Hybrid Bonding Methods forLower Temperature

3D Integration

James HermanowskiOctober 2010

2

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