Modeling of CMP

1
Modeling of CMP
David Dornfeld CMP researchers Jihong Choi,
Sunghoon Lee, Dr. Hyoungjae Kim, Dr. Dan
Echizenya Department of Mechanical
Engineering University of California Berkeley CA
94720-1740 http//lma.berkeley.edu
2
Overview

• Background on modeling
• Review of work to date
• Some new developments
• pattern/feature sensitivity
• pad design

3
New Book on Modeling Chemical Mechanical
Planarization (CMP) Integrated Modeling of
Chemical Mechanical Planarization for Sub-Micron
IC Fabrication From Particle Scale to Feature,
Die and Wafer Scales, J. Luo and D. A. Dornfeld
Written by researchers at UC-Berkeley, this
monograph reviews CMP modeling literature (from
Preston to present day efforts) and develops,
with a strong emphasis on mechanical elements of
CMP, an integrated model of CMP addressing
wafer,die and particle scale mechanisms and
features. Special emphasis is on abrasive sizes,
distributions and resulting material removal
rates and uniformity resulting over all
scales. 175 Figures and 14 tables ISBN
3-540-22369-x Springer-Verlag 2004
For information www.springeronline.com/east/3-54
0-22369-X.
Or contact dornfeld_at_berkeley.edu
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Chemical Mechanical Planarization

• CMP Team in FLCC
• Dornfeld, et al
• Doyle, et al
• Talbot, et al

Mechanical Phenomena
Chemical Phenomena
Interfacial and Colloid Phenomena
5
Scale Issues in CMP
From E. Hwang, 2004
6
CMP Process Schematic
F down force
Oscillation
w w wafer rotation
conditioner
Wafer Carrier
slurry feed
head
Retainer ring
Backing film
Wafer
table
Wall
Pore
pad
w p pad rotation
Pad
Abrasive particle
Electro plated diamond conditioner
Typical pad
7
An overview of CMP research in FLCC
Cu CMP
Bulk Cu CMP
Barrier polishing
W CMP
Oxide CMP
Poly-Si CMP
Bulk Cu slurry
Barrier slurry
W slurry
Oxide slurry
Poly-Si slurry
Abrasive type, size and concentration
Dornfeld model
Doyle
oxidizer, complexing agent, corrosion
inhibitor, pH
Talbot
Chemical reactions
Mechanical material removal mechanism in abrasive
scale
Pad asperity density/shape
Pad mechanical properties in abrasive scale
Physical models of material removal mechanism in
abrasive scale
Topography
Pattern
MIT model
Models of WIDNU
Pad properties in die scale
Slurry supply/ flow pattern in die scale
Pad design
Wafer scale pressure NU
Models of WIWNU
Wafer scale velocity profile
Fabrication
Fabrication technique
Wafer bending with zone pressures
Slurry supply/ flow pattern in wafer scale
Test
Pad groove
8
The 4-component system

• Hypotheses
• all polishing processes can be described as a 4
  component system
• Understanding the components and their
  interactions (pair-wise, triplets, etc) provides
  a structure to catalog our knowledge (and
  ignorance)

Granule? Deliberately sought a word that covers
the range of particles used without implying
anything about size, hardness, or removal
mechanism mm to nm size range from hard
(diamond) to soft (rouge)
Source 86. Evans, J., Paul, E., Dornfeld, D.,
Lucca, D., Byrne, G., Tricard, M., Klocke, F.,
Dambon, O., and Mullany, B., Material Removal
Mechanisms in Lapping and Polishing, STC G
Keynote, CIRP Annals, 52, 2, 2003.
9
Six possible pair-wise interactions

• Fluid-workpiece
• Workpiece-pad
• Workpiece-granule
• Granule-pad
• pad-fluid
• Fluid-granule

10
Three-way interactions (triplets)

• Workpiece-fluid-granule
• Workpiece-fluid-pad
• Workpiece-granule-pad
• Fluid-pad-granule

