1

A Novel Interconnect Design with High Stretchability and Fine Pitch

Capability for Applications in Stretchable Electronics

Yung-Yu Hsu1, 2, Mario Gonzalez1, Frederick Bossuyt3, Fabrice Axisa3, Jan

Vanfleteren3, and Ingrid De Wolf1, 2

1IMEC-IPSI, Kapeldreef 75, 3001, Leuven, Belgium

2Department of Materials Engineering, K.U. Leuven, Belgium

3IMEC-CMST, Gent-Zwijnaarde, Belgium

Abstract

In this paper, the electromechanical properties of a new design for metal

interconnection in stretchable electronics are reported. In this design, a patterned

metal interconnect with a zigzag shape is adhered on an elastomeric substrate. The

electrical resistance remains constant until metal breakdown at elongations beyond

40%. There is no significant local necking in either the transverse or the thickness

direction at the metal breakdown area as shown by both scanning electron microscopy

micrographs and resistance measurements. Micrographs and simulation results show

that a debonding occurs due to the local twisting of a metal interconnect, out-of-plane

peeling and strain localized at the crest of a zigzag structure.

Keywords: stretchable interconnect; debonding; twisting; patterned metal film;

polymer substrate

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1. Introduction

Large area deformable macroelectronics, such as flexible display [1], electronic

skin [2], electronic textile [3], etc., have to withstand various modes of deformation

(e.g., bending, twisting and stretching). Such electronic systems usually are composed

of inorganic parts with limited deformability, and organic parts which can sustain

large deformations. Because of the elastic nature of elastomers, these materials are

often used as substrates for the specific applications mentioned above. In order to

fulfill the demand of deformability, many concepts have been developed. One of these

concepts consists of small rigid islands with active devices or individual thin chips

which are interconnected by thin metal conductor lines. All rigid components are

placed on the small islands to ensure that the strains acting on these brittle

components are small when the structure is subjected to a large deformation. Since the

thin conductor lines have to withstand all these deformations, a proper structural

design is necessary to avoid losing structural integrity and electrical functionality

during this deformation.

Several technologies have been proposed in recent years, such as in-plane

patterned metal conductors [4,5,6], out-of-plane wrinkling metal films [7,8], and

conductive polymers or liquid alloys [9,10]. It has been reported that by depositing a

thin metal strip with a thickness of few nanometers on an elastomeric substrate, the

elongation of the metal can go up to 50% while the strip remains conductive [11].

Upon large strain, the elastic deformation of the elastomeric substrate causes local

debondings of the metal film coevolving with strain localizations [12,13,14,15,

16,17,18,19]. Thanks to this coevolved process, even a ‘straight’ line, deposited on a

polymer substrate, is stretchable. However, a drawback of this unique characteristic is

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that the resistance of the thin metal film changes with elongation, which might be a

disadvantage for certain applications. Another approach for having a stable resistance

is to use bulk metal conductors. Compared to a freestanding bulk metal straight line

which ruptures at strains of 1%~2% [20], an in-plane patterned horseshoe metal

conductor can be stretched up to 100% with a stable resistance before electrical

failure [21,22].

This paper demonstrates a novel in-plane patterned zigzag structure, with

resistance independent of elongation before metal rupture, which can be stretched

beyond 40%. The advantage compared to the horseshoe [21,22], is that due to the

geometrical design, the in-plane zigzag structural interconnects can be applied in a

fine pitch microelectronic device, as shown in figure 1. The experimental

observations on local twisting of the zigzag structure and delamination at the interface

after stretching are discussed and experimental results are supported by numerical

explanations.

2. Finite element modeling

Figure 2 (a) illustrates the design of the patterned zigzag metal conductor

adhering on an elastomeric substrate. Because of the repetition of the zigzag structure,

only the first five crests are shown in this figure. By taking advantage of the

symmetric geometry, half of the zigzag structure was modeled by finite element

simulations. The angle of each crest of the patterned zigzag metal conductor is 60

degree, 3 mm in amplitude. The width and the thickness of the metal track are

0.1 mm and 0.018 mm, respectively. The substrate is a 10 mm × 25 mm × 0.5 mm

block. The uniaxial elongation μ is applied on the end surfaces of the substrate as the

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boundary condition, which corresponds to the experiment. For the convenience of

future analysis, each crest of the zigzag structures is marked (positions A to F),

corresponding with stress and strain concentration points. Out-of-plane deformation

and average equivalent plastic strain will be extracted and analyzed from these points

of the model. The commercial finite element code, MSC.MARC®, was used to

simulate the deformation process of the patterned metal on an elastomeric substrate.

