University of California • Berkeley • San Diego • Los Angeles
Testing of Tribo-Chemical Model for Copper CMP in Acidic Media
Containing Benzotriazole (BTA)Seungchoun Choi* and Fiona M. Doyle
University of California at BerkeleyDepartment of Materials Science and Engineering
210 Hearst Mining Building # 1760Berkeley, CA 94720-1760
[email protected]*Department of Mechanical Engineering
IMPACT Seminar
February 24, 2010
IMPACT • CMP • 2
Outline
Background
Passivation Kinetics in Acidic Benzotriazole (BTA) Solutions
Modeling of Material Removal
Idealized Polishing Pads
Conclusions
University of California • Berkeley • San Diego • Los Angeles
Background
IMPACT • CMP • 4
CMP Overview
ALUMINA PARTICLESaverage size ~ 120 nm
from EKC Tech.
Cross-sectional View ofSUBA 500 Pad, Rodel
Corp. (courtesy Y.Moon)
SLURRY • Abrasive particles• Oxidizer•Complexing agent•InhibitorWafer
Carrier
Slurryfeeder
Polishing Plate
POLISHING PAD
Pressure
Rotation
Polishing pad Pad asperities
Patterned wafer
•With typical pads and rotational speeds, a pad asperity interacts with a given point on the surface about every millisecond
•Most models are empirical, with no systematic basis for modification when operational parameters change
IMPACT • CMP • 5 February 22, 2010
Kaufman’s Model for PlanarizationFor effective planarization, must maintain higher removal at protruding regions and lower removal
at recessed regions on the wafer
1- removal of passivatingfilm by mechanical action
at protruding areas
3- planarization by repetitivecycles of (1) and (2)
Metal Passivatingfilm
2- wet etch of unprotected metal by chemical action.passivating film reforms
Passive films, or corrosion inhibitors, are essential for attaining planarization
Mechanical and chemical mechanisms interact synergistically
IMPACT • CMP • 6 February 22, 2010
Hea d
Plat en
Paddo
wn-
forc
e
slurry supply
rotatio n of
wafer head
Waf
er 4-
12”
Copper
Featu
re
pad asperity
abrasiv
e
particle
s
100nm-10µm
~1µm
1-10µmPad asperity
Abrasive
Pad/Wafer
Die
Feature/Asperity
Abrasive Contact
CMP phenomena at different scales
Here focusing on the smallest scale, where abrasive particles and asperities interact with
copper
IMPACT • CMP • 7 February 22, 2010
At Micro-Nano Scale, Need to Integrate Different Phenomena
Integrated Cu CMP Model
ColloidAgglomeration
OxidizerInhibitor
Complexing agentSurface Film
PadPressure/ Velocity
AbrasiveFor copper
Fluid MechanicsMass TransferNeeded: Needed:
understanding of the understanding of the synergy between synergy between
different componentsdifferent components
Interactions:Interactions:••AsperityAsperity--coppercopper••AbrasiveAbrasive--coppercopper
Fluid pressureContact pressure
University of California • Berkeley • San Diego • Los Angeles
Chemical Mechanical Planarization - Faculty Team
Mechanical Phenomena
Chemical Phenomena
Interfacial and Colloid
PhenomenaJan B. TalbotChemical EngineeringUCSD
David A. DornfeldMechanical EngineeringUCB
Fiona M. DoyleMaterials Science and EngineeringUCB
Kyriakos KomvopoulosMechanical Engineering
UCB
IMPACT • CMP • 9
Copper
Passive film
Pad asperity
AbrasiveAbrasive
Copper CMP: at abrasive scale
2. Mechanical response of passive films
1. Passivation kinetics: the transient oxidation rate of copper after removal of passive film
3. Abrasive-copper interaction frequency & force
All three components need to be individually estimated for modeling
IMPACT • CMP • 10 February 22, 2010
Original Material Removal Model*O
xida
tion
rate
mA
/cm
2 Bare copper
Time (t’) msCopper: transient
passivation behavior i(t’)
Pas
sive
Film
Thi
ckne
ss (L
) (nm
)
1. Passivation kinetics–
Film growth kinetics
Interval between two abrasive-
copper contacts (τ)
Time (ms)
Forc
e (n
N)
Force on an abrasive, nN
Film
thic
knes
s re
mov
ed, Δ
L Å
t0
τ
00 )( dttti
nFMRR Cu
Removal Rate (nm/s)
τ
MCu : Atomic mass of copperρ
: density of coppern : # e-
transferredF : Faraday’s constant*Tripathi, Doyle & Dornfeld, "Tribo-Chemical Modeling of Copper
CMP" 2006 Proceedings of VLSI Multilevel Interconnection Conf.
