Improving the Quality of PVD Cu seed layer for ...

Improving the Quality of PVD Cu seed Improving the Quality of PVD Cu seed Improving the Quality of PVD Cu seed Improving the Quality of PVD Cu seed Improving the Quality of PVD Cu seed Improving the Quality of PVD Cu seed layer for Interconnect Metallizationlayer for Interconnect Metallization

Improving the Quality of PVD Cu seed Improving the Quality of PVD Cu seed layer for Interconnect Metallizationlayer for Interconnect Metallization

A. Dulkin, E. Ko, A. Dulkin, E. Ko, L. WuL. Wu, I. Karim, K. Leeser, and K.J. Park, I. Karim, K. Leeser, and K.J. ParkNovellus Systems, Inc.Novellus Systems, Inc.L. Meng, D.N. L. Meng, D.N. RuzicRuzic

Center for PlasmaCenter for Plasma--Material Interactions, University of Illinois at UrbanaMaterial Interactions, University of Illinois at Urbana--ChampaignChampaign

[email protected]@novellus.com408408--953953--46374637

OutlineOutline

Ø IntroductionØPVD in interconnect metallizationØPVD challenges, evolution of PVD to iPVDØHollow Cathode Magnetron (HCM)

Ø Objective and approaches of the researchØAdvantages and capabilities of HCM

Ø Plasma diagnostic methods and results

2

P. P. 22

Ø Plasma diagnostic methods and resultsØLangmuir probeØGridded Energy Analyzer (GEA)ØQuartz crystal microbalance (QCM)

Ø Process improvement resultsØFilm morphology, stability, filling capability, etc

Ø Conclusions

Interconnect MetallizationInterconnect MetallizationDual Damascene ProcessDual Damascene Process

A)SiN Cu Barrier DepVia Diel Oxide Dep* CMP Diel (buff)

B)SiN Etch Stop DepLine Diel DepARL Dep

C)Resist Via Pattern

D)Via Etch(2 Step)

E)Resist Strip

SiliconSilicon Cu

OxideOxide

SiliconSilicon Cu

OxideOxide

Resist Resist

SiliconSilicon Cu

OxideOxide

Resist Resist

SiliconSilicon Cu

OxideOxideOxide

Resist Resist

OxideOxide OxideOxide


OxideOxide

SiliconSilicon Cu



Resist Resist Resist Resist


P. P. 33 33

H)Resist Strip

SiN Cu Barrier Etch

F)Resist Line Pattern

G)Line Oxide Etch

I)Ta(N) Barrier Dep

J)Cu Seed Dep

K)Electrofill

L)Cu-CMP

M)SiN Cu Barrier Dep

SiliconSilicon Cu

OxideOxide


OxideOxide

Cu

SiliconSilicon Cu

OxideOxide


OxideOxide

Cu

SiliconSilicon Cu

OxideOxide


OxideOxide

Cu

SiliconSilicon Cu



SiliconSilicon Cu

OxideOxide


OxideOxide

SiliconSilicon Cu

OxideOxide


OxideOxide

SiliconSilicon Cu

OxideOxide


OxideOxide

SiliconSilicon Cu



PVD Realm

PVD Cu/barrier seed is PVD Cu/barrier seed is critical for defect free critical for defect free

interconnect metallizationinterconnect metallization

Challenges for PVD ExtendibilityChallenges for PVD Extendibility

è Overhang• Amplified by barrier/seed • Causes “pinch-off” voiding during electrofill

è Sidewall coverage• Thin or discontinuous (agglomerated) lower sidewall coverage can cause bottom voiding.

• Limited due to light-of-sight deposition and overhang growth

è Edge asymmetry

Example

P. P. 44

è Edge asymmetry• Asymmetry and “thinning” of films near wafer edge can cause voiding or barrier breakdown

• Critical with thinner sidewall coverage

è High aspect ratio• Becomes higher after B/S deposition presenting challenges for plating

è Aggressive structure• Dielectrics undercut/OH• Low-k dielectric, damage to dielectric• Rough dielectric surface

è …

Significant challenge to extend PVD to subSignificant challenge to extend PVD to sub--22nm technology22nm technology

