Wafer Bonding using Dielectric Glues for 3D IC Technology

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Wafer Bonding using Dielectric Glues for 3D IC
Technology
Yongchai Kwon, Jian-Qiang Lu, Russell P. Kraft,
John F. McDonald, Ronald J. Gutmann and Timothy
S. Cale
Rensselaer Polytechnic Institute
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Desired Properties of Bonding Glue

• Wafer bonding of fully processed 200 mm wafers
  requires
• intimate contact over the entire wafer
• compatibility with both conventional BEOL
  processes and subsequent thinning and inter-chip
  interconnection
• Approach use of a dielectric "glue" layer with
  the following desired properties

• Good adhesion and cohesion
• - Substrate to glue (adhesive)
• - Glue to glue (cohesive)
• No outgassing during bonding process
• Stability over range of BEOL processing
  conditions
• - high glass transition temperature
• - rigid structure after bonding
• CTE of glue well-matched to that of substrate
• Low stress relaxation and creep
• Low moisture uptake

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Glue Materials and Process

• Low-k materials under evaluation
• - Spin-on amorphous polymers
• Coating and baking with FlexiFabTM
  system
• ? FlareTM (Poly arylene ether)
• ? Accuspin T18 (MSQ Methylsilsesquioxan
  e)
• ? FOX (HSQ Hydrogensilsesquioxane)
• ? CycloteneTM ( DVS-BCB
  Divinylsiloxane-Benzocyclobutene)
• - Vapor-deposited polymer
• Deposition with SCS PDS 1050 system
• ? Parylene-N
• All bonding (curing) processes were carried out
  with a
• EVG 501 bonder system

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EVG 501 Bonder and Cross-Sectional View
Pneumatic cylinder
Uniforce Compliant Membrane
Top and bottom heater
Key programmable parameters - Temperature,
Pressure and Time - Temperature ramp-up and
cool-down speeds - Vacuum 10-4 mbar or inert
gas
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Main Parameters of Bonding Process Using Spin-On
Polymers
Two wafers with glue layers are placed
glue-to-glue, separated by three flags

• Pre-cleaning
• Spin-on recipes
• Baking process to release solvent
• ? Baking temperature
• ? Baking time
• ? Flow of N2 gas
• Bonding(curing) process
• ? Temperature ramp up speed
• ? Final cure temperature
• ? Final cure time
• ? Applied pressure
• ? Pressure vs. time protocol
• ? Cool down speed

Uniforce Compliant Membrane
(a)
Down Force
Top Wafer
Bottom Wafer
Bottom Chuck
Flag
Down Pressure
(b)
Top Wafer
Bottom Wafer
Bottom Chuck
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Bonding Process with Dielectric Glues
Pressure
Polymer resin
Substrate
Flags
Glue interdiffusion
Fully crosslinked glue
After alignment

• Before bonding, two wafers are separated by flags
  on the wafer edge
• During bonding process, glue polymers on each
  wafer interdiffuse when held near the glass
  transition temperature
• Pressure enhances the polymer interdiffusion and
  crosslinking

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FTIR Result of Flare
C-H
C-H

• Solvent (Cyclohexanone) peak (C-H and CO bond)
  disappeared after bake
• No peak difference between after baking and
  after curing of a flare deposited on a prime Si
  Wafer

CO
CO
? No outgassing after final bake
Wavenumbers (cm-1)
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Wafer Bonding Result Using Flare
Large clean void-free bonded area
Small voids may be caused by particles and/or
defects.

• Bonded wafer pair (200mm)
• Si wafer P(100) prime wafer
• Glass wafer Corning 7740

Sufficient for fabrication of via chain test
structure
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Bonding Results Using Other Glues
(Corning glass to prime Si wafer)
Possible void source with MSSQ Outgassing
Void
Possible void source with Parylene-N
Small amount of interdiffused polymers

Parylene-N
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Possible Void Sources with Flare
Polymer structural defect
Particle

• Bonded wafer pair (200mm)
• Si wafer P(100) prime wafer
• Glass wafer Corning 7740

100 ?m
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Cross-Section of Bonded Wafers Using Flare
Glass
Glass
Flare
Flare
Si
Si
SEM micrograph Good adhesion without void
Section of bonded wafer pair
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Bonded Wafer Interface
Void
Non-bonding area
Glass
Glass
Flare
Si
Si
Flare
Cross-section view of Si to glass bonding
Top view of Si to glass bonding
SEM micrographs
Glass
Glass
Glass
Flare
Flare
Si
Si
Flare
Bonding area
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Initial Bonding Integrity Results with Flare

• Objective
• Bond integrity inspection by monitoring
• the change of the number and size of voids
• Methods
• - Thermal Cycling Test
• ? Heating/cooling test of bonded wafer
• (50 oC/min. ramp up, 60 oC/min. cool
  down)
• ? Good bond integrity after thermal
• cycling test
• - Wafer Thinning
• ? Grinding of 200 mm Si wafer in bonded
• wafer pair to 60 ?m thickness
• ? Good bond integrity after silicon wafer
• thinning to 60 ?m

Before cycling
After cycling
After grinding
Before grinding
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Summary and Conclusions

• Dielectric glues for wafer bonding provide high
  bond strength, high bonding yield and
  compatibility with BEOL processes
• Flare has superior bonding properties compared
  to other glues studied to date
• Key parameters of wafer bonding process include
  pre-cleaning, applied pressure and holding time,
  and ramp-up/cool-down speed
• Bonding defects need to be reduced, but current
  process capability is adequate for
  first-generation via-chain test structure

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Future Work

• Reduction of defect density of bonded wafer with
  Flare
• Bonding of damascene patterned wafers for
  via-chain test structure
• Wafer bonding using other dielectric glues (e.g.
  BCB)
• Quantitative evaluation of bonding strength
• Evaluation of bonding integrity including thermal
  and thermomechanical stability