chemistry and electrochemistry of copper cmp – capturing...

October 17, 2005

1

FLCC

Chemistry and Electrochemistry of Copper CMP – Capturing their Influence at the

Feature and Bulk Levels

Chemistry and Electrochemistry of Copper Chemistry and Electrochemistry of Copper CMP CMP –– Capturing their Influence at the Capturing their Influence at the

Feature and Bulk LevelsFeature and Bulk Levels

Ling Wang and Fiona M. DoyleLing Wang and Fiona M. DoyleUniversity of California at BerkeleyUniversity of California at Berkeley

Department of Materials Science and EngineeringDepartment of Materials Science and Engineering210 Hearst Mining Building # 1760210 Hearst Mining Building # 1760

Berkeley, CA 94720Berkeley, CA 94720--17601760

[email protected]@berkeley.edu

October 17, 2005

2

FLCC

OutlineOutlineOutline

•• Introduction Introduction –– objective of FLCC effort in CMPobjective of FLCC effort in CMP•• Passive layers and their importancePassive layers and their importance

–– Anomalous passivity induced by hydrogen Anomalous passivity induced by hydrogen peroxideperoxide

•• EQCM studies of passive filmsEQCM studies of passive films–– Relationship to other literatureRelationship to other literature–– Capturing behavior in modelsCapturing behavior in models

•• Galvanic interactionsGalvanic interactions–– How best to apply in feature scale modelsHow best to apply in feature scale models

•• ConclusionsConclusions

October 17, 2005

3

FLCC

AimAimAim

Insure uniform, global Insure uniform, global planarizationplanarization with no with no defects by means of optimized process defects by means of optimized process recipes and consumablesrecipes and consumablesIdealized singleIdealized single--phase CMP processes are phase CMP processes are now well understood in terms of the now well understood in terms of the fundamental physicalfundamental physical--chemical phenomena chemical phenomena controlling material removalcontrolling material removalThe challenge is to extend this to The challenge is to extend this to heterogeneous structures that are heterogeneous structures that are encountered when processing product encountered when processing product wafers with device featureswafers with device features

October 17, 2005

4

FLCC

ApproachApproachApproach

•• Our approach is to develop integrated Our approach is to develop integrated featurefeature--level process models linked to basic level process models linked to basic process mechanicsprocess mechanics

•• These models will drive process optimization These models will drive process optimization and the development of novel consumables and the development of novel consumables to minimize featureto minimize feature--level defects and pattern level defects and pattern sensitivitysensitivity

•• For this, we will require the capability of For this, we will require the capability of faithfully predicting defects and nonfaithfully predicting defects and non--idealities at feature boundaries. idealities at feature boundaries.

October 17, 2005

5

FLCC

Mechanical Phenomena

Chemical Phenomena

Interfacial and Colloid

Phenomena

Chemical Mechanical

Planarization

October 17, 2005

6

FLCC







October 17, 2005

7

FLCC

Passive films, or corrosion inhibitors, are key to attaining planarization

Passive films, or corrosion inhibitors, are key Passive films, or corrosion inhibitors, are key to attaining to attaining planarizationplanarization

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

October 17, 2005

8

FLCC

Chemistry of CopperChemistry of Copper--Water SystemWater System

-0.8-0.6-0.4-0.20.00.20.40.60.8

0 2 4 6 8 10 12 14 16pH

E, V

vs.

SHE

Cu2+

CuO

22-

Cu

Cu2O

CuO

Potential-pH diagram, with {CuT} = 10-5

Potential-pH diagram, with {CuT} = 10-4 or 10-6, 4% H2O2 or 0.5M hydroxylamineTamilmani, Huang, Raghavan & Small, JECS, 2002

October 17, 2005

9

FLCC

Chemistry of Chemistry of GlycineGlycine--Water SystemWater SystempKa1=2.350 pKa2=9.778

+H3NCH2COOH ↔ +H3NCH2COO- ↔ H2NCH2COO-

Cation: H2L+ Zwitterion: HL Anion: L-

Cu(II) glycinate complexes•Cu(H3NCH2COO)2+ : CuHL2+

•Cu(H2NCH2COO)+ : CuL+

•Cu(H2NCH2COO)2 : CuL2

Cu (I) glycinate complexes•Cu(H2NCH2COO)-

2 : CuL2-

H-O-H

H-O-H

N-H2H2-C

OO=C

C-H2H2-N

C=OOCu2+

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

0 2 4 6 8 10 12 14 16pH

E, V

vs.