Example

Example

Copper Barrier/Seed

Weak Spots

44

Evolution of PVD Evolution of PVD -- Ionized PVD Ionized PVD

M Ar+ M+

Metal neutral depositionPoor Coverage, Shadows, Overhang

formation

Metal ion depositionBetter Nucleation & Surface Mobility, Coverage

enhancement due to Resputtering, Overhang Reduction

P. P. 55

iPVDiPVD addresses some inherent limitations of PVDaddresses some inherent limitations of PVD

Conventional PVD Ionized PVD

55

OutlineOutline




6

P. P. 66



Ø Conclusions

Objective of Research, Hypothesis, and SolutionsObjective of Research, Hypothesis, and Solutions7

Existing problems Hypothesis Solutions

Discontinuous Cu coverage on sidewall

Neutrals with wide angular distribution build Overhang

Film continuity requires thick film.

Overhang increases to shadow sidewall

deposition

Narrow angular distribution by long

throw

Increase ion/neutral ratio

Cu island growth on Ta. Film continuity can be improved by promoting

Minimize scattering

P. P. 77

coverage on sidewall when film is thin ratio

improved by promoting 2D growth

Shift ion balance in the flux to Cu+ from Ar+Cu/Ta adhesion is

easy to compromise

Immiscible metals. Interface can be strengthened by

intermixing of materials Increase number of nucleation sites, implant deposition

species

Increase ion energy

Cu solubility in plating bath, creating voids

Rough, discontinuous film is easy attacked by

plating solution

Hollow Cathode Magnetron

è HCM combines magnetron action with electrostatic confinement of hollowcathode, and magnetic mirror to create very high density plasma in aconcave target

è High density plasma produces high ionization fraction in the depositionmetal flux

e-

Hollow Cathode Magnetron (HCM) Hollow Cathode Magnetron (HCM) iPVDiPVDGeneral Principles of OperationGeneral Principles of Operation

P. P. 88 88

Hollow Cathode Magnetron

e-

ExB

B

E

e-

EM coil

Magnetic separatrix

ions

Magnetic mirror

HCM generates high density plasma source and high metal HCM generates high density plasma source and high metal ion fractionion fraction

A magnetic cusp near the target opening acts as an aperture for plasma extraction into the downstream region

è The target erosion profile can be changed by

changing magnetic field

è Electrical and magnetic fields in the module

modulate the plasma fluxEM Coils

Rotating Magnet

Advantages of HCMAdvantages of HCM9

P. P. 99

è Ion energy can be controlled by changing plasma density which defines the potential drop in the wafer sheath

EM Coils

Deposition flux with high ion fraction

Wafer

Uniform and energetic ion component in deposition flux can be Uniform and energetic ion component in deposition flux can be achieved achieved

ApproachesApproaches

è Increase neutral directionality to improve stepcoverage:§ Long throw mode of deposition by shifting target erosionfrom sidewall to corner by changing magnetic confinement

§ Low pressure to minimize scattering: reduced from0.4mTorr to 0.1mTorr

è Change the primary ion species from Ar to CuErosion

Old process

P. P. 1010

New process

Both plasma diagnostics and film characteristics were performed to Both plasma diagnostics and film characteristics were performed to confirm the Cu seed improvement by modified processconfirm the Cu seed improvement by modified process

è Change the primary ion species from Ar to Cu§By decreasing Ar pressure and utilizing Cu self-sputtering

è Increase the Cu ion/neutral ratio§By increasing ionization via increase of the plasmadensity

è Improve ion flux uniformity and increase ion energy atwafer level§By changing magnetic field configuration at the wafer level

1010

OutlineOutline




11

P. P. 1111



Ø Conclusions

Plasma DiagnosticsPlasma DiagnosticsPlanar Langmuir ProbePlanar Langmuir Probe

Old process New Process

30

40

50

(V)

Vp-Vf

30

40

50

(V)

Vp-Vf

è Planar Langmuir probes developed and placed on wafer level

è ne, Te, ni, Vp and Vf were measured

è Ion energy (estimated as sheath potential Vp-Vf) increased about twice and became more uniform

P. P. 1212

0

10

20

30

0 50 100 150

Vp-V

f(V)

Position (mm)

0

10

20

30

0 50 100 150Vp-V

f(V)

Position (mm)

0

0.2

0.4

0.6

0.8

1

1.2

0 50 100 150

Ion den

sity (a.u.)

Position (mm)

Ion density

-0.3

0.2

0.7

1.2

0 50 100 150Ion den

sity (a.u.)

