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September 25, 2003

Surface Phenomena at Metal-Carbon Nanotube Interfaces

Quoc NgoDusan Petranovic

Hans YoongShoba Krishnan

Cary Y. YangBack

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September 25, 2003

Outline

• Motivation

• Multi-wall carbon nanotube (MWNT) architectures

• Mechanisms of contact resistance

• Characterization of contact resistance

- Side-contacted architecture

- End-contacted architecture

• Conclusion

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Motivation

• Physical limits of copper interconnects and vias will soon be reached i

f scaling trends continue

Chen et al., IEEE Elec. Dev. Lett., 19, 508(1998)

Wire Length:

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September 25, 2003

Motivation

• CNTs provide a feasible alternative due to their superior el

ectrical and mechanical properties

• Full understanding of CNT contact resistance has yet to be

ascertained

• CNT growth processes can be integrated into silicon-based

manufacturing

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September 25, 2003

DiamondC60

Buckyball

Graphite Nanotube

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MWNT Architectures:Side-contacted geometry

• Contacts are either pre-patterned on the substrate, or deposited after th

e nanotube has been dispersed onto a substrate

• Contact is made with the side of the MWNT

Wei, et al., Appl. Phys. Lett., 79, 1172(2001)

Spacing between electrodes ~2.5m

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September 25, 2003

MWNT Architectures:End-contacted geometry*

• Nanotubes are grown vertical

ly from a patterned catalyst fi

lm

• Contact is made with the end

of the MWNT

AFM (current sensing mode) and SEM top view

5μm

*Li et al., Appl. Phys. Lett., 82, 2491 (2003)

200nm

500nm

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Mechanisms of Side-contact ResistanceCopper interconnect:

CNT interconnect:

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Mechanisms of Side-contact Resistance

• Direct or Fowler-Nordheim tunneling between two metals t

hrough a Schottky Barrier (metal-insulator-metal)

• The type of tunneling is dependent on the work function of

the metal, and the applied bias

• Tunneling in an MIM system is approximated by Simmons

(J. Appl. Phys., June 1963)

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0 0.5 1 1.5 2 2.5 30

1

2

3

4

5

x 10-8

Vapp

[V]

Tungsten (W)

M = 4.55eV

Gold (Au)

M = 5.1eV

Platinum (Pt)

M = 5.65eV

0 1 2 3 4 50

1

2

3

4

5

x 10-8

Vapp [V]

Tungsten (W)

M = 4.55eV Gold (Au)

M = 5.1eV

Platinum (Pt)

M = 5.65eV

0(W) = 3.09

0(Au) = 3.64

0(Pt) = 4.19

Work Function Dependence of Side-contact Resistance

Cal

cula

ted

Con

tact

Res

isti

vity

[Ω

-cm

2 ]

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September 25, 2003

Mechanisms of End-contact Resistance

Single MWNT Resistance:

Chromium underlayer

SiO2

MWNT

AFM probe tip

Tungsten probe tip (on ~10μm chromium pad)

Parallel MWNT Resistance:

• AFM tip to MWNT (contact)

• MWNT to metal underlayer

• Metal underlayer sheet resistance

• Probe tip/metal to MWNT (contact)

• MWNTs to metal underlayer

• Metal underlayer sheet resistance

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End-contact Nantotube Characterization

• A statistical approach is taken for calculating resistance of a single M

WNT by measuring many MWNTs in parallel

10μm

• Nanotube diameters = 50-100nm

•~5-6 MWNT per 1μm2

• 100μm2 contains ~500-600 MWNT

• R(single MWNT) 24-29k

-4 -3 -2 -1 0 1 2 3 4-100

-80

-60

-40

-20

0

20

40

60

80

100

R=43 (10x10m contacts)

Voltage [V]

Cur

rent

[m

A]

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September 25, 2003

Metal Underlayer Sheet Resistance

• Chromium sheet resistance is a small percentage of overall resistance in

four-terminal configuration

• Appears to be resistant to high temperature effects of CVD processing

Measurement Type Bare Cr Resistance

(no CVD processing)

Bare Cr Resistance

(post CVD processing)

Two-terminal 12-15Ω 10-15Ω

Four-terminal 2-4Ω ~6Ω

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Importance of Quality Contacts• To demonstrate the importance of quality contacts, we conduct two different measurements:

a) Contacting parallel nanotubes with W probe tip (no contact)

b) Contacting parallel nanotubes through a deposited Cr contact

(a)

(b)

-4 -3 -2 -1 0 1 2 3 4-100

-80

-60

-40

-20

0

20

40

60

80

100

No contact pad (direct probe)With 10x10m contact pad

Voltage [V]

Cur

rent

[m

A]

(b) R=44Ω

(a) R=76Ω

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Conclusion

• Two different metal-CNT contact geometries are studied

• Side-contact resistance is simulated using MIM tunnel jun

ction theory

• End-contact resistance is examined w.r.t. processing effect

s

• Overall resistance for parallel MWNTs demonstrates excel

lent potential for on-chip interconnect applications

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Center for Nanostructures

September 25, 2003

Partners• Center for Nanotechnology at NASA Ames Researc

h Center

- Drs. Meyya Meyyappan, Jun Li, Alan Cassell, La

ura Ye

• National Center for Electron Microscopy (Lawrenc

e Berkeley National Laboratory)

- Dr. Velimir Radmilovic

Publications• Quoc Ngo, et al., “Surface Phenomena at Metal-Carbon Nanotube Interfaces,” IEEE NANO 2003, San Francisc

o, vol. 1, pp. 252-255, August 11-14, 2003.