3. Abrasive-copper interaction force & frequency
2. Mechanical removal response of passive film
LtLtL )()( 00 t0
t0
can be found given L(t’) (fig 1.), ΔL (fig 2.) & τ
(fig 3.)(since L(t’) is concave)
University of California • Berkeley • San Diego • Los Angeles
Passivation Kinetics in Acidic Benzotriazole (BTA) Solutions
IMPACT • CMP • 12 February 22, 2010
Potential-pH diagram for copper-water-glycine system at 25ºC and 1 atm., 0.01M glycine, 10-5M Cu++ [from Aksu]
Acidic slurries need an inhibitor – BTA very common
Neutral slurries actually develop alkaline conditions at surface where peroxide is being reduced
IMPACT • CMP • 13 February 22, 2010
Potentiodynamic polarization curve of copper in pH 4 aqueous solution containing 0.01M BTA and 0.01M glycine using different scan rates
IMPACT • CMP • 14
•D. Tromans and R. Sun, J. Electrochem. Soc., 138, 3235 (1991)
•D. Tromans, J. Electrochem. Soc., 145, L42 (1998).
•B.-S. Fang, C. G. Olson and D. W. Lynch, Surf. Sci., 176, 476 (1986)
•J.-O. Nilsson, C. Tornkvist, and B. Liedberg, App. Surf. Sci., 37, 306 (1989).
•R. Youda, H. Nishihara, K. Aramaki, Electrochim. Acta, 35, 1011 (1990).
•p
H8.
2
•p
H1
IMPACT • CMP • 15
Rapid physisorption of BTA on metallic copper. Totally suppresses reduction of oxygen on copper – not relevant in CMP, because potential is never this low, even if bare metal has just been exposed
Potentiodynamic polarization curve of copper in pH 4 aqueous solution containing 0.01M BTA and 0.01M glycine using different scan rates
Gradual chemiisorption and precipitation of CuBTA on oxidizing copper. Progressively suppresses corrosion rate
Polishing pad Pad asperities
Patterned wafer •With typical pads and rotational speeds, a pad asperity interacts with a given point on the surface about every millisecond•Exposure times in plot below are from 17 s to 500 s to get to the active anodic region•Mainly relevant to recessed topography, not protruding areas where material is being removed
IMPACT • CMP • 16
-0.60
-0.40
-0.20
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.00E-09 1.00E-08 1.00E-07 1.00E-06 1.00E-05 1.00E-04 1.00E-03
Current density (A/cm2)
E (v
s SC
E, V
)
not rotating, in contact1000rpm, in contact1000rpm, not in contact200rpm, not in contact
Potentiodynamic Polarization
Rotation of working electrode promotes adsorption of BTA onto copper by enhanced diffusion
Scan rate: 5mV/s
Adsorbed BTA is removed by abrading action of CMP pad
Patterned wafer
Patterned wafer
IMPACT • CMP • 17
Model Needs Quantification of Current Decay upon Sorption of BTA
Oxidation rate of copper in a passivating solution is a function of thickness/coverage of the passive film.
Passivation kinetics primarily determined by chemistry.
iactive
ipassive
Oxi
datio
n ra
te
Bare copper
Thick/coherent passive film
Time (t’)Copper: passivation kinetics i(t’)
Passivation kinetics of copper can be studied using:
•-Scratch-repassivation –
noisy signal, not explored further•-Potential step passivation
To measure the peak current and the current decay, we must obtain bare copper at passivation conditions.