SHE

Cu2+

CuL2CuL+

CuO

22-

CuO

Cu2OCu

Potential-pH diagram, with {CuT} = 10-5, {LT} = 10-2

October 17, 2005

10

FLCC

Mechanisms for copper passivation and removal

Mechanisms for copper Mechanisms for copper passivationpassivation and and removalremoval

•• The The ““wet etchwet etch”” of copper is an electrochemical of copper is an electrochemical corrosion processcorrosion process–– RedoxRedox reaction in which oxidation occurs at sites separated reaction in which oxidation occurs at sites separated

(albeit at microscopic or (albeit at microscopic or nanoscopicnanoscopic scale) from sites where scale) from sites where reduction occursreduction occurs

–– Cu = CuCu = Cu2+2+ + 2e + 2e -- oxidation, anodic reactionoxidation, anodic reaction–– Ox + Ox + nene = R = R -- reduction, reduction, cathodiccathodic reactionreaction

•• The The Cu(IICu(II) may form a solid phase that ) may form a solid phase that passivatespassivates the the underlying metal. This would be the layer that is underlying metal. This would be the layer that is removed mechanically removed mechanically –– copper itself is very ductilecopper itself is very ductile

•• Regardless, slurry must contain some oxidant, Ox, Regardless, slurry must contain some oxidant, Ox, that is reduced while oxidizing copperthat is reduced while oxidizing copper

October 17, 2005

11

FLCC

•• Consider the dissolution of Consider the dissolution of copper in a slurry copper in a slurry containing hydrogen containing hydrogen peroxide:peroxide:

Cu + HCu + H22OO22 + 2H+ 2H++ = Cu= Cu2+2+ + 2H+ 2H22OO•• Reaction occurs as two Reaction occurs as two

separate reactions, at separate reactions, at different locations:different locations:–– Oxidation of the metal Oxidation of the metal -- called called

anodic dissolution:anodic dissolution:Cu = CuCu = Cu2+2+ + 2e+ 2e

–– Reduction of the hydrogen Reduction of the hydrogen peroxide peroxide -- called called cathodiccathodicreduction: reduction: HH22OO22 + 2H+ 2H++ + 2e = 2H+ 2e = 2H22OO

Polycrystalline copper

Cu =Cu =

CuCu2+2+ + 2e+ 2e

Electrochemical mechanisms of metal Electrochemical mechanisms of metal dissolutiondissolution

HH22OO22 + 2H+ 2H++ + 2e + 2e = 2H= 2H22OO

October 17, 2005

12

FLCC

•• Oxidation is more favorable at high energy Oxidation is more favorable at high energy surface defects, such as emergent surface defects, such as emergent dislocations and grain boundariesdislocations and grain boundaries–– The electrons lower the electrical potential The electrons lower the electrical potential

of these sitesof these sites•• Reduction is Reduction is ““forcedforced”” to dominate at other to dominate at other

areas, where surface sites are available for areas, where surface sites are available for sorption of hydrogen peroxide and sorption of hydrogen peroxide and hydrogen ions, and any reaction hydrogen ions, and any reaction intermediatesintermediates–– The utilization of electrons in the reduction The utilization of electrons in the reduction

process tends to raise the electrical process tends to raise the electrical potential of potential of cathodiccathodic sitessites

•• The potential difference draws electrons The potential difference draws electrons from anodic sites to from anodic sites to cathodiccathodic sites to sites to maintain the reactionmaintain the reaction

Polycrystalline copper

CuCu2+2+HH22OO22

2e2e

2H2H2H2H22OO

Electrochemical mechanisms of metal Electrochemical mechanisms of metal dissolutiondissolution

October 17, 2005

13

FLCC

•• For both the anodic and For both the anodic and cathodiccathodicreactions, the rate of reaction depends reactions, the rate of reaction depends upon the potential relative to the upon the potential relative to the equilibrium potentialequilibrium potential–– potential is a measure of the chemical potential is a measure of the chemical

activity of electrons, one of the activity of electrons, one of the participants in the reactionparticipants in the reaction

•• When the kinetics are dominated by the When the kinetics are dominated by the electron transfer process, the Butlerelectron transfer process, the Butler--VolmerVolmer equation applies:equation applies:

•• May also see kinetics affected by May also see kinetics affected by transport and the availability of reactantstransport and the availability of reactants

∴ i = io { exp -αnFη

RT⎛ ⎝

⎞ ⎠ - exp

1 - α( ) nFηRT

⎛ ⎝ ⎜ ⎞

⎠

Pote

ntia

l, V

(SH

E)

