Pump Induced Slurry Handling Effects on CMP Performance

Rajiv K. Singh, F.C. Chang and S. Tanawade

Materials Science and EngineeringParticle Science Engineering CenterUniversity of Florida, Gainesville FL

OutlineIntroductionCMPOversize Distribution (OSD) and DefectivityExperimental DesignResults and Discussion

Interparticle Force and Particle AgglomerationEffect of pH, Ionic Strength and Particle SizeCorrelation Defectivity- Oversize

Conclusions

CMP of Metals and DielectricsSlurry (mixture of chemicals and particles) is a key enabling technology for metal and dielectric CMP.

Chemical action from aqueous media (sometimes abrasives)Mechanical action from abrasivesCMP regime: increased removal rate and better surface at lower pressures (vs mechanical regime)

ωs

ωp

slurry feed

polishing pad platen

wafer

pressure

Carrierωs

ωp

slurry feed

polishing pad platen

wafer

pressure

Carrier Removal Rate

Incr

ease

Dec

reas

e

Surface Finish

PoorEnhanced

Mechanical Chemical

Chemo-Mechanical

RegimeRemoval Rate

Incr

ease

Dec

reas

e

Surface Finish

PoorEnhanced

Mechanical Chemical

Chemo-Mechanical

Regime

CMP : Polishing MechanismFormation of chemically modified surface layerMechanical removal of this modified layer

Åto

nm

Chemically modified surface layer

Substrate

Thin passivating layer results in lower defectivity and planarization

CMP SlurriesMixture of Particles and ChemicalsTypical Slurry Composition

ParticlesType (silica, alumina, ceria)Size (30 - 200 nm)Size distribution (monosize desirable)ChemistrypH (acidic – basic)Oxidizer (H2O2)Other additives (surfactants, salts)Slurries are distributed by pumpsIssues

- Particle Stability- Particle Size Distribution

Dilution &

mixing station

Day Tank

Slurry Distribution System

CMP Performance – Slurry DependentParticle Characteristics• Size• Size Distribution• Shape• Mechanical Prop. • Chemistry• Dispersability• Concentration

Chemistry• Slurry Chemicals• pH

Down Pressure & Velocity

Pad Characteristics• Mechanical Prop.• Topography

Substrate• Mechanical Prop.• Feature size & density

ωs

ωp

slurry feed

polishing pad platen

wafer

pressure

Carrierωs

ωp

slurry feed

polishing pad platen

wafer

pressure

Carrier

OUTPUT PARAMETERS

Removal ratePlanarizationSurface finishSelectivity

Particle Size and Size DistributionParticle size and size distribution should not change significantlySilica relatively stable over a wide range of pH (2-10) and salt concentration (1mM to 300mM)Very low concentration of oversize tail affects defectivity during polish.

50 100 150 200 250 300-2

0

2

4

6

8

10

12

14

16

pH 10.4 pH 2.03

Diff

eren

tial V

olum

e %

Particle size (nm)0.1 1

0

2

4

6

8

10

12

14

16

Klebosol 12 wt%, pH 10.4

No Salt 0.1 M NaCl 0.3 M NaCl 0.5 M NaCl

Diff

eren

tial V

olum

e (%

)

Particle Size (µm)

OSD OSD

Oversize Distribution

Particle Size

Defectivity

Con

cent

ratio

n

Polishing Rate

Affects defectivity during polishing (even at ppm levels)Oversize tail distribution is not constantChanges during slurry handling

1 100

10,000

20,000

30,000

40,000

50,000

60,000

70,000 0 turnover 500 turnovers 1000 turnovers

Particle Size (µm)

Bellows Pump5wt% Silica / pH 7

Cum

ulat

ive

Con

c(#

/ m

l)

Pump Induced EffectspH 2

5% Silica

Pump Induced Oversize Distribution (OSD)

CentrifugalPump

Positive Displacement Pumps

Commercial low K slurries

0

2

4

6

8

10

12

0 2 4 6 8 10 12

BellowsDiaphragmCentrifugal

R2= 0.90

0

2

4

6

1 1.5 2 2.5 3 3.5normalized oversize particles

scra

tch

den

sity

#/s

q.m

m

Centrifugal pumps significantly reduce OSD effectsReduced defectivity (scratches) in low K polishing

Centrifugal

Positive Displacement

Centrifugal

Positive Displacement

Key Issue – Dynamic Changes in OSDWill all slurries (pH or salt) show similar particle agglomeration response during slurry handling?How do pumping characteristics affect slurry agglomeration ?What are the fundamental issues in slurry design that control the agglomeration characteristics?Is there a direct correlation between oversize and CMP defectivity under all slurry composition conditions?