CMP phenomena lie between these two approaches:
•
Abrasive interactions apply force to remove passive film
•
Applied force does not remove any copper
•
Must also recognize that potential step passivation will involve physisosorption of BTA at low potentials –
not present in CMP
IMPACT • CMP • 18 February 22, 2010
Microelectrode for Studying Passivation Kinetics
(A/c
m^2
)
Microelectrodes offer several advantages over conventional macroelectrodes:
•
Minimal IR drop (due to small total currents)•
Faster capacitive charging•
Faster diffusion (effect can be easily isolated)
Current decay post potential step (from -0.3V to 0.3 V SCE) for a micro and a macroelectrode (in pH12, 0.01M glycine)
•Higher initial peak currents seen.
•Flat IR drop region & charging region eliminated
Microelectrode used: 160µm copper wire coated with enamel
IMPACT • CMP • 19
Electrochemical Impedance Spectroscopy (EIS)
copper-electrolyte interface, held at some DC potential
Apply small AC signal at varying frequencies
Measure corresponding small variations in current (amplitude and phase shift)
Assume equivalent electrical circuit based on physical phenomena
Obtain equivalent electrical parameters
Use these to distinguish between capacitive and Faradaic currents in experimental measurements
IMPACT • CMP • 20 February 22, 2010
Potentiodynamic polarization curve (scan rate 10mV/s) of copper in pH 4 aqueous solution containing 0.01M glycine
EIS was conducted at potentials indicated by red arrows
IMPACT • CMP • 21 February 22, 2010
Current decay after stepping potential from -0.9V to different potentials, copper in pH 4 aqueous solution containing 0.01M glycine (no BTA)
Simulated using electrical parameters determined by EIS
Experimentally measured
Capacitive charging initially, decaying to
purely Faradaic current
IMPACT • CMP • 22 February 22, 2010
Current decay after stepping potential from -1.2V to different potentials, copper in pH 4 aqueous solution containing 0.01M glycine and 0.01M BTA
•
Current decay has a very consistent shape throughout•
Decay rate of 0.5 orders per time decade –
precisely (Cottrell behavior)•
Current decays similarly for ‘cathodic’
potential also (below -0.1V)•
There’s no capacitive charging: RC = 0.2ms
Change in behavior at 1 s appears to correspond to formation of a monolayer of chemisorbed BTA
•There was physisorbed BTA on the surface before stepping potential•Yet there is still diffusion control,•There couldn’t have been much BTA•Only a small fraction of sites on copper are responsible for oxygen reduction•Model below seems unrealistic
BTA must be the species responsible for the decreasing current
University of California • Berkeley • San Diego • Los Angeles
Modeling of Material Removal
IMPACT • CMP • 24 February 22, 2010
Return to Original Material Removal Model*O
xida
tion
rate
mA
/cm
2 Bare copper
Time (t’) msCopper: transient
passivation behavior i(t’)
Pas
sive
Film
Thi
ckne
ss (L
) (nm
)
1. Passivation kinetics–
Film growth kinetics
Interval between two abrasive-
copper contacts (τ)
Time (ms)
Forc
e (n
N)
Force on an abrasive, nN
Film
thic
knes
s re
mov
ed, Δ
L Å
t0
τ
00 )( dttti
nFMRR Cu
Removal Rate (nm/s)
τ
MCu : Atomic mass of copperρ
: density of coppern : # e-
transferredF : Faraday’s constant*Tripathi, Doyle & Dornfeld, "Tribo-Chemical Modeling of Copper
CMP" 2006 Proceedings of VLSI Multilevel Interconnection Conf.
3. Abrasive-copper interaction force & frequency
2. Mechanical removal response of passive film
LtLtL )()( 00 t0
t0
can be found given L(t’) (fig 1.), ΔL (fig 2.) & τ
(fig 3.)(since L(t’) is concave)
IMPACT • CMP • 25
•
At times below a second or so, there isn’t a coherent film to undergo mechanical damage
•
Typical copper removal rates during CMP are in the range of 50 to 600 nm/min.
•
For intervals between two asperity copper contacts of 1 to 10ms, this corresponds to removal of a copper layer of 0.1 to 1Å thick per interaction
•
Due to both dissolution between the two interactions and removal of oxidized copper film by the interaction
•
Atomic radius of copper is 1.4Å•
Means that the likelihood of removal of a single surface copper species is less than unity per interaction
•
The “chemical tooth” model proposed by Cook* seems more appropriate
February 22, 2010
Mechanical Component of Model Clearly Inappropriate
Asperity-wafer interactions happen about every ms.But what passivation time on the curve best represents the starting and ending condition?