Anodic

Cathodic

Eoc

Eoc

Ecorr

log icorr

Kinetics of Electrochemical Reactions Kinetics of Electrochemical Reactions –– Crucial for Reaction ControlCrucial for Reaction Control

System settles at point where anodic and cathodic currents are equal

(electroneutrality), with distinctive corrosion current density and corrosion potential

log i

October 17, 2005

14

FLCC

Magnetic stirrer

Pt counter electrode

Luggin probe & reference electrode

Fritted glassgas bubbler

Rotating Cudisk electrode

Pote

ntia

l, V

(SH

E)log i

Anodic

Cathodic

Eoc

Eoc

Ecorr

Measure current passing between counter electrode and working copper electrode. This is the net difference between the anodic and cathodic curves, as shown

Obtain information on electrochemical Obtain information on electrochemical processes with polarization studiesprocesses with polarization studies

October 17, 2005

15

FLCC

More complex polarization behaviorMore complex polarization behaviorMore complex polarization behavior

•• In aerated solutions used in some In aerated solutions used in some of our experiments, of our experiments, cathodiccathodicreaction principally reduction of reaction principally reduction of oxygen:oxygen:

OO22 + 2H+ 2H22O + 4e = 4OHO + 4e = 4OH--

current limited by transport of current limited by transport of oxygenoxygen

•• Anodic reactions are oxidation of Anodic reactions are oxidation of copper to Cucopper to Cu2+2+, Cu, Cu22O, Cu(OH)O, Cu(OH)22 or or CuOCuO –– current may be limited by current may be limited by transport, blockage of sites or transport, blockage of sites or presence of presence of passivatingpassivating films films

•• Behavior complex if there are Behavior complex if there are multiple anodic or multiple anodic or cathodiccathodicreactions

Pote

ntia

l, V

(SH

E)Anodic

Cathodic

Net

Increasing stirring

Eoc

log ireactions

October 17, 2005

16

FLCC

Polarization Curves in CuPolarization Curves in Cu--GlycineGlycine--HH22OO

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

0 2 4 6 8 10 12 14 16pH

E, V

vs.

SHE

Cu2+

CuL2CuL+

CuO

22-

CuO

Cu2OCu

i, A/m2

10-4 10-3 10-2 10-1 100 101 102 103

E m

V v

s. S

HE

-800

-600

-400

-200

0

200

400

600

800

1000

1200

1400

1600

1800

pH 4pH 9pH 12

{CuT} = 10-5, {LT} = 10-2

{LT} = 10-2

October 17, 2005

17

FLCC

InIn--situsitu Electrochemical Polarization While Polishing Electrochemical Polarization While Polishing

Magnetic stirrer

Pt Counter Electrodes

Luggin Probe & Reference Electrode

Polish pad

Copper Working Electrode

Slurry poolP

Rotator Frame

ω

October 17, 2005

18

FLCC

InIn--situsitu PolarizationPolarization

i, A/m2

10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1

E m

V vs

. SH

E

-800

-600

-400

-200

0

200

400

600

800

1000

1200

1400

1600

1800

No abrasionPolishing with pad onlyPolishing with pad and5 % alumina particles

i, A/m2

10-4 10-3 10-2 10-1 100 101 102 103

E m

V vs

. SHE

-800

-600

-400

-200

0

200

400

600

800

1000

1200

1400

1600

1800


i, A/m2

10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1

E m

V vs

. SH

E

-800

-600

-400

-200

0

200

400

600

800

1000

1200

1400

1600

1800


pH 4

pH 9

pH 12

Aqueous 10-2 M glycine, 27.6 kPa, 200 rpm

October 17, 2005

19

FLCC

Kinetic Experiments Using HKinetic Experiments Using H22OO22

DissolutionDissolution•• Cleaned, weighed, Cleaned, weighed,

copper coupons (50 copper coupons (50 x 25 x 1 mm, x 25 x 1 mm, 99.999%) suspended 99.999%) suspended in stirred solutionsin stirred solutions

•• After tests, dried After tests, dried and weighedand weighed

•• Copper removal rate Copper removal rate determined by determined by weight lossweight loss

PolishingPolishingUsed same Used same equipment used for equipment used for inin--situsitu polarization polarization teststestsElectrochemical Electrochemical information unstable information unstable with peroxidewith peroxidePolishing rates Polishing rates determined from determined from weight loss weight loss measurementsmeasurements

