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Applied Surface Science 427 (2018) 1–9

Contents lists available at ScienceDirect

Applied Surface Science

jou rn al h om ep age: www.elsev ier .com/ locate /apsusc

ull Length Article

ighly adhesive and high fatigue-resistant copper/PET flexiblelectronic substrates

ang Jin Parka, Tae-Jun Kob,d, Juil Yoonc, Myoung-Woon Moonb, Kyu Hwan Ohd,un Hyun Hana,∗

Department of Materials Science and Engineering, Chungnam National University, Daejeon 305-764, Republic of KoreaInstitute for Multidisciplinary Convergence of Materials, Korea Institute of Science and Technology, Seoul 130-650, Republic of KoreaDepartment of Mechanical Systems Engineering, Hansung University, Seoul 136-792, Republic of KoreaDepartment of Materials Science and Engineering, Seoul National University, Seoul 151-742, Republic of Korea

r t i c l e i n f o

rticle history:eceived 1 July 2017eceived in revised form 23 August 2017ccepted 28 August 2017vailable online 31 August 2017

eywords:

a b s t r a c t

A voidless Cu/PET substrate is fabricated by producing a superhydrophilic PET surface comprised ofnanostructures with large width and height and then by Cu electroless plating. Effect of PET surfacenanostructure size on the failure mechanism of the Cu/PET substrate is studied. The fabricated Cu/PETsubstrate exhibits a maximum peel strength of 1300 N m−1 without using an interlayer, and virtually noincrease in electrical resistivity under the extreme cyclic bending condition of 1 mm curvature radiusafter 300 k cycles. The authors find that there is an optimum nanostructure size for the highest Cu/PET

lexible substrateslasma treatmentettability

dhesionatigue

adhesion strength, and the failure mechanism of the Cu/PET flexible substrate depends on the PET surfacenanostructure size. Thus, this work presents the possibility to produce flexible metal/polymer electronicsubstrates that have excellent interfacial adhesion between the metal and polymer and high fatigueresistance against repeated bending. Such metal/polymer substrates provides new design opportunitiesfor wearable electronic devices that can withstand harsh environments and have extended lifetimes.

© 2017 Elsevier B.V. All rights reserved.

. Introduction

Metal/polymer composite films simultaneously possess thendividual advantages of both polymers and metals, including theexibility and light weight of the polymer and the excellent electri-al conductivity and electromagnetic wave shielding of the metal.ccordingly, these composite films have been widely applied inarious industries from food packaging to microelectronics [1–4].mong the various types of polymers, polyethylene terephthalate

PET) exhibits excellent transparency and low density comparedo inorganic glass as well as outstanding thermal, mechanical, andhysicochemical properties, and PET is extensively used in elec-ronic devices and display panels [5–7].

Various metallization processes such as physical vapor deposi-ion (PVD) [8], chemical vapor deposition (CVD) [9], atomic layer

eposition (ALD) [10], electroless plating [11], inkjet printing [12],tc. have been used to fabricate metal/polymer composite films.ith the recent growth in mobile phones and wearable electronics,

∗ Corresponding author.E-mail address: jhhan@cnu.ac.kr (J.H. Han).

ttp://dx.doi.org/10.1016/j.apsusc.2017.08.195169-4332/© 2017 Elsevier B.V. All rights reserved.

there has been increasing demand for metal/polymer compos-ite films with high functionality and increased product lifetime.The major factors determining electronic product lifetime in harshenvironments are the interfacial adhesion between the metal andpolymer, and fatigue resistance against failure from cyclic bend-ing deformation. The need for improvements in these two aspectscontinues to increase [13–18]. However, when metal/polymercomposite films are fabricated using the wet plating method (elec-troplating, electroless plating), the polymers have very low surfacefree energy (�s < 100 mJ m−2) compared to the surface free energyof metals (�s ∼ 500–5000 mJ m−2) [19]. As a result, the plated metalfilm on the polymer is typically not dense due to the low wetta-bility of the plating solution on the polymer substrates accordingto Young’s equation (�sv = �sl + �lv cos�) [20]. This results in lowbending fatigue performance of the metal/polymer composite filmas well as weak adhesion between the plated metal film and thepolymer substrate. To address these limitations, the wettability ofthe plating solution on the polymer substrate should be improved

preferentially, and various studies are currently being carried out[21–24].

