Clarkson University

Abrasive-free Copper Chemical
Mechanical Polishing in an
Orbital Polisher
Qingjun Qin
Advisor: Professor R. Shankar Subramanian
Center for Advanced Materials Processing (CAMP)
Department of Chemical & Biomolecular Engineering
Clarkson University
Potsdam, New York 13699
Outline
¾Objective
¾Background
¾Experimental Work
¾Model development
¾Results and Discussion
¾Conclusions
¾Suggested future work
¾Acknowledgements
Objective
¾ To obtain a fundamental understanding of abrasive-free copper
Chemical Mechanical Polishing in an orbital tool from both
experimental and theoretical perspectives
¾ To develop a model accommodating slurry fluid mechanics, chemical
reaction at the wafer surface, and mechanical removal by pad
asperities to predict removal rate and radial non-uniformity of
removal rates on the wafer surface
¾ To compare predicted removal rate and WIWNU with experimental
data obtained from a SpeedFam-IPEC 676
Background
Special features of orbital tool
Axis of wafer rotation
Wafer
Pad
Center line of
platen/pad
Orbit circle of the
pad center
Wafer
Pad
Platen
Slurry Flow
Reference: Oliver et al. (2004)
Experimental Work
Experiment conditions
¾ Slurry (liquid) composition:
Hydrogen Peroxide - 5.0 wt%; Glycine - 0.2, 0.5, and 1.0 wt%; pH=5.5
¾ Slurry Flow Rate: 200 ml/min
¾ Pad: IC1000 (XY grooved hard pad)
¾ Pressure: 4 psi
¾ Pad Orbit Speed: 100, 200, and 300 RPM
¾ Sample: blanket copper wafer
¾ Polisher: SpeedFam-IPEC AvantGaard 676 CMP System
¾ Cu film thickness measurement: Prometrix Omni RS 35E
Experimental Work
Glycine conc. & pad orbit speed Impact
300
250
200
400
Removal Rate (nm/min)
Removal Rate (nm/min)
350
Slurry Flow Rate = 200ml/min
Pressure = 4 psi
H2O2 Concentration = 5.0 wt%
Pad Orbit Speed (rpm)
100
200
300
150
100
50
0
0.00
0.20
0.40
0.60
0.80
1.00
Glycine Concentration (wt%)
1.20
Flow Rate=200 ml/min
Pressure = 4 psi
Hydrogen peroxide Conc. = 5.0 wt%
Glycine Conc. (wt%)
0.2
0.5
1.0
350
300
250
200
150
100
50
0
0
50
100
150
200
250
300
350
400
Pad Orbit Speed (RPM)
The removal rate increases with both glycine concentration and pad orbit
speed
Experimental Work
500
500
200 ml/min
1.0 wt% Glycine
300 RPM
400
300
Wafer Center
First Circle
Second Circle
Third Circle
200
100
0
1
4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49
Positions
Removal Rate (nm/min)
Removal Rate (nm/min)
Removal rate on radial positions
Orbit Speed (RPM)
100
200
300
400
200 ml/min
1.0 wt% Glycine
4 psi
300
200
Edge
100
0
0
5
Edge
Wafer Center
10
15
20
25 30
35
40
45
Positions
¾ There is a radial non-uniformity of the removal rates on the wafer surface
¾ The removal rate is symmetric about the wafer center with higher values in the
central and edge areas, but lower values in other regions on the wafer surface
50
Model Development
Main idea to predict removal rate
0.127
Y( i)
Y( i)
Y( i)
X ( i)
X ( i)
− 0.127
− 0.127
X( i)
0.127
x = d + ρ cos ( β 0 + ωwt ) − Ro cos (θ 0 + ωo t )
Y( i)
y = ρ sin ( β 0 + ωwt ) − Ro sin (θ 0 + ωot )
X( i)
Ng
Nm
lj
hi
dM 3
Rm = ∑ f ( Ci ) + ∑ f ( C j ) +
dA
i =1 Vi
j =1 V j
( )
Rm = f C +
dM 3
dA
Model Development
Three functional regions on the pad
region-III
region-I
region-II
ρ+Ro
region-III
ρ-Ro
region-I
region-II
η =1
⎛ r 2 + Ro2 − ρ 2 ⎞
η = arccos ⎜
⎟
2
rR
π
o
⎝
⎠
1
Model Development
Assumptions & Simplifications
¾ The slurry flow rate in a pad groove is proportional to the pressure
difference between its two ends Q = K ( P2 - P1 );
¾ The constant K is assumed to be the same for all pad grooves;
¾ The kinetics of copper removal rate as a function of the glycine
concentration at the wafer surface (Zhang & Subramanian, 2001)
2
Rreaction = f ( Cglycine ) = −1.062 × 10−2 C glycine
+ 4.3 × 10−3 Cglycine moles /(m 2 ⋅ s )
¾ The pressure underneath all the holes in the pad is the same, and is
treated as an unknown constant;
(
¾ The resistance to mass transfer in the liquid is neglected; and
¾ Glycine is assumed to be consumed instantaneously by reaction with
copper ions to form a complex.