October 17, 2005

20

FLCC

Effect of HEffect of H22OO22 on Dissolution and on Dissolution and Polish RatesPolish Rates

0

20

40

60

80

100

120

140

160

0 1 2 3 4 5 6H2O2 , wt%

Rem

oval

Rat

e, n

m/m

in Dissolution Rate

Polish Rate

pH 40

102030405060708090

100

0 1 2 3 4 5 6H2O2, wt%

Rem

oval

Rat

e, n

m/m

in

Dissolution Rate

Polish Rate

pH 9

Aqueous 10-2 M glycine, 27.6 kPa, 200 rpm

October 17, 2005

21

FLCC

Effect of HEffect of H22OO22 on Open Circuit on Open Circuit Potential in Aqueous, 10Potential in Aqueous, 10--22 M M GlycineGlycine

0

100

200

300

400

500

600

700

0 1 2 3 4 5 6

H2O2, wt%

EO

C , m

V v

s. S

HE pH 9

-100

0

100

200

300

400

500

0 1 2 3 4 5 6

H2O2 wt%

EOC ,

V v

s. S

HEpH 4

Nernst Equation: E = Eo + 2.303RT/nF log Πaox/ Π ared

October 17, 2005

22

FLCC

Effect of HEffect of H22OO22 on Open Circuit on Open Circuit Potential in Aqueous, 10Potential in Aqueous, 10--22 M M GlycineGlycine

pH 4

pH 9y = 166.03log(x)+518.76

R2 = 0.948

0100200300400500600700

0.1 1 10

H2O2 wt%

EOC ,

mV

vs.

SH

E

y = 59.31log(x)+395.801R2 = 0.996

050

100150200250300350400450500

0.1 1 10

H2O2 wt%

EOC ,

mV

vs. S

HE

October 17, 2005

23

FLCC

Equivalent Polarization CurvesEquivalent Polarization Curves

i, A/m2

10-4 10-3 10-2 10-1 100 101 102 103

E, m

V vs

. SH

E

-800

-600

-400

-200

0

200

400

600

800

1000

1200

1400

1600

1800

Polarization in the absence of H2O2

Dissolution in the presence of H2O2

Polishing in the presence of H2O2

i, A/m2

10-4 10-3 10-2 10-1 100 101 102 103E,

mV

vs. S

HE

-800

-600

-400

-200

0

200

400

600

800

1000

1200

1400

1600

1800

Polarization in the absence of H2O2

Dissolution in the presence of H2O2

Polishing in the presence of H2O2

pH 4

pH 9

October 17, 2005

24

FLCC

Polarization Curves in CuPolarization Curves in Cu--GlycineGlycine--HH22OO

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

0 2 4 6 8 10 12 14 16pH

E, V

vs.

SHE

Cu2+

CuL2CuL+

CuO

22-

CuO

Cu2OCu

i, A/m2

10-4 10-3 10-2 10-1 100 101 102 103

E m

V v

s. S

HE

-800

-600

-400

-200

0

200

400

600

800

1000

1200

1400

1600

1800

pH 4pH 9pH 12

{CuT} = 10-5, {LT} = 10-2

{LT} = 10-2

October 17, 2005

25

FLCC

Why the unexpected Why the unexpected passivationpassivation??

PassivationPassivation induced by Hinduced by H22OO2 2 at moderate at moderate concentationsconcentations cannot be due to colloidal cannot be due to colloidal effects altering abrasiveeffects altering abrasive--substrate substrate interactions, because this effect is seen for interactions, because this effect is seen for dissolution, in the absence of abrasivesdissolution, in the absence of abrasivesNot due to a thermodynamically stable phaseNot due to a thermodynamically stable phaseCuCu22O (O (Hernandez, Hernandez, WrschkaWrschka, and , and OehleinOehlein, J. , J. ElectrochemElectrochem. Soc., 148, G389, 2001). Soc., 148, G389, 2001)CuCu22OO33 or CuOor CuO22 ((PourbaixPourbaix, 1965) , 1965) –– unlikelyunlikelyThe pH at the surface, is much higher than in The pH at the surface, is much higher than in the bulk: Hthe bulk: H22OO22 + 2H+ 2H++ + 2e = 2H+ 2e = 2H22O O –– difficult to difficult to envisage such a dramatic differenceenvisage such a dramatic difference

October 17, 2005

26

FLCC







October 17, 2005

27

FLCC

Quartz Crystal MicrobalanceQuartz Crystal MicrobalanceQuartz Crystal Microbalance

• Sauerbrey equation:

where µq is the shear modulus of the quartz crystal, ρq the density, and f0 the resonant frequency