* L. M. Cook, Journal of Non- crystalline Solids, 120, 152 (1990)
IMPACT • CMP • 26
Establishment of quasi-steady state with less than a monolayer of BTA on copper surface
February 22, 2010
0 0.5 10
0.2
0.4
0.6
0.8
1C
over
age
ratio
,
t / tconst
Coverage ratio, Reduced by abrasion at any given state(e.g. removal of 20% of existing complexes)
t*1t*2
Quasi-steady state
Right after1st reformation of Cu(I)BTARight after
nth reformation of Cu(I)BTA
Right after2nd abrasion
Right after1st abrasion
Right after
nth abrasion
t*n
Abrasion starts
Reformation of a protective film during interval Removal of a protective film by abrasion
= fraction of available sites that are occupied
IMPACT • CMP • 27
Theoretical Analysis
February 22, 2010
00( )Cu
totalMMRR i t t dtnF
itotal
is measured current
0.5
total pass diss mm
ti i i it
mt tfor
( )passdq di t cdt dt
baidiss )1(
0.5
(1 ) mm
d ta b c idt t
0.5
m
m
id t a b adt c t c c
or
( )( )
a b a bt tm m c ci t a b a at erf t e e
c a b a bc a b
IMPACT • CMP • 28
Parameters in model
February 22, 2010
( )( )
a b a bt tm m c ci t a b a at erf t e e
c a b a bc a b
a b
θ
= 1
Derived values for a governing equation of the kinetics of BTA adsorption in a pH 4 aqueous solution containing 0.01 M glycine and 0.01 M BTAPotential
(V)tm
(sec)a
(A/cm2)b
(A/cm2)c
(C/cm2)0.6 2 7.0×10-2 9.4×10-4 7.8×10-5
0.4 4 4.0×10-2 5.2×10-4 8.4×10-5
IMPACT • CMP • 29 February 22, 2010
0 0.2 0.4 0.6 0.8 10
0.2
0.4
0.6
0.8
1
t/tm
Cov
erag
e ra
tio,
10-4
10-3
10-2
10-1
100
0
20
40
60
80
100
i pass
/i tota
l [%]
t/tm
10-4
10-3
10-2
10-1
1000
20
40
60
80
100
i diss
/i tota
l [%]
ipass/itotal
idiss/itotal
Contribution of the current density for forming Cu(I)BTA and the current density for direct dissolution to the total current density
Millisecond scale adsorption kinetics of BTA in pH 4 aqueous solution containing 0.01 M glycine and 0.01 M BTA (tm is 2 s at 0.6V and 4 s at 0.4 V)
University of California • Berkeley • San Diego • Los Angeles
Idealized Polishing Pads
IMPACT • CMP • 31 February 22, 2010
Return to Original Material Removal Model*O
xida
tion
rate
mA
/cm
2 Bare copper
Time (t’) msCopper: transient
passivation behavior i(t’)
Pas
sive
Film
Thi
ckne
ss (L
) (nm
)
1. Passivation kinetics–
Film growth kinetics
Interval between two abrasive-
copper contacts (τ)
Time (ms)
Forc
e (n
N)
Force on an abrasive, nN
Film
thic
knes
s re
mov
ed, Δ
L Å
t0
τ
00 )( dttti
nFMRR Cu
Removal Rate (nm/s)
τ
MCu : Atomic mass of copperρ
: density of coppern : # e-
transferredF : Faraday’s constant*Tripathi, Doyle & Dornfeld, "Tribo-Chemical Modeling of Copper
CMP" 2006 Proceedings of VLSI Multilevel Interconnection Conf.
3. Abrasive-copper interaction force & frequency
2. Mechanical removal response of passive film
LtLtL )()( 00 t0
t0
can be found given L(t’) (fig 1.), ΔL (fig 2.) & τ
(fig 3.)(since L(t’) is concave)
IMPACT • CMP • 32
Preston’s Equation
There isn’t a complete film, and asperities/abrasives pluck CuBTA complexes off the copper surface
So why does Preston’s equation usually seem to be valid, i.e. the material removal rate scales with the applied pressure?