The metal used in this research is copper and is modeled as a plastically deformable

solid, obeying the bi-linear kinematic hardening rule, with the elastic Young’s

Module 0E =117000 MPa, the yielding point at yσ =172.3 MPa, and the Tangent

Module tE =1034.2 MPa. The elastomeric substrate is modeled as an incompressible

hyperelastic Neo-Hookean solid [23] with C10=0.157.

3. Sample preparation

Poly-dimethyl siloxane (PDMS) (Sylgard 186®, Dow Corning) was chosen as

the elastomeric substrate, carrying a patterned metal on top of it. In order to achieve a

stable resistance, low cost and large area fabricating capability, a stand-alone copper

foil with 0.018 mm in thickness was not fabricated in house (which can be done by

sputtering, electroplating, or evaporating copper), but purchased from a printed circuit

board vendor. In order to improve the weak adhesion between the organic and the

inorganic interface, a potassium monopersulfate solution was used for microetching

the copper surface. After microetching, the surface of the copper foil was 2 µm in

roughness as measured by a WYKO interferometer. The prepared copper foil was

then temporarily adhered on a Teflon mold, which is 0.5 mm in thickness. The Teflon

mold has an opening window which is used for casting the PDMS on top of the

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copper foil. The PDMS was prepared in a 10:1 weight ratio of the polymer base and

curing agent at room temperature. After degassing of the air bubbles, the PDMS was

poured in the Teflon window and cured at 60ºC for 12 hours. The conventional

photolithography and wet etching processes were then employed after the Cu/PDMS

lamination was released from the Teflon mold. The Cu/PDMS lamination was cut into

10 mm× 70 mm stripes and two connecting wires were soldered on each pad for

electrical measurements, as shown in figure 2 (b). The distance between the two pads

in free-standing condition was 50 mm (without applied strain).

4. Experimental Setup

The electromechanical properties of the zigzag interconnect on the elastomeric

substrate were evaluated by stretching the substrate on a custom made tensile tester

(Fig. 3). All tests were performed at room temperature and at a constant strain rate of

4 12.5 10 s− −× . During tensile testing, the electrical resistance of the patterned metal

interconnect was recorded in-situ by an Agilent 34420A multimeter. Each data point

of electrical resistance was recorded every 1/3s. The four points probing technique

was employed in the testing so that the resistance of the connecting wires can be

neglected. By this technique, only the resistance of the patterned zigzag interconnect

was measured. The surface morphologies of the fully unloaded specimens were

characterized using a JEOL 5600LV scanning electron microscope (SEM).

5. Result and Discussion

5.1 Electrical resistance under mechanical deformation

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Figure 4 shows that the resistance is independent of the applied uniaxial strain.

The specimen was subjected to a one-time stretching and the experiment was

terminated when the electrical circuit failed at an elongation of 43%. The initial

resistance was Rinitial = 1.06 Ω when the sample was in the relaxed state. During the

tensile test and before the metal break, the electrical resistance remained constant

within a ±2% oscillation. The oscillation is caused by noise originating in the

measurement system. At 43% elongation, there was an abrupt break of the conductor

line resulting in a sudden infinite increase of the resistance (open circuit). The

resistance remained unchanged up to that large elongation because the patterned

zigzag conductor mainly deforms due to opening of the zigzag structure (geometrical

deformation, i.e. opening of the crest), and not due to metal strain (elongation of the

Cu line). The electrical circuit failed due to a metal break at the first crest (position B,

see inset in Fig. 4), although interfacial debonding combined with local twisting was

also observed at other crests. This failure at position B can be explained by the

geometrical non-symmetry and discontinuity of this first crest.

