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6/16/14
1
Stretchable Electronics - shaping electronic circuits -
Prof. Stphanie P. Lacour Lab. For Soft Bioelectronic Interfaces, EPFL
1
What is stretchable electronics?
Video overview https://www.youtube.com/watch?v=jlEIvGzthsk
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Outline
Mechanical concepts Materials Examples of stretchable circuits
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Todays electronic devices
flexible
large-area brittle
trigger foreign body reaction
flat batch
processed
Si technology 18-in. wafer Gen 10: 2850mmx3050mm (2010)
shattered iPhone
Cortical electrode at 1month post-implantation. P. Tresco 2007
Circuits can roll and bend. HP Flexible display
Flex circuits
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Electronics with mechanical freedom
Infineon 2011 power electronics on
40m thick wafer anything thin is flexible
Samsung 2012 flexible OLED display
University of Tokyo 2008
Tactile skin Takao Someya et al.
UIUC 2011 Epidermal electronics
John Rogers et al.
UIUC 2012 Transient
electronics John Rogers et al.
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Background Large Area Electronics
Substrates: Glass Device materials: thin films Technology: planar, thin film processing Applications: Flat Panel Displays (handheld devices,
phones, cameras, monitors, TVs), e-paper
back
plan
e / f
ront
plan
e
Glass carrier: 10m 1mm thick
TFT layer: 1m thick LCD, OLED Display layer
Encapsulation: 1m 1mm thick
circuit diagrams R. Street Adv. Mat. 2009, 21, 1-16
display cross-section 6
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Background - Flexible Electronics
Substrates: Plastic or metallic foils Device materials: thin and thick films Technologies: planar thin-film processing, printing,
electro-plating, hybrid techniques Applications: interconnects, sensors, RF ID tags,
rollable displays, cochlear implants,
Flexible Circuit Technology J. Fjelstad , 2006, 214p.
milFlexCircuit -3M
RF ID tag
All printed TFT on plastic PARC 2009
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On curvatures
A B C
Gaussian curvature: K = 12
A saddle shape: K < 0 B cylinder shape: K = 0 (1 = 0) C spherical shape: K > 0
foil coating
K 0
E 10 GPa bone
E 10-100 kPa skin
Electronic skin wrapped over the fingers
K 0
Electronic tent Orange
K 0 8
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Materials enable applications
Biocompatible + Elastic + Electrically Active + Process compatible
Level of difficulty
Integration of materials with very different properties
Substrates: plastic, elastic Devices: inorganic and/or organic transistors, diodes, sensors, antennas Encapsulation: brittle/plastic composites, elastic
need new design rules that are applicable to all materials
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Pixellated approach structural and physical mismatch
device
IC
IC
device
device
elastic substrate
elastic wiring
top view
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Architecture of stretchable circuitry
Pixellated structure
elastic substrate
rigid platforms hosting devices
elastic wiring
no applied strain 2D stretched
platforms do not deform
elastic wiring stretches
Wagner et al, Phys. E 2004; Lacour et al., Proc. IEEE 2006 11
Stretchable circuits
Stretchable thin-film transistor circuits Imperceptible electronics
Science 325 (2009)
Nature 499 (2013)458
T. Someya
J. Rogers Lacour et al IEEE EDL 25 (2005)
Graz et al APL 98 (2011)
S. Wagner S. Lacour Z. Suo
VDD# Vout#
Vin#
GND#
2mm#
load#TFT#
stretchable#interconnects#
drive#TFT#
J. Vanfleteren
Stretchable microchip
circuits
S. Bauer
Science 344 (2014)
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Deforming a surface
Stress/strain
Elasticity/plasticity
Mechanical failure
Key Mechanical Concepts
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Deforming a surface: Flexibility
Developable surfaces surfaces that can be flattened onto a plane without distortion
cylinder
cone
flat
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Deforming a surface: Stretchability
Deformable surfaces Surfaces that can conform complex shapes once or many times Surfaces that can expand and relax reversibly
stretched inflated
flat
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Stress
Mechanical stress and strain
)Paorm/N(areaload)(stress 2=
%100LL
lengthoriginallengthinchange)(strain ==
5.00;z
y
z
x 0; compressive strain < 0
Poisson ratio
16
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Mechanical strain