• for an AT-cut quartz crystal with a resonant frequency of 5 MHz gives that ∆m/∆f is –1.77 x 10-8

g/cm2Hz

( )( )2

0

21

2 ffm qqρµ

−=∆∆

• The changes in frequency of a piezoelectric quartz crystal, ∆f, are related to changes in mass, ∆m, of a substrate (e.g. Cu) that is attached to the quartz crystal:

October 17, 2005

28

FLCC

Teflon holder1 inch AT-cut polished

5 MHz Cr/Cu crystal

MaxTek, Inc., PM-700 Series Etching Products

Measurement resolution:

Frequency:0.5Hz

Weight: 0.01 µg/cm2

Thickness: 0.001 µm/min ( 0.1 Å/s)

EQCM

October 17, 2005

29

FLCC

pH 4, OCP, 0.01 M glycine premixed in acetate buffer

pH 4, OCP, 0.01 M pH 4, OCP, 0.01 M glycineglycine premixed in premixed in acetate bufferacetate buffer

Temporary loss in Temporary loss in weight, followed by weight, followed by significant gain in significant gain in weight, more weight, more pronounced at higher pronounced at higher concentration of Hconcentration of H22OO22..

October 17, 2005

30

FLCC

pH 9, OCP, 0.01 M glycine added to carbonate buffer after stabilization

pH 9, OCP, 0.01 M pH 9, OCP, 0.01 M glycineglycine added to added to carbonate buffer after stabilizationcarbonate buffer after stabilization

Slow loss in weight Slow loss in weight upon adding upon adding glycineglycine. . Temporary sharp loss Temporary sharp loss in weight after adding in weight after adding peroxide, followed by peroxide, followed by significant gain in significant gain in weight.weight.

October 17, 2005

31

FLCC

Effect of adding additional glycine, afteradding 2.09% hydrogen peroxide

Effect of adding additional Effect of adding additional glycineglycine, , afterafteradding 2.09% hydrogen peroxideadding 2.09% hydrogen peroxide

DeionizedDeionizedwaterwater

pH 9pH 9

October 17, 2005

32

FLCC

Open circuit potential of copper, pH 9, 0.01 M glycine and 2.09% hydrogen peroxide

Open circuit potential of copper, pH 9, 0.01 M Open circuit potential of copper, pH 9, 0.01 M glycineglycine and 2.09% hydrogen peroxideand 2.09% hydrogen peroxide

No No passivationpassivation without without HH22OO2. 2. See that behavior See that behavior is strongly dependent is strongly dependent on history of on history of glycineglycineadditions; oxidized additions; oxidized layers must resist layers must resist dissolutiondissolution

No HNo H22OO22. . Potential Potential same as that same as that induced by induced by HH22OO22

October 17, 2005

33

FLCC

Potential application in CMPPotential application in CMPPotential application in CMP

Polarize copper sample to oxidizing potential, to allow protective film to develop, then add

glycine - little effect

1- removal of protectivefilm by mechanical action

at protruding areasMetal Passivating

film2- freshly created

surface will dissolve actively in

glycine solution. Recessed regions remain protected

October 17, 2005

34

FLCC

Effect of glycine and H2O2 additions at different potentials, pH 9, 0.01 M glycineEffect of Effect of glycineglycine and Hand H22OO22 additions at additions at

different potentials, pH 9, 0.01 M different potentials, pH 9, 0.01 M glycineglycine

Iron diskIron disk--Au ring Au ring electrode. Helectrode. H22OO22produced during produced during reduction of Oreduction of O22is rapidly is rapidly reduced at high reduced at high and low and low potentials, but potentials, but can escape can escape electrode at electrode at intermediate intermediate potentialspotentials

S. S. ZeZeččevievićć, D.M. , D.M. DraDražžiićć, S. , S. GojkiviGojkivićć; ; J. J. ElectroanalElectroanal. . ChemChem, , 265 (1989) 179265 (1989) 179

At controlled At controlled potentials, either potentials, either oxidizing or reducing, oxidizing or reducing, HH22OO22 does NOT lead to does NOT lead to weight increase. weight increase. Protective film must be Protective film must be sensitive to potentialsensitive to potential

However, this is not consistent However, this is not consistent with with passivationpassivation at high at high concentrations of Hconcentrations of H22OO22

October 17, 2005

35

FLCC

Capturing Chemical/Electrochemical Behavior into Models for CMP

Capturing Chemical/Electrochemical Behavior Capturing Chemical/Electrochemical Behavior into Models for CMPinto Models for CMP

•• We know that chemical/electrochemical material We know that chemical/electrochemical material removal rates are relatively slow compared to rates removal rates are relatively slow compared to rates measured while polishingmeasured while polishing–– So is chemistry important at all?So is chemistry important at all?