Dilution &

mixing station

Day Tank

ObjectivesParticle Agglomeration During Slurry Handling with Pump

Role of Slurry Chemistry and Particle SizeRole of pumps (Centrifugal vs Positive Displacement)

Polishing Defects Due to Slurry HandlingFundamentals/ Mechanisms of Particle Agglomeration

Slurry Engineering IssuesSlurries possess a wide range of characteristicsPump Induced Agglomeration (OSD)

External Forces (Pump Characteristics)Repulsive Forces (Slurry Parameters)

Slurry DesignParticle Type Silica (STI, copper, low K, W)Particle Size (30 – 100 nm)Ionic Strength (1 mM – 200 mM)

Interparticle Forces

Electrostatic Force

0

0.02

0.04

0.06

0.08

0.1

0 2 4 6 8 10 12pH

F/R

(mN

/m)

Electrical Double Layer

–

- ------ -

--⊕- -

-⊕

⊕⊕

⊕⊕⊕

⊕ ⊕

⊕⊕⊕

⊕

⊕⊕

⊕⊕

⊕

⊕

⊕

–

–

–

–

–

–

–

⊕

⊕⊕

⊕

⊕

⊕

⊕

⊕

⊕

⊕

-- ------

--⊕- -

-⊕

⊕⊕

⊕⊕⊕

⊕ ⊕

⊕⊕⊕

⊕

⊕⊕

⊕⊕

⊕

⊕

⊕

–

–

–

–

–

–

–

⊕

⊕

⊕

⊕

⊕

⊕

⊕

⊕

⊕

⊕

–

Stern PlaneShear Plane

Interparticle Distance

)exp(/ 2 DRF κκψ −∝∝κ (Ionic Strength)1/2

DZeta Potential

ψ = Surface Potential1/k = Debye Layer

Electrostatic Force

00.010.020.030.040.050.060.070.080.09

0.1

1 10 100 1000Monovalent salt Concentration (mM)

F/R

(mN

/m)

Experimental Protocols1. Slurry variation

pH 2 – 11Size (30 – 60 nm)Ionic Size ( 10-3 M to 0.1 M)

2. Subject slurries via: turnovers in a slurry loopFlow Rate ~ 12 lit/minTurnovers ~ up to 1000Pump ~ Centrifugal and Positive Displacement pumps

3. Conduct CMP ExperimentsIC1000 pad.Low K BD1 substratePressure 1.5 – 3 psiTime (1 minute), 0.6 m/sec

4. Conduct Metrology (Size, OSD, Defectivity)5. AFM Force Measurements

Atomic Force Microscope

Cantilever With Probe Tip

PositionSensitive Detector

Laser

PiezoelectricCrystal

Topographic ImagesForce Measurements:

Silica slurry particle:2R= 1.5 µm

Typical Oversize Distribution with turnovers

Oversize particles significantly increases at low pH values

1 100

20,000

40,000

60,000

80,000

100,000

120,000

140,000

160,000 0 turnover 500 turnovers 1000 turnovers

Cum

m C

onc.

(#/m

l)

Particle Size (µm)

Bellows PumppH-2

5% 30nm silica

1 100

20,000

40,000

60,000

80,000

100,000

120,000

140,000

160,000 0 turnover 500 turnovers 1000 turnovers

Cum

m C

onc.

(#/m

l)

Particle Size (µm)

Bellows PumppH-11

5% 30nm silica

OSD vs Slurry Parameters

Low pH and salt addition significantly enhances particle agglomeration

5 100

5

10

15

20

25 pH 2 pH 7 pH 11

C10

00/C

0 tur

nove

rs

Particle Size (µm)

Bellows Pump5% 30nm silica

5 100

5

10 pH 11 100 mM KCl

C10

00/C

0 tu

rnov

ers

Particle Size (µm)

5% 30nm silica

pH Salt

pH-2

pH-7

pH-11

100mM KCl

0mM KCl

OSD Vs Particle Size

0 5 10

10

20

30

40

50

60

70 30 nm 30 nm with 0.1M KCl 60nm 60 nm with 0.1M KCl

C10

00/C

0 tu

rnov

ers

Particle Size (µm)

1000 turnovers- pH 11

Large particle size cause higher particle agglomeration

60nm-100mM KCl

30nm-100mM KCl

0mM KCl

Measurement of Interparticle Force

HAaVA 12

−=

a

wafer

Cantilever

• Interparticle Potential Energy

• Particle- Plane Surface Potential Energy

HAaVS 6

−=

H

HSEM of 5 micron size silica attached to AFM tip

Inter-particle Force Measurements

Silica-Silica Interparticle Forces

-1

0

1

2

3

4

5

0 5 10 15 20 25 30 35 40

Separation Distance (nm)