11
Stribeck Curve and Characteristics of slurry film
thickness
Slurry
Wafer
Direct contact
Film thickness
Polishing pad
Semi-direct contact
Hydroplane sliding
Direct contact
Elasto- hydrodynamic lubrication
Hydrodynamic lubrication
Friction coefficient
Semi-direct contact
Boundary lubrication
Hydroplane sliding
Stribeck curve
12
Gap effects on mechanics
Eroded surface by chemical reaction --- softening
Silicon wafer
Delaminated by brushing
Small gap
Abrasive particle
Polishing pad
Pad-based removal
Big gap
Slurry-based removal
13
Idealized CMP
Softened surface by chemical reaction
Silicon wafer
Abrasive particle
Polishing pad
Pad asperity
Mechanical Aspects of the Material Removal
Mechanism in Chemical Mechanical Polishing (CMP)
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Interactions between Input Variables
Four Interactions Wafer-Pad Interaction
Pad-Abrasive Interaction Wafer-Slurry Chemical
Interaction Wafer-Abrasive Interaction
Velocity V
Vol
Chemically Influenced Wafer Surface
Wafer
Abrasive particles on Contact area with number N
Abrasive particles in Fluid (All inactive)
Pad asperity
Polishing pad
Active abrasives on Contact area
Source J. Luo and D. Dornfeld, IEEE Trans
Semiconductor Manufacturing, 2001
15
Framework Connecting Input Parameters with
Material Removal Rate
Basic Equation of Material Removal MRR N ? Vol
N
Vol
?
g
Fraction of Active Abrasives X avg-a
Xavg
Force F Velocity
Slurry Abrasive Weight Concentration C
Active Abrasive Size Xavg-a
Fraction of Active Abrasive 1-?((g-Xavg)/ ?)
where g is the minimum size of active abrasives
Average Abrasive Size Xavg
Wafer Hardness Hw / Slurry Chemicals Wafer
Materials
Proportion of Active Abrasives
Down Pressure P0
Pad Topography Pad Material
Abrasive Size Distribution ?
16
Experimental Verification of Pressure Dependence
of Material Removal Rate (MRR)
MRR N Vol K1 1-?(1-K2P01/3)P01/2.
Advantage over Prestons Eq. MRR KePV MRR0
What input variables and how they influence Ke
is predicable
Ke1 (K184148, K2 0.137)
Ke2(K18989, K2 0.3698)
SiO2 CMP Experimental Data from Zhao and Shi,
Proceedings of VMIC, 1999
17
Abrasive Size Distribution Dependence of
MRR Particle Size Distribution 1
Five Different Kinds of Abrasive (Alumina) Size
Distributions for Tungsten CMP
() Frequency
Abrasive Size X (Log Scale)
1. Bielmann et. al., Electrochem. Letter, 1999
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Relationship between Standard Deviation and MRR
Based on Model Prediction
Size influenced
Std dev influenced
19
Pattern-Density Dependency Model
MRR
Up Area
K/density
Time
K
InterLevel Dielectric Case (single material)
Source MIT
Same Pattern Density Different Orientations
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Framework of a CMP Topography Evolution Model
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Dishing and Erosion in Copper Damascene Process
 

 

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Definition of Feature-Scale Topography

 
(a)
(b) (a) Feature scale
topography before dielectric material is exposed
and (b) feature scale topography after dielectric
material is exposed
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Models of Polishing Pad
Linear Elastic and Linear ViscoElastic Models
     
 
Separated Models of Pad Bulk and Asperities
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Three Stages of Wafer-Pad Contact
   
Df
 
SS0
S1Df1
Hcu0
HHcu0Hox0
H Hstage1
  2
  1  
    3
Erosion e
Dishing d
Hox0
Two different materials are removed simultaneously
Only upper part of step is in contact
Both upper and bottom parts of step is in contact
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Simulation Results of Step Height Evolution for
Different Pattern Density
Step height S (nm)
Step height S (nm)
Planarization time (sec)
Planarization time (sec)
Linear Elastic Pad
Linear Viscoelastic Pad
Wcu 100 microns
26
Copper Dishing as a Function of Pattern
Density using commercial pads
27
Copper Dishing as a Function of Selectivity
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Effect of Pattern Density - Planarization Length
(PL)
29
Modeling of pattern density effects in CMP
Planarization length (window size) effect on
Up area
30
Die scale modeling of topography evolution during
CMP
Contact wear model
Initial pressure distribution
MRR model
Iteration with time step
Topography evolution
Contact wear model
New pressure distribution
31
Feature level interaction between pad asperities
and pattern topography
PAD
Z(x,y)
Z_pad
Reference height (z0)
dz
Z(x,y)
Z_pad
z
32
Chip level interaction between pad and pattern
topography
MIT model approximation of contact wear model
40um
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Simulation result
100
50
33
20
50
33
20
34
Pattern orientation effect on on copper dishing
Kinetic analysis of sliding direction during
process time
pad rpm lt wafer rpm
35
Pad Characterization
(SEM, x150)
100µm
(White light Interferometer, x200)