Position (mm)

Ion density

twice and became more uniform across the wafer

è Ion density increased by one order of magnitude on wafer level

New process has higher plasma New process has higher plasma density and ion energy with better density and ion energy with better

uniformity across the waferuniformity across the wafer

1212

No biasing needed

L. Wu, E. Ko, A. Dulkin, K.J. Park, S. Fields, K. Leeser, L. Meng, and D.N. Ruzic,, Rev. Sci. Instrum. November 10, 2010.

è Gridded energy analyzer (GEA) / QCM

� Measure ionization fraction of Cu

� Measure Ar+/Cu+ flux ratio

Plasma DiagnosticsPlasma DiagnosticsGridded Energy Analyzer/QCMGridded Energy Analyzer/QCM

PedestalWater cool

300mm

75mm

Ceramic plate

150mm

QCM

Grids

-+

Ceramic disk 3.2 mm

2.7 mm

3.9 mm

Electron repeller grid

Ion repeller grid

22.2 mm

-+

Ceramic disk 3.2 mm

2.7 mm

3.9 mm

Electron repeller grid

Ion repeller grid

22.2 mm

+

+

+≡

MMM

fractionionMetal0

P. P. 1313

� Measure ion energy distribution

è Typical Bias Setup

� Top grid floating

� Electron repelling grid -45 V (lowerthan Vf)

� Ion repelling grid -75 to 75 V

� QCM to record the metal dep rate

QCMA A

22.2 mm

QCMAA AA

22.2 mm

DeconvolutionDeconvolution of of ArAr++, Cu, Cu++ and Cuand Cu00 fluxesfluxes was achieved by measuring the ion current, Cu was achieved by measuring the ion current, Cu neutral neutral depdep rate and total Cu rate and total Cu depdep rate using the GEA/QCM assembly. rate using the GEA/QCM assembly.

1313L. Wu, E. Ko, A. Dulkin, K.J. Park, S. Fields, K. Leeser, L. Meng, and D.N. Ruzic,,

Rev. Sci. Instrum. November 10, 2010.

Plasma Diagnostics ResultsPlasma Diagnostics ResultsFluxes of Different SpeciesFluxes of Different Species

New processOld process

Cu+ Cu0Cu+&Ar+

P. P. 1414

è Fluxes of different species� Total ion flux (Ar+ and Cu+) increases (higher plasma density)

� Improved ion flux uniformity

� New process has lower Cu0 flux while higher Cu+ flux. More Cu atoms are ionized

1414

New process has higher ion density and Cu ion fluxNew process has higher ion density and Cu ion flux

Plasma Diagnostics ResultsPlasma Diagnostics ResultsIonization FractionIonization Fraction

Ar+/Cu+

Relative Cu ionization fraction

P. P. 1515

New process has higher Cu ionization and lower New process has higher Cu ionization and lower ArAr+/Cu+ ratio+/Cu+ ratio1515

è Cu ionization fraction increases more than twice and is more uniform� Lower at edge due to the diffusion process from source axis (separatrix opening)

è Decreased Ar+/Cu+ ratio.� Energetic Cu ions are preferred over Ar ions for transferring energy to Cu atoms onsurface

Plasma Diagnostics ResultsPlasma Diagnostics ResultsIon Energy DistributionIon Energy Distribution

è Ion energy distribution was resolved with the use of GEA

è The average ion energies agree with the Vp-Vf measurement

è At the center and middle radius, narrow ion energy distributions were observed.

è At the edge, the IED was IED obtained by taking the derivative of the collected current with respect to the energy.

Old process New Process

0

10

20

30

40

50

0 50 100 150

Vp-V

f(V)

Position (mm)

Vp-Vf

0

10

20

30

40

50

0 50 100 150

Vp-V

f(V)

Position (mm)

Vp-Vf

P. P. 1616

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

20 40 60 80

Ion energy distribution

Ion energy on wafer (eV)

Center

Middle

Edge

è At the edge, the IED was broadened and extended to the low energy side. This is caused by the non-perpendicular ion incident angles and the direction of magnetic field lines.

New process

collected current with respect to the energy.

Increased ion energies in the Increased ion energies in the new process were confirmed new process were confirmed with GEA measurementswith GEA measurements 1616

OutlineOutline




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P. P. 1717



Ø Conclusions

18

Area of

Improvement in Cu film CharacteristicsImprovement in Cu film CharacteristicsFilm MorphologyFilm Morphology

New processOld process

P. P. 1818

• Backwall of the seeded via was used as the test structure.