Improving wettability of the plating solution has been gener-ally reported to proportionally increase the interfacial adhesion

2 S.J. Park et al. / Applied Surface Science 427 (2018) 1–9

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ig. 1. Experiment flow chart showing the procedure to fabricate the Cu/PET flexibabricated by electroless plating is not thick enough to peel off, electroplating was p

f the plated metal film to the polymer substrate [25,26]. Poly-er surface modification methods which have been carried out

ecently to resolve the poor adhesion between the metal and poly-er in metal/polymer composite films include laser etching [27],

lasma etching [28–30], ion beam etching [31,32], or chemicaltching [33,34]. These methods increase the roughness of the poly-er surface and consequently increase mechanical interlocking,

r increase chemical bonding by generating chemical functionalroups [35–38]. There are also methods which insert an inorganicdhesion layer such as Ni [39], Cr [40], Ti [41], Ni-Cr [42], or Ni-r-Mo [43] between the metal film and polymer substrate, or anrganic adhesion layer such as amine [44,45] or silane [46,47].

Among the above methods being employed to resolve the poordhesion, increasing the roughness of the polymer surface bytching is most widely used. However, most studies have sim-ly reported that increased surface roughness contributes to the

ncrease in interfacial adhesion [13,34]. Essentially no studies haveeen conducted on how the shape and size of the polymer surfaceanostructures produced by etching influence the interfacial adhe-ion. Also, prior studies have mainly been conducted on ways ofmproving the interfacial adhesion between the metal and polymer,nd there is a shortage of research on fatigue performance.

In this study, we overcome the poor wetting of the copper elec-roless plating solution on the PET surface and the poor adhesionetween the Cu and PET by performing PET surface modificationsing oxygen plasma. To simultaneously maximize the Cu/PET

nterfacial adhesion strength and bending fatigue performanceithout using a tie layer between the Cu and PET, the size and shape

f the anisotropic nanostructures of the PET surface were con-rolled by significantly varying the oxygen plasma etching time. Theffect of sizes and shapes of superhydrophilic anisotropic nanos-ructures on the Cu/PET interfacial adhesion and fatigue propertiesas analyzed, and the failure mechanism of the Cu/PET flexible sub-

trate depending on the shape and size of the nanostructures was

rst studied. In addition, by comparing the changes in wettabilitynd interfacial adhesion according to the oxygen plasma treatmentime, the relationship between wettability and interfacial adhesionas investigated.

strate (a) used for T-peel test (b) and bending fatigue test (c). Because the Cu layermed additionally to increase the thickness of the Cu layer on PET.

2. Experimental procedures

2.1. Oxygen plasma treatment

Flexible PET films with dimensions of 100 mm (l) × 40 mm(w) × 100 �m (t) were cleaned ultrasonically in ethanol and deion-ized water for 5 min each at 25 ◦C, and dried with N2 gas. Thecleaned films were placed on a stainless steel cathode in a vac-uum chamber. The chamber was evacuated to a pressure less than1 mTorr. The oxygen flow rate, oxygen gas pressure, and bias volt-age for the plasma treatment were 20 sccm, 10 mTorr, and −400 V,respectively.