• Ra 12.5µm
• Rz 96.7µm
• Pore diameter 3050 µm
• Peak to Peak 200300µm

45µm -45µm
100µm
300µm
500µm
200300µm
36
Pad modeling
37
3 Dimensional analysis
Reaction region
Transition region
Reservoir region
38
2D and 3D image of reaction region
2 dimensional image (w/o pressure)
3 dimensional image (w/o pressure)

• Contact area 10-50µm
• Ratio of real contact area 10-15

• Spherical or conical shape edge
• Stress concentration when compressed

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Reaction region ILD CMP
10 50 µm
Reaction region (asperity)
Defects of a conventional pad

• Over polishing on recess area
• Smoothing, not planarization

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Reaction region Cu CMP
Stress concentration
Cu-CMP defects (due to stress concentration in
conventional pad)
Pad asperity
wafer
wafer
Pressure
Fang
Erosion
Dishing
Avg. contact pressure
Nominal pressure
wafer
Position
41
Pad degradation
New
In 3minutes
In 5minutes
In 7minutes
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Design rules for a pad
Design rules for a pad Design rules for a pad Design rules for a pad
Macro scale Micro scale Nano scale
Stacked layer (Hard/soft) Slurry channel Constant contact area (width10-50um) The ratio of real contact area (13-17) Conditioning-less CMP High slurry efficiency Compatible features to abrasive Constant re-generation of nano scale surface roughness
43
A pad design based on the rules
Channel
Nano scale features
50-200µm
Hard Layer (i.e. high stiffness)
50-70µm
Soft Layer (i.e. low stiffness)
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Expectations
ILD CMP
Advantages
Pad

• Conditioning-less process
• High planarity good uniformity in ILD CMP
• Without stress concentration
• Less defects in Metal CMP

Wafer
Cu CMP
Pad
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Design of new pads
Type 1 Without slurry guidance
Type 2 With slurry guidance
50µm
Slurry flow direction
20µm
46
Simulation result
Type 2
Type 1

• Area 4.294-10 m2
• Flow rate 3.24-10 kg/sec

• Area 4.3-10 m2
• Flow rate 3.93-11 kg/sec

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Pad fabrication
New pad
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Performance of a new pad Planarity in ILD CMP
Polishing machine
Experiment condition
Pad IC1000/SUBA400 New pad New pad
Pad 60rpm 60rpm 60rpm
Wafer 3inch wafer (12-100 density,1.7µm SiO2) 3inch wafer (12-100 density,1.7µm SiO2) 3inch wafer (12-100 density,1.7µm SiO2)
Wafer 30rpm 30rpm 30rpm
Slurry D-7000 (Cabot Co.) D-7000 (Cabot Co.) D-7000 (Cabot Co.)
Slurry 100ml/min 100ml/min 100ml/min
Pressure 1.6psi 1.6psi 2.7psi
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Density 20 - under same pressure1.6psi
IC1000/SUBA400 (1.6psi)
New pad (1.6psi)

• Time 40minutes
• Over Polishing 400Å

• Time 17minutes
• Over Polishing 2200Å

Good planarity
High removal rate
50
Density 20 - under different pressure1.6psi
2.7psi
IC1000/SUBA400 (1.6psi)
New pad (2.7psi)

• Time 17minutes
• Over Polishing 2200Å

• Time 20minutes
• Over Polishing 800Å

Good planarity removal rate