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IMPROVING CLEAN PERFORMANCE FOR ADVANCED PACKAGINGCHRISTOPHE LORANT, SIMON BRAUN, SAMUEL SUHARD, FRANK HOLSTEYNS

IMEC, Kapeldreef 75, 3001 Heverlee, Belgium

+32 016 28 32 95

Christophe.lorant@imec.be

OUTLINE

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1. 3D interconnect technology landscape

2. Hybrid collective die to wafer bonding

3. Cleaning of dies on carrier

4. Towards wet process optimization

THE 3D INTERCONNECT TECHNOLOGY LANDSCAPE

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Eric Beyne, “The 3-D Interconnect Technology Landscape”, IEEE Design & Test, 2016 , Vol. 33 NR 3 , pp. 8 – 20

Eric Beyne, “3D System Integration Technologies” , 2006 International Symposium on VLSI Technology, Systems, and Applications, 2006, pp.1-9

HYBRID BONDING

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CU-CU DIRECT BONDING

Eric Beyne et al., “Scalable, sub 2μm pitch, Cu/SiCN to Cu/SiCN hybrid wafer-to-wafer bonding

technology”, 2017 IEEE International Electron Devices Meeting (IEDM), 2017, pp. 32.4.1 - 32.4.4

HYBRID COLLECTIVE DIE TO WAFER BONDING

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PROCESS FLOW

Tape

Protective layer

TapeTape

Particle creation

DicingDie Carrier

Adhesive (TBM)

Clean dies on TBM

Die Carrier

(wet) glue strip

Die pick D2W placement on TBM

Target wafer Target wafer

W2W bonding

(wet) protective layer strip

Die Carrier Die Carrier

Die Carrier

PROTECTIVE LAYER FOR DEFECT REDUCTION

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MOTIVATION

Without protective layer With protective layer

After population After protective layer strip After bonding

▪ Defects can be generated either during

dicing or die handling

▪ Si particles generation

▪ Contact defects during pick & place

▪ Particulate contamination cause large

voids after bonding when they are not

removed

➢ A protective layer is a key enabler to

achieve a low defect die surface and

successful bonding10x10 mm Particle

Void

Target waferSAM after bonding

PROTECTIVE LAYER FOR DEFECTIVITY REDUCTION

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PROCESS REQUIREMENTS

Clean dies on TBM

▪ Protective layer and particles must be removed

from the die to enable successful bonding

▪ Protective layer is removed in a wet strip

process compatible with TBM

▪ The clean needs to remove sub 0.1 µm particles

to avoid bonding voids

Bearda T. et al., JJAP 44 (10), 7409, 2005

Die Carrier

Particles Protective layer

Die Carrier

Wafer deflection as a function of silica particle size

CLEANING OF DIES ON A CARRIER

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TEST MATERIAL

Increasing die thickness

Carrier

Die

PR

TBM

50 µm

Die

PR

TBM

775 µm 25 % 100 %

Increasing die population density

▪ 30 µm adhesive layer (TBM)

▪ 14 µm photoresist as protective layer

▪ 7x7 mm dies, 3 mm spacing

▪ Photoresist is stripped in a single wafer wet tool

Thinned dies: Good cleaning performance on thinned dies, residues free surface

Thick dies: strong process non-uniformity observed and increases with population density

CLEANING OF DIES ON A CARRIER

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PROCESS UNIFORMITY AFTER PROTECTIVE LAYER STRIP

775 µm dies (25% pop.)

▪ No PR residues visible

V

775 µm dies (100% pop.)

▪ X-like pattern is clean

▪ PR residues in 4 “corners”▪ No PR residues visible

50 µm dies (25% pop.)

X

Standard TMAH/DMSO based resist strip on a TEL Cellesta+

CLEANING OF DIES ON A CARRIER

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FLOW PATTERN DURING CENTRAL DISPENSE ON FULLY POPULATED WAFERS

Channeled liquid flow

Blocked liquid flow

Most difficult dies to clean

▪ Outside the dispense zone, the expected liquid film height on a flat surface is < 300 µm

▪ Thin dies (50 um) are wetted but thick dies (775 um) oppose a resistance to the liquid flow

depending on their orientation and position

➢ Die thickness impacts process uniformity

Computed liquid film height

as a function of wafer rotation speed (Q = 1l/min)

Computed based on:Prieling D., Computational investigation of liquid films on rotating disks, phD thesis, 2013

CLEANING OF DIES ON A CARRIER

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LIQUID SPLASHES DURING CENTRAL CHEMICAL DISPENSE ON THICK DIES

▪ Liquid jets and bubbles formation are observed below the dispense nozzle

➢ Risk of nozzle contamination!

