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ERCERCERCParticle Science & Technology

Stability of Colloidal Suspensions for CMP Applications

Brij M. Moudgil Department of Materials Science and Engineering, and

Particle Engineering Research CenterUniversity of Florida, Gainesville, FL 32611, USA

Levitronix CMP Users’ ConferenceSanta Clara, CA

February 17, 2005


Applications of Nanoparticulate Dispersions

• Advanced structural ceramics• Controlled drug delivery systems• Microelectronics- abrasives for chemical mechanical polishing

• Coatings• Inks• Nanocomposite materials• Cosmetics (suntan lotions, creams,

toothpastes, etc)


CMP ProcessCMP Process

SlurryWafer Carrier

Platen

Polish Pad

WaferLoad

CMP requirementsHigh removal rate (~400nm/min)Global planarityLow roughness (<1nm)Selectivity (~100)Low corrosion

Three Main Components of CMPSurface to be polishedSlurryPolishing Pad


Surface Chemistry Challenges in Formulating Optimally Performing CMP Slurries

• Stability under extreme processing conditions

- normal forces 10 - 100 mN/m- shear rates >10,000 s-1

- reactive additivesK3Fe(CN)6, KIO3, H2O2, BTA- ionic strength > 0.1 M- pH > pH 10


How strongly do we need to disperse the CMP slurry ?


CMP-Agglomerates Affect Surface Morphology-- silica polished with 200 nm silica at pH 10.5 --

100

0

-100

nm

Baseline/Dispersed0.85 nm RMS25 nm MAX

100

0

-100

nm

Aggregated (hard)2.66 nm RMS65 nm MAX

• Even soft agglomerates may adversely impact surface roughness and defects during CMP. Robust dispersion is required

100

0

-100

nm

Flocculated (soft)1.44 nm RMS45 nm MAX

1000

-100

nm

Coagulated (soft)2.76 nm RMS120 nm MAX


Inorganic Dispersant- Sodium Silicate -

Sodium Silicate Dosage (mg/g)0 20 40 60 80 100

Turb

idity

(NTU

)

0

200

400

600

800

Stable Slurry Turbidity

200 nm SilicapH 4, 100 mM NaClafter 60 minutes

• For nanoparticles in high ionic strength solution, even large dosages of inorganic dispersant may not stabilize the suspension.


Polymeric Dispersant- Darvan C -

Electrolyte Concentration (M)0 1 2 3 4 5

Turb

idity

(NTU

)

0

100

200

300

400

500

600

700

800200 nm SilicapH 6; NaCl5 mg/g Darvan Cafter 60 minutes

• As ionic strength increases, polyelectrolyte dispersants such asDarvan C may become less effective.


5 M Salt

Electrolyte Concentration (M)0 1 2 3 4 5

Rel

ativ

e Tu

rbid

ity

0.0

0.2

0.4

0.6

0.8

1.0

200 nm silica pH 4with 32 mM of C12TABafter 60 minutes

Surfactants as Dispersants-- Nanoparticulate Stability --

• Even at 5 M NaCl the suspension is stable in the presenceof surfactants.


How do Surfactants act as Dispersants for CMP Slurries ?


Separation Distance (nm)0 5 10 15 20

Inte

ract

ion

Forc

e (n

N)

-1

0

1

2

3

4

5

32 mM C12TABNo Surfactant

AFM Tip/MicapH 4; 0.1 M NaClAggregates

Break

Rigidity of Self-Assembled Films

• Self-assembled ionic surfactant layers lead to a significant steric barrier.- Silica suspensions stabilized with C12TAB even in 5 M NaCl solutions.

A

A

BB

C

C


What is the source of particle-particle repulsion in the presence of surface active dispersants ?

Hemi-Micelle Formation

Self-Assembled Surface Aggregates

Steric repulsion by self-assembled surfactant aggregates on abrasive particles


What is the optimal concentration of dispersant ?


C12TAB Concentration (mM)0 10 20 30 40 50

Max

imum

Rep

ulsi

ve F

orce

(nN

)

0

1

2

3

Susp

ensi

on T

urbi

dity

(NTU

)

100

200

300

400

500

600

700

AFM Tip/SilicaSilica 200 nm

pH 4; 0.1 M NaClτ after 60 minutes

Correlation of Suspension Stability and Barrier Onset

• There is a correlation between the onset of suspensionstability and force barrier.


Optimal concentration is governed by steric forces

Below TransitionConcentration

Above TransitionConcentration


Polishing performance with surfactant dispersant

Silica ParticleSilica Particle Silica ParticleSilica Particle

0.6 M NaCl + 1mMC12TAB

+ 8mMC12TAB

+ 32mMC12TAB

Baseline0

1

2

3

4

5

Mea

n Pa

rticl

e Si

ze (µ

m)

Slurry Stability

♦ Adequate particle-particle repulsion is achieved with C12TAB for high ionic strength silica slurries leading to stable suspension.