strain () = change in lengthoriginal length
=LL
100%
R0 h
bend
surface bending strain = = h2 R0
tensile strain > 0
compressive strain < 0
h/2
neutral plane
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Stress (strain) curve
stre
ss
strain
plastic
elastic
linear regime
permanent deformation
tensile strength
)Pa(E
= Youngs modulus
fracture
unload
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Typical Stress(strain) curves
strain (%)
0 0.05 0.1 0.15 0.2 1 5 25
stre
ss (M
Pa)
0
50
100
150
200
250
300
350
400
450 Brittle (glass-like) materials SiNx, SiO2, Si High stress, low fracture strain
Plastic deformation metals, stainless steel, polyimide High stress, higher fracture strain
SiNx SiOx
Si
stainless steel
polyimide
metal
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rubber
Mechanical Failure
Brittle behaviour
Crack propagates
Semiconductors Dielectrics Metal oxides
Ductile behaviour
Film necks
Metals
Z. Suo, Harvard University, http://www.seas.harvard.edu/suo/
stre
ss
strain
brittle ductile
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Substrates Elastomers
Device materials Brittle materials
Encapsulation Materials Plastics or elastomers
Materials for Stretchable Electronics
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Selection of substrates
Application-dependent RF ID tag OLED display implantable interface
Physical form dependent Lightweight conformable rollable stretchable
Manufacturing process dependent Batch R2R
PARC flex photosensors
Polyonics
Holst Centre LG Display
GlobalSolar Michigan State Univ.
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Main substrate types for stretchable applications
Polymer substrates flexible, optical transparency, rolls or spinable poor thermo-mechanical stability, rough surface, not elastic
Elastomeric substrates reversible stretchability, spinable, moldable poor thermal stability thus process compatibility, chemical susceptibility
Biodegradable substrates stiff enough for manipulation, degrades quickly cannot process directly of these materials
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Plastic substrates PolyEthylene Terephthalate (PET)
PolyEthylene Naphthalate (PEN)
Thermoplastic polymer Widely available Low cost
Polyester polymer More recent material Very good barrier properties
Teonex
Mylar
Polyimide (PI)
Thermoplastic Very good thermal stability Kapton
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Elastomeric substrates
Elastic polymers Long polymer chains cross-link during curing
Covalent cross-linkage
Natural and synthetic materials Rubber Silicones, acrylics, polyurethanes Biodegradable materials (silk, gels)
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PDMS Sylgard 184 Dow Corning
Polydimethylsiloxane 2-part polymer to mix in weight ratio E = 0.5-3MPa, ~ 0.5 Coef. of thermal expansion:
CTE = 310ppm/C Transparent Dielectric constant: 2.65
Preparation Casting, spin-coating Molding
Biocompatible grades
0 5 10 15 20 25 30-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
stre
ss (M
Pa)
s tra in (% )
E=2MPa
Silicon: Glass: Polyimide:
E = 160GPa E = 70GPa E = 3GPa
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Paper as a substrate
Packed cellulose fibers (+fillers) Thickness range: 0.1mm Light weight (10s g/m2) Opaque to translucent
Hygroscopic material Good electrical insulator ( [1010 - 1014.cm]; 1.3-4 @ 1MHz)
Often coated with protective materials R2R compatibility Low-cost, recyclable, biodegradable
Tobjork et al Adv. Mat. 2011 23 1935 Zschieschang et al Adv. Mater. 2011 23 654
http://www.handprint.com/
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Biodegradable substrate
From the drug-delivery field or natural materials Thermoplastic polyesters PLGA, Ecoflex (BASF) Potato/corn starch based materials Caramelized glucose Silk (water-soluble and enzymatically degradable)
Bettinger et al Adv. Mat. 2010 22 651 Irimia-Vladu et al Adv. Funct. Mater. 2010 20 4069
PLGA degradation over time in vitro
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Active device materials
1. Inorganic materials materials from the microelectronics and MEMS fields In thin film formats (
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Inorganic/organic brief comparison
Property Silicon Pentacene
Atomic bonding Covalent bond Molecular bond
Crystal symmetry Single crystal form Face-centered cubic
Amorphous, polycrystalline
Dimensionality (electronic prop.)
isotropic anisotropic
Deposition techniques High temperature High costs (epitaxy, etc.)