2.2. Fabrication of Cu/PET flexible substrates

Fig. 1(a) shows the experiment flow chart showing the pro-cedure to fabricate the Cu/PET flexible substrates used for T-peeltest and bending fatigue test. The plasma-treated surfaces of thePET films were coated with copper layers by electroless plating.The composition of the electroless copper plating solution wasCuSO4·5H2O (10.0 g L−1) as a copper ion source, HCHO (5 mL L−1)as a reducing agent, NaKC4H4O6·4H2O (28.25 g L−1) as a com-plexing agent, 2,6-diaminopyridine (1 mg L−1) as an accelerator,2,2′-dipyridyl as a stabilizer (1 mg L−1), NiSO4·6H2O (1 mL L−1) asa stress-relief agent, sodium dodecyl sulfate (SDS, 1 mL L−1) as asurfactant, and de-ionized water (remainder). The pH and temper-ature of the plating solution were maintained at 12.5 using NaOHand 33 ◦C, respectively. The thickness of the Cu films after electro-less plating for 10 min was 200 nm. After electroless plating, thePET films coated with Cu were washed with de-ionized water toremove the remaining plating solution and then dried in a vacuumoven.

2.3. Adhesion test (3M tape test and 90◦ T-peel test)

To measure the adhesion strength between the Cu coating layerand PET substrate film, peel-off-tests were carried out using a uni-versal testing machine (MTS Insight 1) with an attached 90◦ T-peel

S.J. Park et al. / Applied Surface Science 427 (2018) 1–9 3

Fig. 2. SEM micrographs of the (a) plasma-etched PET surfaces, (b) surfaces of the electroless-plated copper layers after oxygen plasma treatment for 0, 7, 15, 30, 60, 90, and1 gyen

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20 min, and (c) cross sections of the Cu/PETs following electroless plating after oxf the electroless plating solution on the plasma-treated PET surfaces.

est rail at a peel-off-rate of 4.0 mm min−1. Because the Cu layernitially fabricated by electroless plating was not thick enough to

eel off, additional electro-plating was performed to increase thehickness of the Cu layer on the PET. The thickness of the Cu layerfter the additional electro-plating was 100 �m, which includes thehickness of the electroless plating.

plasma treatment for 7, 15, 60, and 120 min. The insets in (a) show the wettability

The dimensions of the PET films used for the peel-off-test were100 mm (l) × 20 mm (w) × 100 �m (t). When a thick Cu layer is

peeled off during a peel-off-test, the peel strength cannot be exactlymeasured because of the energy loss due to bending of the thick Culayer. In this study, therefore, the PET layer was peeled off as shownin Fig. 1(b), and no peeling occurred at the interface between the

4 urface Science 427 (2018) 1–9

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lectroless plating layer and the electroplating layer. However, thedhesion of the Cu film on the non-plasma-treated PET was too lowo perform peel test. Instead, a scotch tape test using 3M #610 tapeas carried out to compare with the effects of the oxygen plasma

reatment.

.4. Bending fatigue test

The fatigue performance of the Cu/PET flexible substrates wasvaluated by measuring changes in the electrical sheet resistancef the Cu layer on the PET films during repetitive bending, using

bending fatigue test system as shown in Fig. 1(c). The bendingurvature (r) and frequency were 1 mm and 1 Hz, respectively. Thelectrical sheet resistance was measured up to 300 k cycles at 20 kycle intervals along the direction normal to the bending line, using

4-point probe (CMT-SR2000N, AIT).

.5. Microstructure characterization

A field emission scanning electron microscope (FE-SEM, S-800, HITACHI) was used to observe the shapes and sizes of thenisotropic nanostructures on the plasma-etched PET surfaces,nd the topologies of the electroless-plated copper films on thenisotropic PET nanostructures. To observe the microstructure ofhe Cu/PET interfaces, cross-sections of the Cu/PET substrates wererepared using a focused ion beam (FIB). In addition, plane andross-section views of the fractured surfaces after T-peel tests andending fatigue tests were observed using a FE-SEM.