•• Dornfeld group has successfully modeled various Dornfeld group has successfully modeled various aspects of mechanical behavioraspects of mechanical behavior

•• Talbot group has added colloidal behavior to the Talbot group has added colloidal behavior to the LuoLuoand Dornfeld model for material removal rates; and Dornfeld model for material removal rates; accounts for effect of solution chemistry on accounts for effect of solution chemistry on agglomeration of abrasive particlesagglomeration of abrasive particles–– Would there be large errors from ignoring chemistry?Would there be large errors from ignoring chemistry?

•• Yes!Yes!

October 17, 2005

36

FLCC

Chemistry also interacts synergistically with mechanical/colloidal phenomena

Chemistry also interacts synergistically with Chemistry also interacts synergistically with mechanical/colloidal phenomenamechanical/colloidal phenomena

Mechanical forces on copper introduce defects, increasing reactivity

Mechanical properties of films appear to be strongly dependent on chemistry, and probably potential

Chemistry affects degree of aggregation of abrasive particles.

Copper nanoparticleshave dramatic effect

October 17, 2005

37

FLCC

Adding chemistry/electrochemistry to modelsAdding chemistry/electrochemistry to modelsAdding chemistry/electrochemistry to models

•• Obtain quantitative information on Obtain quantitative information on mechanical properties of surface films under mechanical properties of surface films under different chemical conditionsdifferent chemical conditions–– Add to modelsAdd to models

•• Use curveUse curve--fitting to capture major chemical fitting to capture major chemical detailsdetails–– add to models as additive material removal rate add to models as additive material removal rate

over and above mechanical phenomenaover and above mechanical phenomena•• Model material removal with Model material removal with ““chemical chemical

toothtooth”” approach, and add kinetic data to approach, and add kinetic data to estimate rates for reforming passive filmsestimate rates for reforming passive films

October 17, 2005

38

FLCC







October 17, 2005

39

FLCC

How are electrode kinetics affected by multiple half-cell reactions?

How are electrode kinetics affected by How are electrode kinetics affected by multiple halfmultiple half--cell reactions?cell reactions?

Pote

ntia

l, V

(SH

E)Anodic

CathodicEcorr

• Metal A, which is more active, corrodes at E’corr

• Metal N, which is more noble, corrodes at a higher potential, Ecorr

• When metals A and N are in electrical contact, they develop a common corrosion potential, E”corr

• At E”corr, the corrosion current of N is lower than for the isolated metal, but the corrosion current of A is greater than for the isolated metal

• This effect is called galvanic corrosion

E’corr

E”corr

A

N

log i

October 17, 2005

40

FLCC

• Galvanic cells may form when polished patterned copper wafers with Ta or Ti-based barriers are immersed in CMP slurries; either copper or the barrier layer is corroded preferentially by galvanic corrosion

• Galvanic corrosion may impair the quality and reliability of copper interconnects. This effect will become increasingly pronounced with smaller-scale features

• Galvanic corrosion can be observed microscopically, measured by electrochemical techniques and controlled by modifying the slurry chemistry, temperature etc.

• Therefore, it is becoming increasingly important to understand the galvanic cells that develop during copper CMP, and incorporate this understanding into models

MotivationMotivation

October 17, 2005

41

FLCC

What is the effect of different electrode What is the effect of different electrode areas?areas?

•Total anodic CURRENT must equal total cathodic current•A small anode:cathode area ratio causes more severe galvanic corrosion, because• The total anodic current is concentrated over a smaller area • Hence the anodic dissolution rate increases

Jones, Denny A., Principles and prevention of corrosion, 2nd ed.