Polishing with C12TAB Slurries

Material Removal RateMaterial Removal Rate

0

2000

4000

6000

Mat

eria

l Rem

oval

Rat

e(Å

/min

)

Baseline 0.6 M NaCl + 1mMC12TAB

+ 8mMC12TAB

+ 32mMC12TAB

♦ Slurries stabilized with C12TAB yield good surface quality but negligible material removal.

Max. Surface D

eformation

(nm)

0

20

40

60

80

100

120

140StableUnstable

0

2

4

6

8

10

12

14


+ 8mMC12TAB

+ 32mMC12TAB

RM

S Su

rfac

e R

o ugh

n ess

(n

m)

Surface QualitySurface Quality


PPARTICLEARTICLE-- SSUBSTRATEUBSTRATE

IINTERACTIONSNTERACTIONS

WaferWafer

PadPad


WaferWafer

Strength of Surfactant Structures

Adler J. J., Singh P.K., Patist A., Rabinovich Y.I., Shah D.O., Moudgil B.M., Langmuir, 16, p. 7255-7262 (2000).Singh, P. K., Adler, J. J., Rabinovich, Y. I., and Moudgil, B. M., Langmuir, 17, 468-473 (2000).

♦ Self-assembled surfactant aggregates introduce a repulsive force barrier.

How the repulsive force barrier affects material removal?

Possible Mechanisms Impacting Particle-Substrate Interactions

WaferWafer

Lubrication Effect

J. Klein, E. Kumacheva, D. Mahalu, D. Perahia and L. J. Fetters, Nature, 370, p.634, (1994).

♦ Surfactants result in lubrication between the abrasive and the surface to be polished and decrease the frictional force.

Surface lubrication will decrease the material removal.


Repulsive Force Barriers

Separation Distance (nm)

Forc

e/R

(mN

/m)

-2

0

2

4

6

8

-10

0

10

20

30

40

0 5 10 15 20

32 mM C12TAB + 0.6 M Salt

140 mM C8TAB + 0.6 M Salt68 mM C10TAB + 0.6 M Salt

Baseline W/O Salt

Baseline W 0.6 M Salt

pH=10.5

1.5µm particle-wafer

Force per 0.2 µm Particle (nN

)

Barrier: Baseline W/O salt, C10TAB, C12TAB (Stable)

No Barrier: Baseline W salt, C8TAB (Unstable)

♦ Stable slurries were obtained for the systems with repulsive barriers.


Polishing with C8TAB Slurries

10

20

30

40

50

60

Max. Surface D

eformation

(nm)

Surface QualitySurface Quality

0

1

2

3

4

5

6

RM

S S u

rfac

e R

o ugh

n ess

(nm

)


+ 35mMC8TAB

+ 140mMC8TAB

UnstableUnstable

Material Removal RateMaterial Removal Rate10000

Mat

eria

l Rem

oval

Rat

e(Å

/min

)

0

2000

4000

6000

8000


+ 35mMC8TAB

+ 140mMC8TAB

♦ Magnitude of repulsive force barrier controls polishing. Weak particle-substrate, particle-particle repulsion yields high material removal but induces defects.


Stability & Material Removal Rate

Parti

cle

Size

(µm

)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

+ 0.6 M NaCl& 140mM

C8TAB

+ 0.6 M NaCl& 68mM C10TAB

+ 0.6 M NaCl& 32mM C12TAB

Unstable Stable

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

Materia l R

e mova l R

a te (Å/m

in )

Particle size

Material removal rate

♦ Slurry stability is directly correlated to the repulsive force barrier.

♦ To correlate the material removal rate response, normal force per particle must be known.


Particle-Surface Interactions with C8TAB, C10TAB, C12TAB

Normal force per particleNormal force per particle

??7 PSI

Silica Wafer

Silica Particle Silica Particle

Repulsive force barrier (nN/0.2µm)C8TAB C10TAB C12TAB

0 2.2 6.0

♦ Normal force per abrasive particle needs to be estimated to determine the effect of surfactants on pad-particle-substrate interactions.


Particle-Surface Interactions with C8TAB, C10TAB, C12TAB

Normal force per 0.2µm particle

750 750 ±± 150150 nN [41.7 x 10[41.7 x 1066 particles/inchparticles/inch22]nN ]

Repulsive force barrier (nN)C8TAB C10TAB C12TAB

0 2.2 6.0

Silica Wafer

Silica ParticleSilica Particle

♦ The repulsive force barrier is overcome by the applied normal force per particle.


WaferWafer

Strength of Surfactant Structures

♦ Self-assembled surfactant aggregates introduce a repulsive force barrier.