. Results and discussion

.1. Nanostructures on plasma-treated PET surface

The oxygen plasma treatment of a PET surface produces aanostructure morphology comprised of nanoprotrusions on theET surface. This nanostructure morphology provides sites for theechanical interlocking of the copper layer that is subsequently

oated on top of the nanostructure [30,48,49]. Additionally, as isell known, the PET surface becomes activated with polar groups

ike C O, C O, COOH, and O C O, which form on the PET sur-ace due to the oxygen plasma treatment [13,50]. As a result ofhe nanostructure formation and surface activation by the oxygenlasma treatment, the PET surface becomes hydrophilic [51], andhis wettability improvement increases the adhesion of Cu layery suppressing the occurrence of defects such as voids which mayorm in Cu/PET interlayer during copper electroless plating and by

aking the copper film dense.Up to now, most research on enhancing the Cu/PET interfacial

dhesion by modifying the PET surface roughness has used shortlasma treatment times. As a result, no distinct nanostructuresith significant aspect ratios have been established on the PET sur-

ace, and the surface roughness is typically low, with most RMSoughness below 100 nm [13,28,29]. In this study, to investigatehe changes in the Cu/PET interfacial adhesion depending on thehape and size of the PET surface nanostructures rather than sim-ly the effect of increasing surface roughness, the plasma treatmentime was significantly increased up to 120 min.

Fig. 2(a) shows SEM images of the PET surface nanostructuresroduced after oxygen plasma etching of the PET surface for 7, 15,0, 60, 90, and 120 min. As the SEM images indicate, the shapes andizes of the PET surface nanostructures are changed significantly by

ncreasing the plasma treatment time. At first, after a short plasmareatment time (7 min), it was observed that small nanostructuresere formed on the surface of the PET. As the plasma treatment

ime increased, the diameter of the nanostructures did not change

Fig. 3. C1s XPS spectrum of the oxygen plasma-treated PET surface for 30 min.Oxygen plasma treatment decreased C C bonds and increased C O and O C Obonds.

significantly, but their height gradually increased, and the nanos-tructures changed into a finely divided hairy morphology with highaspect ratios. The gaps between the nanostructures also graduallyincreased. An XPS analysis was also conducted to confirm the for-mation of polar groups by the oxygen plasma treatment. The XPSresults showed that C C bonds decreased and that C O and O C Obonds increased by the oxygen plasma treatment of the PET surfaceas shown in Fig. 3.

To measure the wettability change of electroless plating solu-tion on the PET surface according to the oxygen plasma treatmenttime, the copper electroless plating solution was dropped onto thenanostructured PET surface as shown in Fig. 2(a), and the changein wettability of the electroless plating solution on the PET surfacewas measured according to the oxygen plasma treatment time. ACCD camera was used to observe the wetting behavior of the plat-ing solution and the insets in each subfigure in Fig. 2(a) show theresults. The wetting angle of the copper plating solution on the PETsurface without plasma treatment was approximately 43◦ while thewetting angle following 7 min of plasma treatment was a greatlyreduced 5◦. As the plasma treatment time increased beyond 15 min,the specimen exhibited superhydrophilicity with wetting angles of0◦. This wettability enhancement was caused by the formation ofthe nanostructure on the PET surface and surface activation by theoxygen plasma treatment [21,51].

3.2. Microstructure of the copper/PET substrate

Fig. 2(b) shows SEM images of the surfaces of the ∼200 nmthick copper layers coated by electroless plating on the plasma-etched PET surfaces shown in Fig. 2(a). As can be observed, thenanostructures formed on the plasma-etched PET surfaces havebeen completely covered by the deposited nanosized copper par-ticles. It was noted that the particles gradually increased in sizeas the plasma treatment time increased. However, after 60 min,there were no observable changes. This phenomenon was relatedto the gaps between the nanostructures. When the nanostructuregaps were small (t ≤ 60 min), the diameters of the deposited cop-per particles decreased as the nanostructure gap decreased. Thisoccurs because the growth of a single copper particle deposited ontop of a nanostructure is hindered by the copper particles that are

deposited on the tops of nearby nanostructures. However, when thenanostructure gaps were greater than the diameter of the depositedparticles (t > 60 min), the copper particles were not hindered bythe surrounding nanostructures and were able to grow during the

S.J. Park et al. / Applied Surface

Fig. 4. Variations in the height, width, and gap of the surface nanostructure of thepf

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for 120 min. This result was attributed to the superhydrophilicity

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lasma-treated PETs for different times. The nanostructure sizes were measuredrom the 2D image of Fig. 2(c), and increased with plasma treatment time.

lating process without interference. As a result, as the plasmareatment time increased, the copper particle diameters becameniform regardless of the nanostructure gap.