)
Model Development
Slurry fluid mechanics
pad
i, j+1
Ph ( i , j )
y
x
i-1, j
i, j
Rubber pad backer
z
i, j-1
Q = K ΔP
⎧
⎪⎪128a 2 ∞ 1
K =⎨ 3
∑ 3
⎪ π μ L n =1,3,... n
⎪⎩
i+1, j
Pd
steel pad backer
Qin + qsource = Qout
⎡
⎛ nπ b ⎞ ⎤ ⎫
sinh
⎜
⎟ ⎪
⎢ ab 2a 2
2a ⎠ ⎥ ⎪
⎝
⎢
⎥⎬
− 2 2
⎢ nπ n π cosh ⎛ nπ b ⎞ ⎥ ⎪
⎜
⎟
⎢⎣
⎝ 2a ⎠ ⎥⎦ ⎪⎭
Pi , j +1 + Pi +1, j + Pi −1, j + Pi , j −1 − 4 Pi , j = 0
Pi , j +1 + Pi +1, j + Pi −1, j + Pi , j −1 − 4 Pi , j = −
Reference: Shah and London (1978)
q(i , j )
K
Ph (i , j ) −
∑q
qi , j
Kh
(i , j )
− Pd = 0
= Qtotal
Model Development
Pressure Distribution
¾ Pressures near the pad center are much higher than those in the pad edge area
¾ The pressure drops fast near the pad edge area
¾ Slurry flow rates in each groove can be obtained by using this pressure distribution
and Q = K ( P2 - P1 )
Model Development
Glycine concentration in a pad groove
QAB ⎡⎣C ( x ) − C ( x + Δx ) ⎤⎦ = 2Wdxf ( C ( x ) )
4.3×10−3CA
CB =
4.3×10−3 exp( 2bWL QAB ) −1.062×10−2 CA ⎡⎣exp( 2bWL QAB ) −1⎤⎦
L' =ηL
Model Development
Computation of glycine concentration distribution
C1
C3
C2
C1
C1
C1
C2
C2
C0
Calculation of the glycine concentrations at all intersections from 61 known
concentrations at holes is an iterative computation
Model Development
Glycine concentration distribution
¾ Glycine is consumed rapidly near the pad center
¾ On average, glycine concentration is larger at the edge than in the pad central area
Model Development
Average glycine concentration in path annulus
C0 = 133.2 mol/m3
3
Glycine Concentration (mol/m )
150
100
C0 = 66.6 mol/m3
50
C0 = 26.6 mol/m3
0
0.00
0.02
0.04
0.06
Radial Position (m)
∑C A
=
∑A
gi
Cg
g
i
g
i
0.08
0.10
Model Development
Mechanical removal by pad asperities
150
Mechanical Removal Rate
(nm/min)
Mechanical Removal Rate
(nm/min)
150
Glycine concentration (wt%)
0.2
0.5
1.0
100
50
0
0
50
100
150
200
250
300
350
400
Pad orbit speed (rpm)
100
200
300
100
50
0
0.00
0.20
Pad Orbit Speed (rpm)
0.40
0.60
0.80
1.00
Glycine Concentration (wt%)
Rmech = Rexpt − f ( Cmodel )
R = f ( Cmodel ) + 2.16CmodelVwp
1.20
Results and Discussion
Overall Removal Rate
350
300
Pad orbit speed (rpm)
Removal Rate (nm/min)
Removal Rate (nm/min)
350
100
200
300
250
200
300 rpm
150
200 rpm
100
100 rpm
50
Glycine concentration (wt%)
0.2
0.5
1.0
300
250
1.0 wt%
200
150
0.5 wt%
100
50
0.2 wt%
0
0.00
0.20
0.40
0.60
0.80
1.00
Glycine Concentration (wt%)
1.20
0
0
50
100
150
200 250 300
Pad Orbit Speed (rpm)
350
400
¾ The overall removal rate increase linearly with pad orbit speed but non-linearly
with glycine concentration
¾ The increasing slope is attributed to a synergistic action between the chemical
reaction and the mechanical removal
Results and Discussion
Radial Variation of Removal Rate on Wafer Surface
350
200
150
100
50
wafer edge
0
wafer center
wafer edge
1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49
Sites
(a)
Removal Rate (nm/min)
350
300
250
200
150
wafer edge
wafer center
wafer edge
50
0
1
4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49
Sites
(c)
300
250
200
150
100
50
0
wafer edge
wafer center
wafer edge
1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49
Sites
(b)
400
100
Removal Rate (nm/min)
Removal Rate (nm/min)
250
liquid flow rate = 200 ml/min
pressure = 4.0 psi
H2O2 concentration = 5.0 wt%
glycine concentration = 1.0 wt%
(a) 100 rpm
(b) 200 rpm
(c) 300 rpm
Results and Discussion
Radial Variation of Removal Rate on Wafer Surface
200
Removal Rate (nm/min)
