Post on 06-Jan-2016
description
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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
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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.