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Micromechanics of macroelectronics

Zhigang Suo

Harvard University

Work with

Teng Li, Yong Xiang, Joost Vlassak (Harvard University)Sigurd Wagner, Stephanie Lacour (Princeton University)

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Displays

Sony e-Reader

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Roll-to-roll printing Low cost, large area

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polymer

inorganicdefect

Challenges to the mechanics of materials and structures

•Large structures•Hybrid materials (organic/inorganic)•Small features

Polymer substrate

Active device

Hermetic seal

Thin-film transistor (TFT)Al

undoped a-Si:HSiNxTi/Cr

100 nm

100 nm360 nm100 nm

180 nmSiNx(n+) a-Si:H 50 nm

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How to make brittle materials flexible?

310mm10

μm10 R

ctop

Thin substrateStrain caused by bending

2

621110

m10N/m10

N/m10

Eac

Small flawsStrain to cause fracture

R

Neutral plane

c

top

Suo, Ma, Gleskova, Wagner Appl. Phys. Lett. 74, 1177-1179 (1999).

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G

S D TFT island

a-Si thin-film transistor (TFT)on polyimide substrate

0

1

-3 -2 -1 0 1 2 3Strain (%)

TensionCompression

n /

n0

High strain and fractureGleskova, Wagner, Suo Applied Physics Letters, 75, 3011 (1999)

Cracks

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How to make stretchable circuits?

Islands, linked by interconnects•Fracture at crossovers•Fatigue of metals•Small island size

Most microelectronic materials fracture at small strains (less than about 1%)

Polymer substrate

Springs•3D microfabrication

Hsu, Bhattacharya, Gleskova, Huang, Xi, Suo, Wagner, Sturm, APL 81, 1723 (2002).

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G

S D TFT island

a-Si thin-film transistor (TFT)on Kapton substrate

Debonding and cracking

Gleskova, Wagner, Suo Applied Physics Letters, 75, 3011 (1999)

Cracks

SiN island on Kapton substrate

Bhattacharya, Salomon, WagnerJ. Electrochm. Soc. 153, G259 (2006)

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Metal on polymer

nm100~Al, Cu, Au

•Metal film deforms plastically (Ho, Kraft, Arzt, Spaepen…)•What is the rupture strain of the metal film?

Kapton, Silicone

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Ductile vs. brittle film

metal film

Rupture by necking

ceramic film

Rupture by breaking atomic bonds

ceramic film

polymer substrate

metal film

polymer substrate

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FEM: large-amplitude perturbation

023.0

8.0

Free-standing

Substrate-bondedLong-wave perturbation

Conclusion from nonlinear analysis:Substrate retards perturbation of ALL wavelengths.

Substrate-bondedshort-wave purturbation=0.8

Li, Huang, Suo, Lacour, Wagner, Mechanics of Materials 37, 261 (2005)

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Al film on Kepton substrate

5000 Å Al film, 7 % Strain 5000 Å Al film, 10 % Strain

Gage, Phanitsiri (2001)

Chiu, Leu, Ho, (1994)

Alaca, Saif, Sehitoglu (2002)

Channel cracks start at ~2% strain

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Possible causes for small rupture strains of metal on polymer

• The film is brittle.

• The film debonds from the substrate.

• The substrate is too compliant.

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Co-evolution: necking and debonding

      

  

Coupled rupture and debond

EE10.4 Thursday 2:30pm, Teng Li Ductility of thin metal films on polymer substrates modulated by interfacial adhesion.

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MPa5max

T22

35%

37.3%

38%

38.5%

T12

35%

37.3%

38%

38.5%

Li, Suo, IJSS (2006)

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Xiang, Li, Suo, Vlassak,

APL 87, 161910 (2005)

100nm Cu /10nm Ti/Kapton, strained to 10%

100nm Cu /20nm C/ Kapton, strained to 6%

Effect of adhesion

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170nm Cu /10nm Ti/ Kapton strained to 30%

Xiang, Li, Suo, Vlassak, APL 87, 161910 (2005)

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The effect of substrate stiffness

Esub = 2 MPa, = 2.8%

Esub = 300 MPa, = 47%

Esub = 150 MPa, = 37%

Li, Huang, Suo, Lacour, Wagner, Appl. Phys. Lett. 85, 3435 (2004)

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Au film on PDMS substrate survives large elongation

0 10 20 30 40 50 60 700.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

R/R

init

time (h)

•electron-beam evaporation•101 cycles of elongation by 35%

PDMS (1mm)

Cr (5 nm)

Au (25 nm)

Lacour, Wagner, Huang, Suo,, APL 82, 2404 (2003).

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Au film is cracked from the beginning, but…

1µm

stretching direction

b 1µm

stretching direction

c

1st cycle to 35% strain

1µma As deposited

101st cycle to 35% strain

Lacour, Li, Chen, Wagner, Suo, APL 88, 204103 (2006).

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Other Compliant Patterns

When pulled, the sheet elongates by buckling

Y-shaped cracks

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The importance of being compliant

5 10 15 20 25 301E-7

1E-6

1E-5

1E-4

1E-3

0.01

stretch and out-of-plane buckle

pure in-plane stretch

J/Ea

Engineering strain (%)

L a

A

L aL aL a

A

46

8

11

9

1010

10

10

10

m

m

Pa

Pa

Ea

h

Ea

J Yc

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Serpentine:a compliant pattern of a stiff material

Top surface

Bottom surface

Li, Suo, Lacour, Wagner, JMR 20, 3274 (2005)

Large elongation, small strainA platform for devices

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Summary

A stiff polymer substrate can retard necking in a metal film.

A compliant polymer substrate can accommodate large displacement of a patterned film.