The cross sections of Cu/PET specimens were examined by SEMo observe the changes in the size and shape of the nanostructuresased on the plasma treatment time, and to investigate how well

he copper filled the high aspect ratio hairy nanostructures afterhe copper electroless plating. Figs. 2 (c) and 4 demonstrate thathe height, width, and gap of the nanostructures increased as the

ig. 5. (a) Variation in the peel strength of the Cu layer on plasma-treated PET for differu/PETs plasma-treated for 0 and 7 min. (b) SEM fractographs of copper layers after T-peelreatment for 7, 15, 30, 60, 90, and 120 min.

Science 427 (2018) 1–9 5

plasma treatment time increased. However, the variation in nano-structure widths showed a trend somewhat different from thatobserved in Fig. 2(a). In Fig. 2(a), the nanostructures had a colum-nar shape before the electroless plating, and the nanostructurewidths (i.e., diameters) did not change greatly as the plasma treat-ment time varied. However, after the copper electroless plating, thenanostructures had a circular cone shape, and the nanostructurewidths increased with the plasma treatment time, as shown in Figs.2 (c) and 4 . The changes in the shape of nanostructures after elec-troless plating were caused by the entanglement of the dried hairynanostructures after exposure to the acid solution in the pretreat-ment steps of electroless plating. This phenomenon became moresevere as the nanostructure height increased. Fig. 2(c) also showsthat the copper filled the spaces between the nanostructures with-out voids through the electroless plating, even for specimens withnanostructures with large heights such as the specimen that under-went plasma treatment for 120 min. This result was attributed tothe superhydrophilicity induced by the oxygen plasma treatment. Itenabled the pretreatment solution and the plating solution to effec-tively penetrate the gaps between the nanostructures with largeheights.

3.3. Adhesion between Cu and PET (Cu/PET failure mechanism)

After surface treatment of the PET using oxygen plasma, Cu/PETsubeights such as the specimen that underwent plasma treatment

induced by the oxygen plasma treatment. It enabled the pretreat-ment solution and the plating solution to effectively penetrate thegaps between the nanostructures with large heights.

ent oxygen plasma etching times. The insets show the 3M tape test results of the tests of the Cu/PET substrates fabricated by electroless plating after oxygen plasma

6 S.J. Park et al. / Applied Surface Science 427 (2018) 1–9

Fig. 6. SEM images of the fractured Cu surfaces (a, e, i) and cross sections (b, f, j) of the fractured specimens after T-peel tests of the Cu/PETs fabricated by electroless platinga (i) re(

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fter oxygen plasma treatment for 60 (a–d), 90 (e–h), and 120 min (i–l). (d), (h), andg) indicates pores induced by crazing of PET.

.3. Adhesion between Cu and PET (Cu/PET failure mechanism)

After surface treatment of the PET using oxygen plasma, Cu/PETubstrates were fabricated using electroless plating of copper. Tovaluate the adhesion of copper layer coated on PET surface, peeltrength was measured using a 90◦ T-peel test equipment. Fig. 5(a)hows the variation in the peel strength of the copper layer withxygen plasma treatment time. The adhesion of the Cu/non-treatedET interface was too weak to even carry out the 90◦ T-peel test, so

