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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.
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