerancesTo prevent oxide formation, it is more desireable to load wafers at lower temperatures into the bond chamber
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Gold-Gold bond at 300°C for 30 min. Au layer is 350nm, Cr is 50nm thick
0.5μm
AuAu
AuAu
CrCr
CrCrSiSi
SiSi
InterfaceInterface
0.5μm
AuAu
AuAu
CrCr
CrCrSiSi
SiSi
InterfaceInterface
Surface roughness is important
to maintain intimate contact and
good bonds.
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Thin (400nm) Cu/Cu bonds at 300°C for 30 min.
1μm
Si
Si
Cu
Cu
Interface Interface
1μm1μm
Si
Si
Cu
Cu
Interface Interface
Ultra smooth surfaces allow
better molecular intermixing
and deliver good bond quality
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Common Polymer Material Choices
Company Dow Toray Sumitomo Sumitomo Dow Corning HD-Micro HD-Micro MicroChemTrade Name Cyclotene PWDC-1000 CRC-8000 CRX 2580P WL-5000 HD-2771 HD-3003XP SU8Material BCB PI PBO PI Silicone PI PI EpoxyPhotoPatternable Both Negative Positive Positive Yes Yes Negative Yes Negative Both NegativeResidual Stress (MPa)
28 28 60 <6.4
Moisture Uptake (%) 0.23 0.6 0.3-0.9 0.06 ~0.2 >1.0 0.08%Coefficient of Thermal Expansion (ppm/°C)
52 36 51 100 <236 42 124 52
Glass Transistion Temperature (°C)
>350 295 294 188 50-55
Cure Temperature (°C)
210-250 250+ 320 200 <250 >350 220 95
Dielectric Constant 2.65 2.9 2.65 <3.3 3 3.4Modulus (GPa) 2.9 2.9 2.9 1.6 0.15-0.335 2.7 2.4 4Thermal Stability (%loss at 350C/1hr)
2 <1 5 <6 <1 <1
Shrinkage During Cure (%)
2.5 <2 40-50 <0.04%
Minimum Thickness (µm)
1 3 3 10 2 4 1 5
Storage Temperature (°C)
-15 4 -15 r.t. or -18
Shelf Life (mos.) 6 6 6 12 @ -18C
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BCB Phase Diagram: Tailored Solutions
Liquid
Solid
• Control of bond kinetics allows interface to become more or less compliant to device layers and structures.
• Control of phase transformation and gaseous byproducts happens during both the pre-cure and the final bond process.
65°C 100°C 125°C 150°C 175°C
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Hybrid Cu/BCB BondsSample # Lx Ly Rx Ry
92 -1.45 -0.75 -0.55 0.40
Benefits to maintaining alignment while bonding metal with BCB as a supporting layer and
interlayer dielectric
Equipment for Permanent & Temporary Bonding for 3D Integration
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Permanent BondingCu-Cu Bonding
Polymer / Hybrid Bonding
Fusion Bonding
Temporary Bonding/De-bonding capability
Thermoplastics Process (eg. HT10.10)
3M WSS Process
Dupont / HD Process
Thin Materials AG (TMAT) Process
Total Process Flexibility for 3D Applications
XBC300 Standardized Platform
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XBC300 Configuration Examples
SC300For
adhesive coating
Module 3
PL300
(TMAT)Laser
moduleDB300
Tape onframe
LF300SC300
for cleaning(optional)
Temporary Bonding De-bonding
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True Modular Design
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True Modular Design
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True Modular Design
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True Modular Design
True Modular Design
Lowers investment riskIdeal for changing technology requirements
Lowers COOSmall footprint, high throughput
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BA300UHP Aligner
CB300 Bonder
CP300 Cool Plate
SC300 Spin Coater
PL300T Surface Prep
LF300 Low Force Bonder
DB300 Debonder
Temporary Bonding
Permanent BondingCL300 Wafer Cleaning
PL300 Plasma Activation
Process Flexibility: Complete Line of Process Modules
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Permanent Bonding Configurations
BA300UHP
Aligner
CB300
Bonder
CP300
Cool Plate
Fusion Bond Configuration Cu-Cu and Polymer Bond Configuration*
BA300UHP
Aligner (if alignment
with keys required)
PL300
Plasma Activation
CL300
Wafer Cleaning
*Optional Die to Wafer Collective Bonding
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Permanent Bond Configurations
BA300UHP Bond Aligner – submicron alignment accuracyCB300 Bond Chamber – temperature & force uniformityCP300 Cool Plate – controlled cool rate
*Optional Die to Wafer Collective Bonding
Cu-Cu and Polymer Bond Configuration*
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Sub Micron Alignment AccuracyPath to 350nm PBA for Cu-Cu bondingPath to 150nm PBA for Fusion bondingISA alignment mode for face to face alignmentAllows smaller via diameters and higher via densities
Built in Wedge Error Compensation (WEC) to make upper and lower wafers parallel prior to alignment
Eliminates wafer shift during wafer clamping
Closed loop optical tracking of mechanical movements
Void free bonding in the BA with RPP™Patent pending RPP™ creates an engineered bond wave for propagationEliminates need for bond module
BA300UHP Bond Aligner Module
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Fusion Bonding in the BA300UHP
Wafers are loaded and vacuum held against SiC chucks
Chucks and the vacuum or pressure, that can be controlled between the chuck and the backside of the wafer, “engineers” the shape of the bonding surface
The chucks are used to align and bring the wafers into contact
The chucks are also used to engineer the bond wave from center to edge using RPP (Radial Pressure Propagation).
