Testing of Tribo-Chemical Model for Copper CMP in Acidic ...cden.ucsd.edu/archive/secure/archives/seminars/presentations/2010… · 1.00E-09 1.00E-08 1.00E-07 1.00E-06 1.00E-05 1.00E-04

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


Background


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


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


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


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


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


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)


Passivation Kinetics in Acidic Benzotriazole (BTA) Solutions


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


Potentiodynamic polarization curve of copper in pH 4 aqueous solution containing 0.01M BTA and 0.01M glycine using different scan rates


•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


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


-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


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


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


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


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


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


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


Modeling of Material Removal


Return to Original Material Removal Model*O

xida

tion

rate

mA

/cm

2 Bare copper



Pas

sive

Film

Thi

ckne

ss (L

) (nm

)





Time (ms)

Forc

e (n

N)


Film

thic

knes

s re

mov

ed, Δ

L Å

t0

τ

00 )( dttti

nFMRR Cu

Removal Rate (nm/s)

τ







LtLtL )()( 00 t0

t0




•

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)


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


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


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


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)


Idealized Polishing Pads


Return to Original Material Removal Model*O

xida

tion

rate

mA

/cm

2 Bare copper



Pas

sive

Film

Thi

ckne

ss (L

) (nm

)





Time (ms)

Forc

e (n

N)


Film

thic

knes

s re

mov

ed, Δ

L Å

t0

τ

00 )( dttti

nFMRR Cu

Removal Rate (nm/s)

τ







LtLtL )()( 00 t0

t0




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


Replace Stochastic Distribution of Interaction Force and Frequency with Well-Defined Values

Oxi

datio

n ra

te m

A/c

m2 Bare copper



Pas

sive

Film

Thi

ckne

ss (L

) (nm

)





Time (ms)

Forc

e (n

N)


Film

thic

knes

s re

mov

ed, Δ

L Å

t0

τ

00 )( dttti

nFMRR Cu

Removal Rate (nm/s)

τ







LtLtL )()( 00 t0

t0




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


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


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


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


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


Conclusions


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

Documents

Testing of Tribo-Chemical Model for Copper CMP in Acidic ...cden.ucsd.edu/archive/secure/archives/seminars/presentations/2010… · 1.00E-09 1.00E-08 1.00E-07 1.00E-06 1.00E-05 1.00E-04