•This effect could be really problematic in CMP if diffusion barriers are more active than copper

October 17, 2005

42

FLCC

Galvanic Series and Sensitivity to Galvanic Series and Sensitivity to Solution ChemistrySolution Chemistry

•• A galvanic series is a series in which the corrosion A galvanic series is a series in which the corrosion potential of different metals and alloys is placed in potential of different metals and alloys is placed in ascending orderascending order

•• Comprehensive galvanic series have been published Comprehensive galvanic series have been published for a large number of metallic materials in sea water for a large number of metallic materials in sea water and aerated fresh waterand aerated fresh water

•• Much less information is available for copper and Much less information is available for copper and barrier materials in solution chemistries barrier materials in solution chemistries representative of CMP slurriesrepresentative of CMP slurries

•• Anticipate significant variations in potentials of Anticipate significant variations in potentials of copper and barrier materials according to the copper and barrier materials according to the precise precise complexingcomplexing agents and oxidantsagents and oxidants

October 17, 2005

43

FLCC

Polarization tests in solutions containing H2O2 and glycine

Polarization tests in solutions containing Polarization tests in solutions containing HH22OO22 and and glycineglycine

• Electrodes: Cu, Ti, Ta polished down to 0.25 polished down to 0.25 µµmmusing diamond pasteusing diamond paste• Electrolytes:

–Solution containing H2O2 and 0.01M glycine–Solution pH controlled using buffer solution

• Acetate buffer pH=3• Carbonate buffer pH=9• KOH solution pH=12

–Varying H2O2 concentration•• PotentiodynamicPotentiodynamic polarization measurements were polarization measurements were made on the Ta and Cu rotating disk electrodes, made on the Ta and Cu rotating disk electrodes, scanning at 1mV/s, rotating at 650 rpmscanning at 1mV/s, rotating at 650 rpm

October 17, 2005

44

FLCC

Ta-Cu, Ti-Cu, pH 3 buffer + 0.01M glycine+ 0.65 M H2O2

TaTa--Cu, TiCu, Ti--Cu, pH 3 buffer + 0.01M Cu, pH 3 buffer + 0.01M glycineglycine+ 0.65 M H+ 0.65 M H22OO22

-600

-400

-200

0

200

400

600

800

1000

1200

1400

1600

1800

2000

-10 -8 -6 -4 -2 0

LogI, A/cm^2

E vs

SC

E, m

V

Ta-0.65%wt H2O2

Cu-0.65%wt H2O2

-400

-200

0

200

400

600

800

1000

1200

1400

1600

1800

2000

-10 -8 -6 -4 -2 0

LogI, A/cm^2

E vs

SC

E, m

V

Ti-0.65%wt H2O2

Cu-0.65%wt H2O2

October 17, 2005

45

FLCC

Reminder – what you are seeing with polarization curves

Reminder Reminder –– what you are seeing with what you are seeing with polarization curvespolarization curves

Pote

ntia

l, V

(SH

E)

Anodic

Cathodic

Eoc

Eoc

Ecorr

Pote

ntia

l, V

(SH

E)

Net cathodic

Net anodic

Ecorr

log i log i

October 17, 2005

46

FLCC

Polarization tests Polarization tests –– commercial slurrycommercial slurry

•• ElectrodesElectrodes: : Ta rotating disk electrode (6.25mm in Ta rotating disk electrode (6.25mm in diameter) and Cu rotating disk (10mm in diameter), diameter) and Cu rotating disk (10mm in diameter), polished down to 0.25 polished down to 0.25 µµmm using diamond pasteusing diamond paste

•• ElectrolytesElectrolytes::–– Mixture of commercial slurry A + HMixture of commercial slurry A + H22OO22 (final concentration: (final concentration:

1.0%wt H1.0%wt H22OO22 or 6.0%wt Hor 6.0%wt H22OO22), referred to as Cu), referred to as Cu--1.0%H1.0%H22OO22, , CuCu--6.0%H6.0%H22OO22, Ta, Ta--1.0% H1.0% H22OO22 ,Ta,Ta--6.0% H6.0% H22OO22

–– 50 times dilution of the above mixture, referred to as Cu50 times dilution of the above mixture, referred to as Cu--1.0%1.0%--diluted, Cudiluted, Cu--6.0 %6.0 %--diluted, Tadiluted, Ta--1.0 %1.0 %--diluted,Tadiluted,Ta--6.0 %6.0 %--diluteddiluted

–– The above electrolytes had a measured pH of 4.00The above electrolytes had a measured pH of 4.00•• PotentiodynamicPotentiodynamic polarization measurements were polarization measurements were

made on the Ta and Cu rotating disk electrodes, made on the Ta and Cu rotating disk electrodes, scanning at 1mV/s, rotating at 650 rpmscanning at 1mV/s, rotating at 650 rpm