5.2 Local twisting and out-of-plane deformation caused debonding

Figure 5 shows the simulated equivalent plastic strain εpl of the zigzag structure

at 40% elongation. In order to compare the local strain at different crests, the

normalized average equivalent plastic strain, calculated from these simulations, is

plotted in Figure 6(a) at the different crest positions (A to F, see Fig. 2a). When the

zigzag structure is subjected to μ=40% elongation, position B has the highest local

plastic strain. This can be explained by the fact that B has an asymmetrical geometry,

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with one short and one long arm. Also A has an unsymmetrical geometry, however

the change of the angle between the arms upon elongation is smaller in A than in B.

For this reason A has a lower equivalent plastic strain than B, nevertheless, higher

than C, D etc. Figure 6 (b) shows the deformed zigzag interconnect during stretching.

Both simulation and experimental results show that position B has a larger

geometrical opening than the other positions which results in the highest local plastic

strain and explains why the Cu line fails first in position B (see Fig. 4, inset). The

plastic strain reaches a constant value starting from position F because of the stable

internal stress/strain distribution.

The local plastic strain at each crest is not only caused by the geometrical

opening of the arms, but also by local twisting and out-of-plane deformation during

the stretching. Indeed, upon deformation the inner, concave side of the crest tends to

move downwards, pushing the substrate locally down; while the outer, convex side of

the crest moves upwards, pulling the substrate up. As a result, the Cu line is not lying

flat anymore but is locally tilted (twisting). The forces acting on the substrate can

plastically deform the substrate at the inside (pushed down), while they can result in

delamination of the Cu from the substrate at the outer side (peeling-off). Figure 6 (c)

shows the simulation results of the normalized out-of-plane deformation at the outer

side of the crests. Since the metal is adhered to the substrate, the out-of-plane

deformation is confined as long as debonding does not take place, and thus in-plane

opening dominated the deformation.

Figure 7 shows a scanning electron microscopy (SEM) micrograph taken at

position A (see Fig. 2a) on the relaxed sample after the tests shown in Fig. 4, i.e. after

detecting the open circuit. It clearly shows a local cohesive debonding at position A,

but the Cu line is still intact. This debonding is caused, as explained above, by the

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local out-of-plane deformation and twisting. Some PDMS at the debond region is still

adhered to the Cu, indicating that the PDMS is locally tearing. This is because the

adhesion strength between copper and PDMS is larger than the strength of the PDMS.

The rough porous looking surface of the PDMS substrate is caused by the surface

roughness of the copper foil since the PDMS was casted on the copper surface. The

rough copper surface enhances the interfacial adhesion, and thus the crack went

through the porous region of the PDMS substrate because of its weak structural

integrity.

On the inner edge of the crest at position A, due to the local twisting

deformation, the copper interconnect compressed the PDMS substrate. The Cu was

plastically deformed and as such did not return back to its original geometry.

Figure 8 (a) and (b) show a set of SEM micrographs taken with different tilted

angles of position B. At this position, there is not only twisting, delamination and

plastic deformation of the Cu, in addition the metal broke and caused the electric

circuit failure. Not only the Cu cracks, there is also a crack in the PDMS at the same

position. No significant necking was observed at the cross-section of the metal rupture

area. When the specimen is being stretched to a large deformation, the crack of the

copper interconnect near the crest probably initiated and propagated too fast without

allowing large plastic necking. This phenomenon can also be attested by the

resistance measurements. If there were plastic necking during stretching, the

resistance would increase substantially before failure instead of remaining constant.

From the observation of micrographs and from simulation results, it is found

that the local twisting coevolved with the out-of-plane deformation at all the arms and

the crests. Figure 9 illustrates the typical deformation of the arms and the crest on a

side view (30o tilted) SEM micrograph of position C. The twisting arms caused local

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debonding from the substrate in one edge and compression on the PDMS substrate in

the other. The crest was peeled off from the substrate due to out-of-plane deformation.

It should be noted that, although there are local debonding and out-of-plane

deformation, no local necking or crack was observed at position C and beyond.