3M tape test was used to simply compare with the adhesion ofhe Cu/plasma-treated PET interface. As shown in Fig. 5(a), mostf the copper layer coated on the non-treated PET (0 min) waseeled off after the 3M tape test, but the copper layer coated onhe 7 min plasma treated PET was not peeled off at all after theM tape test. From this result, it could be concluded that despitelasma treatment of the PET surface for only a short time (7 min),he interfacial adhesion between the Cu and plasma-treated PETas greater than the adhesion between the Cu and 3M tape as well

s the interfacial adhesion between the Cu and non-treated PET.s the plasma treatment time increased, the Cu/PET interface peeltrength increased in a stable manner, but after 60 min, there was

drastic increase, reaching a peak value of 1300 N m−1 at 90 min,

ollowed by a small decrease at 120 min. Lu et al. [47] reportedhe highest Cu/PET peel strength of 1670 N m−1, but this result wasbtained using silane as the interlayer between the Cu/PET. Thereave been few studies reporting a Cu/PET interface peel strength

present fracture modes depending on the size of the nanostructures. The arrows in

greater than the 1300 N m−1 obtained in this study without usingan interlayer.

In order to understand the change in the peel strength accord-ing to the oxygen plasma treatment time, the microstructure of theCu/PET fracture surfaces and cross sections of the fractured spec-imens after the peel tests was observed using a SEM. In addition,the change in the Cu/PET film failure mechanism was investigatedbased on the size and shape of the PET surface nanostructures.Fig. 5(b) shows SEM images of the fracture surface of the Cu layeron the fractured specimens after the peel tests. When the plasmatreatment time was shorter than 60 min, craze fibrils [52–54],which are generally formed during deformation and fracture ofpolymer, were observed, and it was found that the size of thefibrils increased as the plasma treatment time increashyperlinkAP-SUSC37032FIG00255 (b) and 6, it can be observed that the fracturebehaviors were similar for plasma treatment times up to 60 min;fractured craze fibrils can be observed, and failure occurred as thecrack propagates along the Cu/PET interface (Fig. 6(a)–(d)). How-ever, for specimens treated for more than 90 min, failure in the formof multiple cracking can be observed, showing that cracks propa-gated in the PET surrounding the Cu/PET interface as well as in theCu/PET interface (Fig. 6(e)–(h)). The 120 min treatment case shows

crack propagation within the PET, and not at the Cu/PET interface(Fig. 6(i)–(l)). The ridged surface in Fig. 6(i) was formed when thecrack cut through the middle of the fibril, and a smooth surface wasformed as the crack propagated along the crazy boundary, i.e., the

S.J. Park et al. / Applied Surface Science 427 (2018) 1–9 7

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ig. 7. (a) Changes in electric sheet resistance of Cu films on PET substrates during batigue tests of 300 k cycles of the Cu/PETs fabricated by electroless plating after ox

nterface between the craze and the surrounding bulk material. Thisbservation is in agreement with the fracture surface reported byanrattanakul et al. [55] From these results, it was observed thathe Cu/PET failure mode changed as the plasma treatment timencreased, that is, as the PET surface nanostructure size increased.

.4. Bending fatigue property

Bending fatigue tests were performed with a curvature radiuset to 1 mm, to evaluate the fatigue properties of the Cu/PET flexibleubstrates under cyclic conditions with extremely large curva-ures. Fig. 7(a) shows the electrical sheet resistance measurementesults for the copper coating layer according to the bending cycle.or the non-plasma-treated specimen (0 min), the sheet resistancerastically increased as the bending cycle increased. However, forhe 7, 15, and 30 min plasma-treated specimens, the increase inheet resistance tended to decline as the plasma treatment timencreased. The 60 min plasma-treated specimen showed outstand-ng bending fatigue performance, with virtually no change in sheetesistance up to 300 k cycles.

To determine the reason for such bending fatigue performance,he microstructure of the copper coating layer of each specimenfter fatigue tests was observed using a SEM, and the results arehown in Fig. 7(b). For the non-plasma-treated specimen, a large

rea of the Cu layer was peeled off after the fatigue test. As thelasma treatment time increased, both the area of delaminatedopper and the crack opening distance decreased. Although cracknitiation was observed on the Cu coating layer surface for the

g fatigue tests and (b) SEM images of the electroless plated Cu surface after bendinglasma treatment for different times.