October 17, 2005

47

FLCC

TaTa--Cu: Commercial slurry + 1% HCu: Commercial slurry + 1% H22OO22

-1000-800-600-400-200

0200400600800

10001200140016001800

-10 -8 -6 -4 -2 0

LogI, A/cm^2

E vs

SC

E, m

V

Cu-1.0%-diluted

Ta-1.0%-diluted

October 17, 2005

48

FLCC

TaTa--Cu: Commercial slurry + 6% HCu: Commercial slurry + 6% H22OO22

-800-600-400-200

0200400600800

100012001400160018002000

-10 -8 -6 -4 -2 0

LogI, A/cm^2

Evs

SCE,

mV

Cu-0.6%-dilutedTa-6.0%-diluted

October 17, 2005

49

FLCC

Results of electrochemical measurements

Results of electrochemical Results of electrochemical measurementsmeasurements

0 200 400 600 800 1000-2.0x10-6

0.0

2.0x10-6

4.0x10-6

6.0x10-6

8.0x10-6

1.0x10-5

1.2x10-5

1.4x10-5

1.6x10-5

H2O2

Gal

vani

c C

urre

nt,A

Time,s

Ta:Cu=2.6:6.0 (cm2) Ta:Cu=3.9:6.0 (cm2) Ta:Cu=3.9:4.5 (cm2)

0 200 400 600 800 1000-0.300

-0.250

-0.200

-0.150

-0.100

-0.050

0.000

0.050

0.100

0.150

0.200

Pote

ntia

l, V

vs S

CE

Time, s

Ta:Cu=3.9:6.0 (cm2) Ta:Cu=2.6:6.0 (cm2) Ta:Cu=3.9:4.5 (cm2)

Area ratio effect, without polishing

Start with DI water, then add buffer, then glycine, then H2O2 to create pH=9 carbonate buffer, 0.01M glycine and 0.3%H2O2

October 17, 2005

50

FLCC

Results of electrochemical measurementResults of electrochemical measurementResults of electrochemical measurementEffects of [H2O2], pH=9, without polishing

0 300 600 900 1200

0.0

5.0x10-6

1.0x10-5

1.5x10-5

2.0x10-5

Gal

vani

c C

urre

nt, A

Time,s

0.3% 0.1% 0.6% 1.0%

0 300 600 900 1200-0.300

-0.200

-0.100

0.000

0.100

0.200H2O2, 30 wt%

stock glycine sltn

carbonate buffer

Mix

ed P

oten

tial,

V vs

SC

E

0.3% 0.1% 0.6% 1.0%

in pH=9 carbonate buffer, 0.01M glycine and varying [H2O2]

October 17, 2005

51

FLCCsample assembly capable electrochemical control and measurement

Cu Ta

Pt

Cu wires underneath for electrode connection

Galvanic currents and potentials will be measured with or without polishing, to investigate the effects of surface modification by chemicals and mechanical force on the polarity and galvanic current; area ratio of Cu:Ta is to be varied using different sample assemblies with different area ratios

Experimental setup and samplesExperimental setup and samplesExperimental setup and samples

•sample carrier

Potentiostat

Polishing pad

Rotating platen

Slurry film

Sample assemblyconditioner

slurrypressure

rotation

Electrical connector to Potentiostat leads

Reference electrode /Pseudo RE connecting to Potentiostat

October 17, 2005

52

FLCC

How to model galvanic effectsHow to model galvanic effects• Modest galvanic currents can have devastating

effects on quality if localized• Need to focus on feature level

• Basic electrochemical theory suggests that the corrosion current in a galvanic couple can be predicted• Relative areas of anodic and cathodic metals can

then be invoked to predict damage due to galvanic interactions

• But the corrosion potentials of Cu, Ta and Ti differ in different solution chemistries; in some cases the relative nobility changes• Hence the polarization behavior of each material

must be determined for different slurries to provide input for modeling copper CMP

October 17, 2005

53

FLCC







October 17, 2005

54

FLCC

ConclusionsConclusionsConclusions

•• The chemical and electrochemical mechanisms in The chemical and electrochemical mechanisms in copper CMP are complexcopper CMP are complex–– Sometimes the behavior is not fully understoodSometimes the behavior is not fully understood

•• However, the subtleties of chemical behavior alone However, the subtleties of chemical behavior alone may be of second order, compared to synergism may be of second order, compared to synergism between mechanical, chemical and colloidal between mechanical, chemical and colloidal behaviorbehavior–– A mechanistic understanding is invaluable in tackling A mechanistic understanding is invaluable in tackling

modeling of this synergismmodeling of this synergism•• Galvanic effects are different; here models must Galvanic effects are different; here models must

focus on feature scale effectsfocus on feature scale effects

chemistry and electrochemistry of copper cmp – capturing...

Documents