How the repulsive force barrier affects material removal?

Possible Mechanisms Impacting Particle-Substrate Interactions

WaferWafer

Lubrication Effect

♦ Surfactants result in lubrication between the abrasive and the surface to be polished and decrease the frictional force.

Surface lubrication will decrease the material removal.

♦ Lubrication provided by the surfactants controls the material removal.


Lateral Atomic Force Microscopy

FL~ϕ ~(C-D)

ϕ

FL

DCA

B

C - D

0

Twis

t (V

)

Distance (µm)


Effect of Surfactant Addition on Frictional Forces

Material Removal Rate(Å/min)

Loading Force (nN)

Late

ral F

orce

(nN

)

0

50

100

150

200

0 300 600 900 1200 1500

Base Line140 mM C8TAB68 mM C10TAB32 mM C12TAB

pH 10.5

(Without Salt)Baseline 4300 ± 290

C8TAB(140 mM) 61 ± 36

C10TAB(68 mM) 53 ± 33

WaferWafer

C12TAB(32 mM) 56 ± 46

♦ In the absence of salt surfactants form lubrication layers leading to minimal friction force and material removal.


Effect of Surfactant & Salt Addition on Frictional Forces

Material Removal Rate(Å/min)(With Salt)

0

50

100

150

200

0 300 600 900 1200 1500Loading Force (nN)

Late

ral F

orce

(nN

)

Base Line 140 mM C8TAB+0.6M NaCl68 mM C10TAB+0.6M NaCl32 mM C12TAB+0.6M NaCl

pH 10.5

Baseline 7058 ± 302*

C8TAB(140 mM) 6167 ± 864*

C10TAB(68 mM) 650 ± 187

♦ Addition of salt results in loosely adhered surfactant layers and more friction between the particle and the substrate.

WaferWafer⊕ ⊕ ⊕ ⊕⊕

C12TAB(32 mM) 66 ± 27

* Unstable Slurries


Manipulation of the Frictional Forces: Effect of Bivalent SaltManipulation of the Frictional Forces: Effect of Bivalent SaltSlurry Stability & Material Removal Rate with NaCl and CaCl2

010002000300040005000600070008000900010000 M

ateria l Re m

o v al Rate (Å

/mi n)

Material removal rate

Baseline Salt Salt + 32 mM C12TAB

00.20.40.60.81.01.21.41.61.82.0

Parti

cle

Size

(µm

)Baseline0.6 M NaCl0.24 M CaCl2

Particle size

♦ Bivalent CaCl2 salt resulted in material removal even in the presence of 32 mMC12TAB surfactant and yielded acceptable surface quality.

Surface Quality(CaCl2)

RMS

Rmax

0.8 ± 0.4

253.10 ± 1.39

60

0.47 ± 0.7

16


Can we achieve selectivity in planarization with surfactants ?


Selectivity inSelectivity in PlanarizationPlanarization: Research Approach: Research Approach▶▶ Design ConceptDesign Concept

SiO2

Si3N4

Si

Si3N4passivation layer

•• Passivation of SiPassivation of Si33NN44 by by Selective Surfactant AdsorptionSelective Surfactant Adsorption•• Lower DefectivityLower Defectivity by SiOby SiO22 abrasives than CeOabrasives than CeO22


▶▶ Influence of SodiumInfluence of Sodium DodecylDodecyl Sulfate (SDS) Sulfate (SDS) Addition on MRR and SelectivityAddition on MRR and Selectivity

1 2 3 4 5 6 7 8 9 10 11 12

0200400600800

10001200140016001800200022002400260028003000

2 4 6 8 10 12

0

400

800

1200

1600

2000

2400

2800 (a)

pH

with SDS MRR SiO2 MRR Si3N4

MRR SiO2 MRR Si3N4

MR

R (A

/min

)

1 2 3 4 5 6 7 8 9 10 11 12 130

5

10

15

20

25

30

2 4 6 8 10 121 2 3 4 5 6 7 8 9 10 11 12 13

Selectivity

Sele

ctiv

itypH

Selectivity, with SDS

•• Selectivity increase in entire pH range: More SDS on SiSelectivity increase in entire pH range: More SDS on Si33NN44•• At pH 2, MRR of SiAt pH 2, MRR of Si33NN4 4 minimized: Maximum selectivity more than minimized: Maximum selectivity more than 2525•• Below pH 5, MRR of SiBelow pH 5, MRR of Si33NN4 4 steeply decreases: Conc. of adsorbed SDS steeply decreases: Conc. of adsorbed SDS increaseincrease PassivationPassivation layer on Silayer on Si33NN4 4 by SDSby SDS