60 min treated specimen, no cracks were observed for specimenswith treatment times of 90 min and greater. Based on these results,the improvement in bending fatigue performance with increasingplasma treatment time was attributed to the increase in Cu/PETinterfacial adhesion with increasing plasma treatment time as dis-cussed above.

3.5. Relationship between wettability and interfacial adhesion

It was observed from Figs. Fig. 22(a) and Fig. 55(a) that thewettability and Cu/PET interfacial adhesion were increased, respec-tively, by the oxygen plasma treatment, and that changes in boththe wettability and Cu/PET interfacial adhesion were related to theincrease in the size of the nanostructures. Accordingly, the relation-ship between wettability and interfacial adhesion was evaluated inrelation to variation in the sizes of the nanostructures formed by theoxygen plasma treatment. The enhancement in wettability follow-ing plasma treatment was thought to be caused by the changes ininterfacial energy and the formation of the nanostructure, while theimprovement in interfacial adhesion was caused by the improve-ment in wettability and nanostructure formation. Fig. 8 shows thevariations in wetting angle and peel strength as a function of plasmatreatment time. There was a prominent improvement in interfacialadhesion at treatment times of 60 min and greater, when there was

no observable wettability change. In other words, there was a sig-nificant increase in adhesion with increase in nanostructure size,even without change in wettability. Therefore, it was revealed thatwhile the Cu/PET interfacial adhesion can be influenced by wetta-

8 S.J. Park et al. / Applied Surface

Fig. 8. Variations in wetting angle of electroless plating solution on the plasma-tt

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reated PET surface and peel strength between Cu and PET as a function of plasmareatment time.

ility, the formation of nanostructure at the interface was a moreominant factor for improvement of interfacial adhesion.

. Conclusions

A superhydrophilic PET surface comprised of nanostructuresith large width and height was produced using long oxy-

en plasma treatment time. The Cu/PET interfacial adhesion wasbserved to increase as the plasma treatment time was increased,eaching its peak value of 1300 N m−1 after an optimum plasmareatment time of 90 min, which was previously impossible with-ut using interlayers. It was revealed that the failure mechanismf the Cu/PET substrate was dependent on the size and shape ofhe PET surface nanostructure (i.e., plasma treatment time). Theending fatigue performance was significantly enhanced by thexygen plasma treatment. When the nanostructure size was suf-ciently large (t ≥ 60 min), the electrical sheet resistance showedlmost no change up to 300 k bending cycles. While the Cu/PETnterfacial adhesion can be influenced by wettability, the forma-ion of nanostructure at the interface was a more dominant factoror improvement of interfacial adhesion.

cknowledgements

This research was supported by Basic Science Research Programhrough the National Research Foundation of Korea (NRF) fundedy the Ministry of Education (2016940411).

eferences

[1] R. Guo, Y. Yu, Z. Xie, X. Liu, X. Zhou, Y. Gao, Z. Liu, F. Zhou, Y. Yang, Z. Zhang,Matrix-assisted catalytic printing for fabrication of multiscale flexible,foldable, and stretchable metal conductors, Adv. Mater. 25 (2013) 3343–3350.

[2] S. Yao, Y. Zhu, Nanomatarial-enabled stretchable conductors: strategies,materials and devices, Adv. Mater. 27 (2015) 1480–1511.

[3] S. Khan, L. Lorenzelli, R.S. Dahiya, Technologies for printing sensors andelectronics over flexible substrates: a review, IEEE Sens. J. 15 (2015)3164–3185.

[4] Y.S. Rim, S.-H. Bae, H. Chen, N. De Marco, Y. Yang, Recent progress in materialsand devices toward printable and flexible sensors, Adv. Mater. 28 (2016)4415–4440.

[5] S. Sawada, Y. Masuda, P. Zhu, K. Koumoto, Micropattering of copper onpoly(ethylene terephthalate) substrate modified with self-assembled

monolayer, Langmuir 22 (2006) 332–337.