▪ Droplets formed will fall back on the wafer

▪ Liquid jets an bubbles are not observed when processing 50 um thick dies

t = 0 ms 125 ms100 ms75 ms50 ms25 ms

CLEANING OF DIES ON A CARRIER

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LIQUID SPLASHES AT INCREASED ROTATION SPEED

𝝎

Liquid accelerates Splash when hitting a dieDispense

▪ Multiple parameters play a role in splashing

(rotation speed, arm position, die

population, die thickness...)

▪ Splashes can contaminate the tool chamber

➢ Rotation speed to be limited when

processing thick dies to avoid splashing

▪ The entire chamber is wet▪ Splashes are visible below the nozzle ▪ Splashes reaches the shield

Increasing spin speed

25% die pop.

CLEANING OF DIES ON A CARRIER

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DIE SHIFT OR RELEASE DURING PROCESS

𝝎

Increasing centrifugal force

600 s strip▪ Initial shear strength >> centrifugal

force: die will not move in normal

conditions

▪ Local defects (particles, bubbles, etc.)

can lead to bond weakening

▪ Chemical compatibility with TBM can

also affect bond strength

▪ Dies can shift or lift off due to

centrifugal and hydrodynamic forces

acting on them (drying step is the most

critical, 𝜔 > 1000 rpm)

▪ Die shift or release will result in

bonding failure and possible tool damage

Initial die shear strength

Max. centrifugal force

CLEANING OF DIES ON A CARRIER

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NON-UNIFORM DRYING ON DIE SURFACE

▪ Liquid remains trapped on top of the dies even after extended DIW and spin-drying time due to

local low centrifugal forces

▪ Trenches between dies appears to be dry

▪ An IPA rinse combined with N2 blow is used to dry die top surface

Standard DIW rinse IPA rinse + N2 blow

WET CLEANING CHALLENGE SUMMARY

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1. Thick dies interacts strongly with the flow pattern

2. Splashes may occur when the wafer is populated with thick dies

3. Local defects may result in die shift under centrifugal and hydrodynamic forces

4. An IPA rinse/N2 blow combination is required to dry the populated wafer succesfully

TOWARDS WET PROCESS OPTIMIZATION

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IMPROVED PROCESS PARAMETERS

Thickness 775 um 775 um 775 um

Population 100 % 100 % 100 %

Dispense 3 min – central dispense 3 min – central dispense 3 min – tailored scan

Rot. speed Normal Low Low

Resist strip Incomplete, X-like pattern Incomplete, radial pattern Complete

TOWARDS WET PROCESS OPTIMIZATION

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CFD SIMULATIONS TO UNDERSTAND FLUID BEHAVIOR

𝛻 ⋅ 𝑣 = 0 (mass conservation)

𝑣 ⋅ 𝛻 𝑣 = −𝛻𝑝 +𝜇

𝜌𝛻2𝑣 (momentum conservation)

Navier-Stokes Equation: ▪ Expected liquid film thickness?

▪ Impact of dies on the flow pattern?

▪ ...

Improve the process with

minimized risk!Volume-of-fluid (VoF) scheme:

Thin film approximation:

V

X Computationally demanding

Complex 3D geometries

Detailed solution

V

XStrong hypothesis

Only valid for simple geometries

Fast computation▪ Reduced dimensionality: Film thickness 𝑡 is much

smaller than other dimensions

▪ Vertical velocity is neglected

▪ Pressure is mainly hydrostatic

▪ 2 distinct phases (air and liquid) such that

𝜙𝑎𝑖𝑟 + 𝜙𝑙𝑖𝑞 = 1

▪ Interface tracking scheme

▪ Identical velocity

P. Vita et al., THIN FILM FLOWSIMULATION ON A ROTATING DISC, ECCOMAS, 2012

SUMMARY

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1. Die singulation and transfer with a low defectivity level is required to enable successful

die-to-wafer bonding

2. Protective layer in combination with a dedicated clean is investigated as a route to yield

clean die surface

3. Thick dies reveal multiple challenges linked to liquid splashes, non-uniform process and

die drying

4. The interaction between liquid flow and dies is complex and requires tailored process

parameters to enable full protective layer strip

5. CFD numerical simulations offer the possibility to understand and further optimize the

process

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