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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Challenges to the mechanics of materials and structures
•Large structures
•Hybrid materials (organic/inorganic)
•Small features
Thin-film transistor (TFT)
Al
SiNx
(n+) a-Si:H
undoped a-Si:H
SiNx
Ti/Cr
Hermetic seal
Active device
Polymer substrate
defect
100 nm
180 nm
50 nm
100 nm
360 nm
100 nm
inorganic
polymer
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How to make brittle materials flexible?
top
Thin substrate
Strain caused by bending
 top
c
Neutral plane
c 10μm
 
 10 3
R 10mm
R
Small flaws
Strain to cause fracture
c 


Ea
10 N/m
2

10
1011 N/m 2 10 6 m



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Suo, Ma, Gleskova, Wagner Appl. Phys. Lett. 74, 1177-1179 (1999).
n / n0
Gleskova, Wagner, Suo Applied Physics Letters, 75, 3011 (1999)
High strain and fracture
Cracks
G
S D
a-Si thin-film transistor (TFT)
on polyimide substrate
1
0
TFT island
Compression
-3
-2
Tension
-1
0
1
Strain (%)
2
3
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How to make stretchable circuits?
Most microelectronic materials fracture at small strains
(less than about 1%)
Polymer substrate
Islands, linked by interconnects
•Fracture at crossovers
•Fatigue of metals
•Small island size
Springs
•3D microfabrication
Hsu, Bhattacharya, Gleskova, Huang, Xi, Suo, Wagner, Sturm, APL 81, 1723 (2002).
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Debonding and cracking
Cracks
G
S D
SiN island on Kapton substrate
Bhattacharya, Salomon, Wagner
J. Electrochm. Soc. 153, G259 (2006)
TFT island
a-Si thin-film transistor (TFT)
on Kapton substrate
Gleskova, Wagner, Suo Applied Physics Letters, 75, 3011 (1999)
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Metal on polymer
Al, Cu, Au ~ 100nm
Kapton, Silicone
•Metal film deforms plastically (Ho, Kraft, Arzt, Spaepen…)
•What is the rupture strain of the metal film?
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Ductile vs. brittle film
 Rupture by necking
metal film
 Rupture by breaking atomic bonds
ceramic film
metal film
ceramic film
polymer substrate
polymer substrate
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FEM: large-amplitude perturbation
Free-standing
  0.023
Substrate-bonded
Long-wave perturbation
  0.8
Substrate-bonded
short-wave purturbation
=0.8
Conclusion from nonlinear analysis:
Substrate retards perturbation of ALL wavelengths.
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
Gage, Phanitsiri (2001)
Chiu, Leu, Ho, (1994)
Alaca, Saif, Sehitoglu (2002)
5000 Å Al film, 10 % Strain
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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Coupled
rupture and debond
Co-evolution:
necking and debonding
EE10.4 Thursday 2:30pm, Teng Li
Ductility of thin metal films on polymer substrates modulated by interfacial adhesion.
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 max  5MPa
T22
Li, Suo, IJSS (2006)
T12
35%
35%
37.3%
37.3%
38%
38%
38.5%
38.5%
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Effect of adhesion
100nm Cu /20nm C/ Kapton, strained to 6%
Xiang, Li, Suo, Vlassak,
APL 87, 161910 (2005)
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100nm Cu /10nm Ti/Kapton, strained to 10%
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 = 150 MPa,  = 37%
Esub = 300 MPa,  = 47%
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Li, Huang, Suo, Lacour, Wagner, Appl. Phys. Lett. 85, 3435 (2004)
Au film on PDMS substrate
survives large elongation
Lacour, Wagner, Huang, Suo,, APL 82, 2404 (2003).
5.0
4.5
Au (25 nm)
4.0
3.5
R/Rinit
Cr (5 nm)
3.0
2.5
2.0
1.5
1.0
0.5
PDMS (1mm)
0.0
0
10
20
30
40
50
60
70
time (h)
•electron-beam evaporation
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•101 cycles of elongation by 35%
Au film is cracked from the beginning, but…
a
b
1µm
As deposited
1µm
Lacour, Li, Chen, Wagner, Suo, APL 88, 204103 (2006).
c
stretching
direction
1st cycle to 35% strain
1µm
stretching
direction
101st cycle to 35% strain
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Other Compliant Patterns
Y-shaped cracks
When pulled, the sheet elongates
by buckling
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The importance of being compliant

L a
A
0.01
p u re
ne s
in-pla
tretch
1E-3
J/Ea
s tr e tc h
1E-4
a
-of-pla
nd o ut
ne b uc
kle
J c  Y h  109 Pa  108 m 

  11  6   104
Ea Ea  10 Pa  10 m 
1E-5
1E-6
1E-7
5
10
15
20
Engineering strain (%)
25
30
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Serpentine:
a compliant pattern of a stiff material
Large elongation, small strain
A platform for devices
Top surface
Bottom surface
Li, Suo, Lacour, Wagner, JMR 20, 3274 (2005)
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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.
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