6. Conclusions

In summary, a zigzag shape patterned copper interconnect, with fine pitch

capability, supported by an elastomeric polymer substrate was fabricated by using a

large area and low cost process. Our experiments show that, with a proper patterned

structure design of the metal interconnect such as this zigzag structure, the metal

interconnect can deform without rupture when subject to large uniaxial tensile strain:

The electrical resistance of the interconnect remains constant up to a strain level of

43%, upon which the electric circuit fails due to breaking of the Cu in crest B. Both

the experimental and simulation results indicated that out-of-plane deformation on the

crests and local twisting coevolved. Although there was a debonding between the

metal and polymer substrate, local plastic necking of the metal interconnect was not

observed.

It is shown that this zig-zag design offers promising possibilities for stretchable

interconnects, especially for fine pitch applications. We demonstrated by FEM

simulations, by experiments and by SEM inspection that the weak point is clearly at

crest B. Therefore, by optimizing the design at that position using FEM, it is expected

that we can further improve the stretchability of this zigzag Cu interconnect structure.

Acknowledgements

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This work was supported by European Commission, under the research project

of STELLA (Contract Number 028026). The authors would like to thank Frederic

Duflos and Veerle Simons for their support of equipment setup and experiment.

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Figure 1. Schematic of the pitch comparison between zigzag and horseshoe

interconnects; “d1“denotes to the pitch of zigzag interconnect and “d2” denotes the

pitch of horseshoes. Obviously, “d1” is smaller than “d2”.

d1

d2

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(a)

(b)

Figure 2. (a) Schematic of the geometrical design of the zigzag structure on an

elastomeric substrate which is subject to uniaxial elongation. Only five crests are

shown here because of repetition. (b) A freestanding specimen with connecting wires

soldered on each pad for electrical measurements.

A

B C

D E

F

μ

Pad

Elastomeric substrateUniaxial elongation

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Figure 3. Experimental setup for uniaxial tensile test.

Stretching Direction Fixed End

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0 10 20 30 40 50 600.9

1.0

1.1

1.2

1.3

1.4

1.5

Res

ista

nce(Ω)

Uni-axial Strain (%)

Infinite Resistance

Figure 4. Electrical resistance of the Cu zigzag connector as a function of the applied

uniaxial tensile strain. The connector fails (open) at 43% strain.

metal break

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Figure 5. Three dimensional finite element model before and after deformation. The equivalent plastic strain distribution is visualized on the Cu line only.

Pad

A

B

C

D

E

F

Stretching Direction

A

B C

D

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A B C D E F0.0

0.5

1.0

1.5

2.0

Nor

mal

ized

Ave

rage

Equ

ival

ent P

last

ic S

train

Position (a)

(b)

A B C D E F0

1

2

3

4

5

Nor

mal

ized

Out

of P

lane

Def

orm

atio

n

Position (c)

Figure 6. (a) Column graph of normalized average equivalent plastic strain at crests A

to F (see 2a) at 40% elongation. The results are obtained by finite element simulations.

(b) The deformed zigzag interconnect when subjected to stretching. (c) Column graph

of normalized out-of-plane deformation at crests A to F when at 40% elongation.

A B

C

D

E

F

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Figure 7. Relaxed state of Cu interconnect at position A after stretching up to 43%

strain. The top edge Cu compresses the PDMS substrate and the bottom edge peels

from the PDMS substrate. The narrow part of Cu is not necking but twisting. The

micrograph is taken with the sample tilted at 30º.

compressing

Position A

peeling

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(b)

Figure 8. SEM pictures taken at different angles of the broken Cu interconnect at

position B (see Fig. 2) after 43% strain (see Fig. 4). (a) Cu is broken and the PDMS

shows caving at the same location. Transverse necking in width is not pronounced.

The crest is debonded and deformed out-of-plane. (b) Broken Cu interconnect with

debonding and twisting. The inner radius part of the crest is pushing into the PDMS

substrate.

(a)

metal twisting

Position B

Position B

substrate caving

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Figure 9. A typical deformation pattern of a zigzag crest at position C (see Fig. 2)

after 43% uniaxial strain. The arms are twisting with one side debonding and another

side pushing down the PDMS substrate. The top of the crest is peeling off from the

PDMS substrate and deforming out-of-plane.

Position C