▶▶ Roughness by AFM Roughness by AFM

Si3N4pH 2 + SDS pH 2 pH 3.04 pH 4.8 pH 7.98 pH 10.4 pH 11.50.0

0.5

1.0

1.5

2.0

Si3N4 wafer, CMP with Klebosol 12 wt%

Rou

ghne

ss (n

m)

RMS Rmax

SiO2 pH 2 + SDS pH 2 pH 4.81 pH 7.98 pH 10.4 pH 11.50.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Rou

ghne

ss (n

m)

SiO2 wafer, CMP with Klebosol 12 wt%

RMS Rmax

•• Roughness improves at lower pH Roughness improves at lower pH •• Surfactant addition improves RoughnessSurfactant addition improves Roughness•• At pH 11.5: HighAt pH 11.5: High defectivitydefectivity for for SiOSiO22 due to dissolution, not for due to dissolution, not for SiSi33NN44


What are the design parameters for optimal slurry performance?


Optimization of Slurry Particulate PropertiesOptimization of Slurry Particulate Properties

Central Composite DesignCentral Composite Design

Design variables: -1.68 -1 0 +1 +1.68

♦ Particle Size (µm) 0.2 0.3 0.5 0.8 1.0

♦ Solids Concentration (wt%) 0.5 6 15 24 30

♦ Applied Download (N) 34 54 74 94 114

Analyzed Responses

♦ Material Removal Rate > 2000 (Å/min)

♦ Surface Roughness (RMS) < 1.5 (nm)

♦ Maximum Depth of Scratches or Pits < 40 (nm)


Optimization ResultsOptimization Results54.00 N Applied load

0.20 0.40 0.60 0.80 1.000.00

7.50

15.00

22.50

30.00

Particle Size (µm)

Solid

s Con

cent

ratio

n (%

wt)

MRR: 2000

RMS: 1.5RMS: 1.5

Rmax: 40

Solid

s Con

cent

ratio

n (%

wt)

Particle Size (µm)0.20 0.40 0.60 0.80 1.00

0.00

7.50

15.00

22.50

30.00RMS: 1.5

MRR: 2000

RMS: 1.5

Rmax: 40

94.00 N Applied load

Material removal rate (MRR) is less than 2000 Å/min.

Surface roughness (RMS) is higher than 1.5 nm.Maximum surface deformation is higher than 40 nm.

♦ Low applied load regime limits the material removal, whereas at high loads surface quality becomes the limiting response.

Optimal operation regime.


Optimal CMP ConditionsOptimal CMP Conditions76.19 N Applied Load

Particle Size (µm)

Solid

s Con

cent

ratio

n (%

wt)

0.20 0.40 0.60 0.80 1.000.00

7.50

15.00

22.50

30.00

MRR: 2000

RMS: 1.5

RMS: 1.5

Rmax: 40

♦ For optimal polishing response small particle size, medium solids concentration slurries must be used at medium pressure range.


Slurry Design CriteriaSlurry Design CriteriaParticle Size Distribution

Monosized Slurry

Acceptable

Surface Quality

CMP

Material Removal

Optimal Polishing

Performance

Hard Agglomerates

Filtration

Soft Agglomerates

Dispersion/Stabilization

Interparticle Repulsion

Frictional Interactions(Salt addition,

surfactant chain length)

No Material Removal

Optimize Slurry Particulate Properties(size, concentration)

No Material Removal

Unacceptable


ConclusionsConclusions

♦♦ Monosized slurries must be used for CMP applications by avoidingMonosized slurries must be used for CMP applications by avoidingthe hard and soft agglomerates.the hard and soft agglomerates.

♦♦ In developing robust dispersion schemes for CMP slurries:In developing robust dispersion schemes for CMP slurries:♦ Control of particle-particle interactions is necessary but not sufficient

♦ Pad-particle-substrate interactions must also be controlled by;

♦ Manipulating the frictional interactions by means of surfactant adsorption/desorption

♦ Surfactant type♦ Chain length♦ Slurry ionic strength♦ Valency of added salt


♦♦ Particle size, slurry solids concentration and applied head loadParticle size, slurry solids concentration and applied head loadgovern frictional interactions in the system (abrasive sliding ogovern frictional interactions in the system (abrasive sliding or r rolling)rolling)

♦♦ Optimization of slurry characteristics lead to enhanced CMP Optimization of slurry characteristics lead to enhanced CMP performanceperformance

♦ Smaller particle size♦ Medium solids concentration♦ Medium applied load


Acknowledgments

The authors would like to acknowledge the financial support of the Particle Engineering Research Center (PERC) at the University of Florida, The National Science Foundation (NSF) grant #EEC-94-02989, and the Industrial Partners of the ERC.

brij m. moudgil · l r e m o v a l r a t e (Å / m i n) 0 2000 4000 6000 8000. baseline 0.6 m nacl...

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