Click icon forRPP Presentation
XBC300 Wafer Bonder RPP (Radial Pressure Propagation)
in the BA300UHP Aligner Module
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Si C Chuck & Tool Fixture (Patent Pending)
Transports aligned pair from BA300 to CB300
Delivers reproducible submicron alignment capabilities
Maintains wafer to wafer alignment throughout all process and transfer steps
No exclusion zone required for clamping
Maintains alignment accuracy through temperature ramp
Chuck CTE matches Si CTE
Increases throughput by reduction of thermal mass
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CB300 Bond Chamber ModuleProduction Requirement Closed Bond ChamberContamination Free
Open chamber lid introduces air-turbulence and particles into bond chamber
Uniform heatOpen chamber lid causes temperature gradient between the front and back
3 Post Superstructure takes force, not bond chamber
Chamber lid is the structural force carrying element in clam shell design–this causes force distortion
SafetyOpening chamber lid exposes user to high temperatures
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CB Chamber Force Uniformity
Excellent Force UniformityWithin ±5% pressure uniformityPatented Pressure Column Technology for up to 90kN of bond forceLoad Cell VerificationBond Force options
Standard: 3kN to 60kNHigh Force Option: 3kN to 90kN
Traditional PistonTraditional Piston
Bond-Interface
SUSS Pressure Column TechnologySUSS Pressure Column Technology
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CB Chamber Thermal Design
Superior Thermal PerformanceWithin ±1.5% temperature uniformityFast ramp (to 30°C/min) and cool rate (to 20°C/min)Matched top and bottom stack assemblies
Perfect symmetryMulti-zone, vacuum-isolated heaters
Dramatically reduces hot spots and burnoutsEliminates edge effects
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CB Chamber Structural Design
Best-in-Class Post Bond Alignment
±1.5µm post bond alignment for metal bonds
Rigid superstructureSolid alignment stability
High planarity silicon carbide chucksMaintains long term planarity for superior post-bond alignment accuracy
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CP300 Cool Plate Module
Fixture and wafer coolingUnclamp, unload, and optional fixture load
Queuing and buffer station for fixtures and wafers
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CL300 Wafer Cleaning Module for Fusion Bonding
Wet spin process for wafer cleaning
Twin ultrasonic headIR Assisted DryingNH4OH chemistry
Simultaneous clean, mechanical align and bond two wafers
Bond initiation integrated into CL300
Closed process chamber for maximum particle protection
Rated for particle sizes down to 100nm
Design based on CFD(computational fluid dynamic) modeling
Example of KLA data w/ no adders down to 100nm
CFD modeling of chamber
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PL300 Plasma Activation Module for Fusion Bonding
Cleaning & surface conditioning for fusion bonding
Simple operation with plasma activation times in <30 seconds
Enables high bond strength at low annealing temperatures
Vacuum chamber based plasma system
Uniform glow plasmaPower supply options for frequency and power level
Ex: 100kHz/300W; 13.56MHz; 2.4GHzAutomatic tuningInput gases with up to 4 MFCsRadially designed high conductance plenum and vacuum system
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Summary
Metal, fusion and hybrid bond processes have been reviewedHybrid bond processes require mixture of metal bond processing with either oxide bonding or polymer bond process modules
Tool flexibility is importantMetal hybrid bonding processes are being implemented as the next generation solutionAlthough metal hybrid bonding processes have many advantages over other approaches they also require much more from the process equipment
For example much more stringent specs for force and thermal controlProcess equipment proven to satisfy these requirements has been presented