RT-low temperature Low costs (spin-coating, evaporation)
carrier mobility cm2/V.s
10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103
amorphous films
polycristalline films Molecular crystals
silicon a-Si:H polycryst. cryst.
orga
nics
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Thin is Flexible
1-atom thick materials Carbon Nanotubes CNTs and Graphene
http://ipn2.epfl.ch/CHBU/NTbasics1.htm 32
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Biocompatible, biodegradable organic materials
semiconductors
dielectrics
substrate
Advanced Functional Materials, 2010, 20 4069 33
Biodegradable, biocompatible inorganic materials
Mg, MgO, ZnO Degrade in water or biofluids into metal hydroxydes e.g. Mg(OH)2, Zn(OH)2
J. Rogers group, Small April 2013 34
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Liquid metals
Electrically conductive metallic fluid encapsulated in microchannels (Mercury) EGaIn: 3.104S/cm Low melting point: 17C In elastic conduits
Patternable Self-healing
Advanced Functional Materials, 23(18), 2308-2314 Advanced Functional Materials, March 2013
35
Deported/stiffening contacts
Electrical Probing
R. Carta et al. Sens. Act. A 2009 online stretchable section
stiffened contact
Contacts do not deform
Contacts stretch along
stretchable contact pads
5mm
S.P. Lacour, EPFL
Compliant contacts
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Results from the lab. LSBI, EPFL
37
relaxed!
stretched!
-1.2
-1
-0.8
-0.6
-0.4
-0.2
0-60 -50 -40 -30 -20 -10 0
VDS (V)!
I DS (
A)!
VGS = 0,-20V!
VGS = -40V!
VGS = -60V!appl = 12.6%"
Stretchable circuits
Applied Physics Letters, 2011, 98, 124101 Journal of Applied Physics, 2014, 115, 143511.
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The soft-to-hard challenge
PDMS"
stretched PDMS = 20%"
Cross-sections
non deformable or stiff layer"
PDMS"stretched PDMS = 20%"
sharp peak strain" >> 20%"
39
stretch
stretchable substrate
device island
elastic interconnect
On uniform stretchable substrate
Al2O3 disk Au film Al2O3 disk Au film
before stretching at 20% strain after stretching
Applied Physics Letters, 2013, 102, 131904
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embedded stiff platform
stretchable substrate
device island
elastic interconnect
stretch
D
t d
S
g h
Rigid embedded platforms
Al2O3 disk Au film
embedded SU8 platform
Al2O3 disk Au film
embedded SU8 platform
before stretching at 20% strain after stretching
Applied Physics Letters, 2013, 102, 131904
PDMS SU8
S=4D
100m 50m
D
S=4D
100m 50m
D
SU8 P-PDMS
D
PDMS SU8
S=4D
100m 50m
D
d
G
UV Cr mask sample cross-section
SU8
in P
DM
S Sl
oped
SU
8 in
PD
MS
SU8
in P
-PD
MS
D d D
G=2mm
Optimizing further
A. Romeo et al. Proc. of the SPIE 2014 (in print)
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Engineered Substrate- Strain Profile
A. Romeo et al. Proc. of the SPIE 2014 (in print)
Conformable and stretchable transducers
44
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Stretchable electronics new combinations of very different materials Micro/nanofabricated electronic circuits embedded in
ultra-compliant matrices Design of the enabling mechanical architecture for
stretchable circuitry Strain in semiconductors < 0.3% Elastic interconnects; devices on rigid platforms
An opportunity and many remaining challenges Exploring the science and technology for biomimetic
man-made interfaces.
Summary & Outlook
45
Acknowledgment
Laboratory for Soft Bioelectronic Interfaces, EPFL Ivan Minev, Katherine Musick, Tero Kulmala, Aaron Gerrat, Swati Gupta Cdric Paulou, Amlie Guex, Alessia Romeo, Arthur Hirsch, Hadrien Michaud
Zhigang Suos lab, Harvard University Qihan Liu