• Continuous seed is required for electrofill.

• Smooth film becomes continuous at the smaller thickness.

Area ofinterest

Improved seed morphology can result in continuous film Improved seed morphology can result in continuous film with lower thicknesswith lower thickness

~100 nm ~100 nm

19

40

Oxide

Improvement in Resistance to oxidation

Oxidation plasma grown oxide

Improvement in Cu film CharacteristicsImprovement in Cu film CharacteristicsFilm StabilityFilm Stability

P. P. 1919

20

Oxidethickness (A)

Old New

Smooth film with the less defined grains demonstrates Smooth film with the less defined grains demonstrates improved resistance to oxidationimproved resistance to oxidation

0

Resistance to dewetting

Barrier/seed interface wassubjected to H2 anneal totest the adhesion andstability of it.

Wafer Center: No Cu

Old process New process

Improvement of Cu SeedImprovement of Cu SeedInterface Stability and AdhesionInterface Stability and Adhesion

center center

500 nm

P. P. 2020

Wafer Center: No Cudewetting from the barrierfor both processes

Wafer Edge: Cu dewettedand agglomerated in oldprocess case while the newone demonstrated adhesionsimilar to the center

New process ensures that seed adhesion is uniform across New process ensures that seed adhesion is uniform across the waferthe wafer 2020

edge

r

edge

500 nm

4s 6s4s6sCold plating stability

DD features with seed layer

Old New

Improvement of Cu seedImprovement of Cu seedSeed StabilitySeed Stability

21

center

center

P. P. 2121

6s4s 4s 6s

DD features with seed layer(800A field thickness) were“cold” plated with variabledelay time.

New seed demonstratedlonger “life” in the platingbath as a result of betterfilm continuity

Better stability in the plating bath indicates improved seed Better stability in the plating bath indicates improved seed continuity and results in the wider continuity and results in the wider electrofillelectrofill process windowprocess window

edge

edge

Improvement of Cu seedImprovement of Cu seedElectrofillElectrofill of High Aspect Ratio of High Aspect Ratio ViasVias

Center

1488A 665A643A1453A

Edge Center Edge

Old New

P. P. 2222

1388A 1367A 542A 543A

The new process ensures The new process ensures electrofillelectrofill with 2x thinner seed. with 2x thinner seed. Uniformity is improved across the waferUniformity is improved across the wafer

2222

ConclusionsConclusions

Ø The emphasis of research was placed on improving Ta/Cubarrier/seed interface characteristics such as stability, adhesion, andmorphology on feature sidewalls with good uniformity across thewafer.

Ø Deposition flux was alternated by simple changing of magneticconfinement and operating pressure.• Planar Langmuir probe, GEA and QCM diagnostics have been utilized in assistingof changing the parameters in desired direction.

23

P. P. 2323

Ø Improved deposition was achieved due to• Long throw effect for the neutral flux• Increased plasma density and high metal ion fraction in the total deposition flux• Increased ion energy• Improved uniformity of plasma parameters• Enhanced Cu ion fraction in total ion flux

Ø Resulting seed demonstrated improved performance, which allowedfor better and more uniform electrofill with less seed thickness, aswell as better resistance to oxidation and agglomeration, which cantransform into better resistance to electro and stress migration.

ReferencesReferences

q L. Wu, E. Ko, A. Dulkin, K.J. Park, S. Fields, K. Leeser, L. Meng, and D.N. Ruzic, “Flux and Energy Analysis of Species in Hollow Cathode Magnetron Ionized Physical Vapor Deposition of Copper”, The Review of Scientific Instruments, 81, 123502, (2010)

q L. Meng, R. Raju, R. Flauta, H. Shin, D.N. Ruzic, and D.B. Hayden, J. Vac. Sci. Technol. A 28, 112 (2010)

P. P. 2424 2424

Thanks for your attention!

Old process New process

53% 64%

Improvement in Cu film CharacteristicsImprovement in Cu film CharacteristicsStep CoverageStep Coverage

Backup slide

P. P. 2525 2525

53%

12%

12%

14%

45%

64%

12%

28%

16%

76%

Improved step coverage as a result of more directional Improved step coverage as a result of more directional deposition fluxdeposition flux

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