Removal Rate (nm/min)
150
100
50
wafer edge
0
wafer center
(a)
Removal Rate (nm/min)
200
150
100
0
wafer center
wafer edge
1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49
Sites
(c)
50
wafer edge
0
wafer center
wafer edge
1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49
Sites
(b)
250
wafer edge
100
wafer edge
1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49
Sites
50
150
liquid flow rate = 200 ml/min
pressure = 4.0 psi
H2O2 concentration = 5.0 wt%
glycine concentration = 0.5 wt%
(a) 100 rpm
(b) 200 rpm
(c) 300 rpm
Results and Discussion
80
80
70
70
Removal Rate (nm/min)
Removal Rate (nm/min)
Radial Variation of Removal Rate on Wafer Surface
60
50
40
30
20
10
0
wafer edge
wafer center
wafer edge
1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49
Sites
(a)
Removal Rate (nm/min)
80
60
40
0
wafer edge
wafer center
wafer edge
1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49
Sites
(c)
50
40
30
20
wafer edge
10
0
1
wafer center
wafer edge
4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49
Sites
(b)
100
20
60
liquid flow rate = 200 ml/min
pressure = 4.0 psi
H2O2 concentration = 5.0 wt%
glycine concentration = 0.2 wt%
(a) 100 rpm
(b) 200 rpm
(c) 300 rpm
Conclusions
¾ The discrepancy between the experimentally measured removal rate
and the predicted chemical removal is attributed to mechanical
removal of the reacted film by the pad asperities on the mesas
¾ The overall removal rate of copper from the wafer surface depends
approximately linearly on the pad orbit speed and non-linearly on the
glycine concentration
¾ There appears to be a synergy between the chemical action and
mechanical action during CMP, which reinforces material removal
and can be enhanced by increasing the glycine concentration
Conclusions
(Continued)
¾ The shape of the radial variation of the removal rate depends on the
nominal glycine concentration in the experiment, but is independent
of the nominal glycine concentration or the pad orbit speed in the
modeling
¾ For glycine concentrations of 0.5 wt% and 1.0 wt%, in most cases,
the agreement between the predicted radial variation and the
experimental radial variation is good near the wafer edge but not
good in the central region of the wafer
¾ Other factors can influence the radial variation of the removal rates
Suggested Future Work
¾ The constant K in the assumption Q = K ( P2 - P1 ) will be different for different pad
grooves, and needs to be calculated individually
¾ Mass transport in the groove should be accommodated
¾ For an accurate prediction of the radial non-uniformity of the removal rate on the
wafer surface, the incoming-wafer film uniformity, down-force, wafer curvature,
backside pressure, wafer-to-retaining-ring protrusion, retaining ring pressure, pad
conditioning, etc. should be considered
¾ The mechanical aspects of removal in abrasive-free polishing are worth pursuing
in-depth
¾ A better understanding of the slurry delivery system is needed to improve the
accuracy of the predictions from the model
Acknowledgements
¾ Professor R. Shankar Subramanian for his guidance throughout this
research program both personally and professionally
¾ Novellus Systems, Inc. (SpeedFam/IPEC) and Rohm and Haas Electronic
Materials CMP Technologies for supplies and advice
¾ New York State Office of Science, Technology, and Academic Research
(NYSTAR) for financial support
¾ Professor Yuzhuo Li and members of his group for their help with the
experiments