Possibly due to deformation of asperities, leading to a larger contact area with increasing pressure, and hence more plucking of oxidized species from surface
Test using ideal pad with deformation-resistant asperities
Also use this to test modelFebruary 22, 2010
Patterned wafer
IMPACT • CMP • 33 February 22, 2010
Replace Stochastic Distribution of Interaction Force and Frequency with Well-Defined Values
Oxi
datio
n ra
te m
A/c
m2 Bare copper
Time (t’) msCopper: transient
passivation behavior i(t’)
Pas
sive
Film
Thi
ckne
ss (L
) (nm
)
1. Passivation kinetics–
Film growth kinetics
Interval between two abrasive-
copper contacts (τ)
Time (ms)
Forc
e (n
N)
Force on an abrasive, nN
Film
thic
knes
s re
mov
ed, Δ
L Å
t0
τ
00 )( dttti
nFMRR Cu
Removal Rate (nm/s)
τ
MCu : Atomic mass of copperρ
: density of coppern : # e-
transferredF : Faraday’s constant*Tripathi, Doyle & Dornfeld, "Tribo-Chemical Modeling of Copper
CMP" 2006 Proceedings of VLSI Multilevel Interconnection Conf.
3. Abrasive-copper interaction force & frequency
2. Mechanical removal response of passive film
LtLtL )()( 00 t0
t0
can be found given L(t’) (fig 1.), ΔL (fig 2.) & τ
(fig 3.)(since L(t’) is concave)
IMPACT • CMP • 34
Testing of Preston’s Equation
The reason for increased MRR at higher applied pressure will be elucidated by the experiment shown below
Preston’s equation predicts that the MRR for those cases should be identical, but it would be different unless the rate of copper dissolution were constant
If the MRR is different, our mechanistic copper CMP model can explain it while Preston’s equation cannot
1 psi1 m/s 2 psi0.5 m/s
Pattern defined CMP pad
Copper
IMPACT • CMP • 35
Validation of Our Mechanistic Cu CMP Model
Our mechanistic copper CMP model includes both asperity-copper interaction frequency and effectiveness of removal of oxidized species from the copper surface
Our model will be examined by the following experiments
If the MRR for both cases is identical, our model can be validated (Preston’s equation cannot give any explanation for this case)
1 psi1 m/s 1 psi0.5 m/s
IMPACT • CMP • 36
Fabrication of Pattern Defined CMP Pad
Asperity features with well defined geometry were fabricated on a stack of silicon wafer and SU-8 layer by lithography
Procedure
Si wafer
100 µm
10 µm
20 µmSU-8
Spin Coat Soft Bake Exposure
Development Hard BakePost Exposure Bake
IMPACT • CMP • 37
Experimental Setup
Solution: pH4 aqueous solution containing 0.01M BTA, 0.01M Glycine and 10-4M Cu(NO3
)2
Pt Counter Electrodes
Luggin Probe & Reference Electrode
Polish pad
Copper Working Electrode
Solution
Rotator Frame
Load cell
CopperInsulation coating
Copper Electrode
Fabricated CMP Pad
IMPACT • CMP • 38
Fabrication of Pattern Defined CMP Pad
SU-8 structure was hard baked after development for enhanced adhesion to the substrate and mechanical strength
After 10 seconds of polishing under 3 psi applied pressure and 0.5 m/s linear velocity
Abraded region
University of California • Berkeley • San Diego • Los Angeles
Conclusions
IMPACT • CMP • 40 February 22, 2010
Conclusions
BTA protects copper under both reducing and oxidizing conditions, but by different mechanisms
Under oxidizing conditions, it forms a monolayer of Cu-BTA in about a second. Thereafter, thicker Cu-BTA films form
Each interaction of an asperity with a given point on the copper surface removes only a fraction of a monolayer of Cu-BTA
Most of the material removal is due to direct dissolution of copper ions into the slurry
The fundamental basis for Preston’s equation in the presence of BTA is not apparent
Tests are now underway to elucidate the apparent mechanical phenomena in CMP and validate the model