F/R

(mN

/m) pH 11

pH 7pH 2

Interparticle force increases with pH

pH 11

pH 2pH 7

Particles Agglomeration Vs pH

0

1

2

3

4

5

6

7

8

9

10

pH 2 pH 7 pH 11

C10

00/C

0 tur

nove

rs

Normalized OversizeParticles

02468

10

pH 2 pH 7 pH 11

C10

00/C

0 tur

nove

rs

0246810

F/R

(mN

/m)

Normalized OversizeParticlesInterparticle Forces

Particle agglomeration increases as pH decreases

Particle agglomeration is higher for lower interparticle forces

1 2 3 4 5 6 7 8 9 100

5

10

15

20

25

30

35

40

Centrifugal Bellows

C10

00/C

0 tu

rnov

ers

Particle Size (µm)

5wt% Silica / pH 2

1 2 3 4 5 6 7 8 9 100

5

10

15

20

25

30

35

40

Centrifugal Bellows

C10

00/C

0 tu

rnov

ers

Particle Size (µm)

5wt% Silica pH 2/ 0.1M KCl

Positive Displacement vs Centrifugal Pumps

Centrifugal Pump causes significantly less particle agglomeration

Polishing Defectivity – Positive Displacement Pump

Bellows-pH 2

0

5

10

15

20

25

30

As-received slurry 1000 turnovers 1000 turnovers- 0.1MKCl

C10

00/C

0 tur

nove

rs

Normalizedoversize particles

1000 turnovers 1000 turnovers- 0.1M KCl

Positive displacement pump significantly increases in defectivity

As-received slurry

Defectivity- Positive Displacement Pumps

Bellows-pH 2

0

0.5

1

1.5

2

2.5

0 5 10 15 20 25 30Normalized Oversized Particles

RM

S (n

m)

0510152025303540

Rm

ax (n

m)

RMSRmax

•As oversize particle distribution increases, Rmax and RMS increase.

•Defectivity higher as lower pH and high salt concentration.

Bellows-pH 11

00.20.40.60.8

11.21.41.61.8

0 5 10 15 20 25Normalized Oversized Particles

RM

S (n

m)

0

5

10

15

20

25

30

35

Rm

ax (n

m)

RMSRmax

Polishing Defects – Centrifugal Pump (pH 11)

Centrifugal-pH 11

0

2

4

6

8

As-received slurry 1000 turnovers 1000 turnovers-0.1M KCl

C10

00/C

0 tur

nove

rs

Normalizedoversize particles

As-received slurry

1000 turnovers 1000 turnovers-0.1M KCl

Centrifugal pump does not increase polishing defectivity

Defectivity – Centrifugal Pumps ( pH 2)

Centrifugal-pH 2

00.10.20.30.40.50.60.70.8

0 1 2 3 4 5 6 7 8 9Normalized Oversize Particles

RM

S (n

m)

0

5

10

15

20

25

30

35

Rm

ax (n

m)

RMSRmax

Centrifugal pump does not increase polishing defectivity

No Handling

Oversize Particles-pH 2

050,000

100,000150,000200,000250,000300,000350,000400,000450,000500,000

pH 2 pH 2 with 0.1M KCl

Cum

Con

c. (#

/ml)

BellowsCentrifugal

RMS-pH 2

0

0.5

1

1.5

2

2.5

pH 2 pH 2 with 0.1M KCl

RM

S (n

m)

BellowsCentrifugal

Rmax-pH 2

05

10152025303540

pH 2 pH 2 with 0.1M KCl

Rm

ax (n

m)

BellowsCentrifugal

Centrifugal vs Positive Displacement Pumps (Acidic Slurries)

Centrifugal pump causes significantly less particle agglomeration and lower defectivity (surface roughness) at pH 2

Oversize Particle-pH 11

050,000

100,000150,000200,000250,000300,000350,000400,000

pH 11 pH 11 with 0.1M KCl

Cum

Con

c. (#

/ml)

BellowsCentrifugal

RMS-pH 11

00.20.40.60.8

11.21.41.61.8

pH 11 pH 11 with 0.1M KCl

RM

S (n

m)

BellowsCentrifugal

Rmax-pH 11

0

5

10

15

20

25

30

35

pH 11 pH 11 with 0.1M KCl

Rm

ax (n

m)

BellowsCentrifugal

Centrifugal pump results in significantly less OSD and lower defectivity (surface roughness) at pH 11

Centrifugal vs Positive Displacement Pumps (Basic Slurries)

Conclusions

Slurry composition plays a critical role in increase in oversize fraction during slurry handling with conventional positive displacement pumps.

OSD increases under acidic pH conditionHigh ionic strengthLarger particle size

Centrifugal pumps significantly reduced oversize particle formation under all slurry conditions.The increased number of oversize particles correlates to higher defectivity during CMP polishing of low k dielectrics.