[6] M. Mas-Torrent, C. Rovira, Novel small molecules for organic field-effecttransistors: toward processability and high performance, Chem. Soc. Rev. 37(2008) 827–838.

[

Science 427 (2018) 1–9

[7] H. Su, M. Zhang, Y.-H. Chang, P. Zhai, N.Y. Hau, Y.-T. Huang, C. Liu, A.K. Soh,S.-P. Feng, Highly conductive and low cost Ni-PET flexible substrate forDye-sensitized solar cells, ACS Appl. Mater. Interfaces 6 (2014) 5577–5584.

[8] B. Wang, W. Eberhardt, H. Kuck, H, Plasma pre-treatment of liquid crystalpolymer and subsequent metallization by PVD, Vacuum 81 (2006) 325–328.

[9] T. Duguet, F. Senocq, L. Laffont, C. Vahlas, Metallization of polymer compositesby metalorganic chemical vapor deposition of Cu: surface functionalizationdriven films characteristics, Surf. Coat. Technol. 230 (2013) 254–259.

10] K.L. Jarvis, P.J. Evans, Growth of thin barrier films on flexible polymersubstrates by atomic layer deposition, Thin Solid Films 624 (2017) 111–135.

11] Y.-C. Liao, Z.-K. Kao, Direct Writing Patterns for Electroless Plated Copper ThinFilm on Plastic Substrates, ACS Appl. Mater. Interfaces 4 (2012)5109–5113.

12] K. Woo, D. Kim, J.S. Kim, S. Lim, J. Moon, Ink-jet printing of Cu-Ag based highlyconductive tracks on a transparent substrate, Langmuir 25 (2008) 429–433.

13] J.H. Kim, Y.G. Seol, N.-E. Lee, Adhesion properties of electroless-plated Culayers on polyimide treated by inductively coupled plasmas, J. Korean Phys.Soc. 51 (2007), S819-S192.

14] X.J. Sun, C.C. Wang, J. Zhang, G. Liu, G.J. Zhang, X.D. Ding, G.P. Zhang, J. Sun,Thickness dependent fatigue life at microcrack nucleation for metal thin filmson flexible substrates, J. Phys. D: Appl. Phys. 41 (2008) 195404.

15] H. Hayden, E. Elce, S.A.B. Allen, P.A. Kohl, Adhesion enhancement betweenelectroless copper and epoxy-based dielectrics, IEEE Trans. Adv. Packag. 32(2009) 758–767.

16] K. Alzoubi, M.M. Hamasha, S. Lu, B. Sammakia, Bending fatigue study ofsputtered ITO on flexible substrate, J. Disp. Technol. 7 (2011) 593–600.

17] B.-J. Kim, Y. Cho, M.-S. Jung, H.-A.-S. Shin, M.-W. Moon, H.N. Han, K.T. Nam,Y.-C. Joo, I.-S. Choi, Fatigue-free, electrically reliable coper electrode withnanohole array, Small 8 (2012) 3300–3306.

18] H.-N. Lee, Y. Han, J.-H. Lee, J.-Y. Hur, H.K. Lee, Aging effect on adhesionstrength between electroless copper and polyimide films, Mater. Trans. 54(2013) 1040–1044.

19] M. Stamm, Polymer Surfaces and Interfaces, Springer, Berlin, Germany, 2008,p. 113.

20] G. Palasantzas, J. Th, M. De Hosson, Wetting on rough surfaces, Acta Mater. 49(2001) 3533–3538.

21] J. Bico, U. Thiele, D. Quere, Wetting of textured surfaces, Colloids Surf. A 206(2002) 41–46.

22] Y. Wu, M. Kouno, N. Saito, F.A. Nae, Y. Inoue, O. Takai, Patternedhydrophobic-hydrophilic templates made from microwave-plasma enhancedchemical vapor deposited thin films, Thin Solid Films 515 (2007) 4203–4208.

23] T. Ishizaki, H. Sakurai, N. Saito, O. Takai, Control of site-selective adsorptionreaction on a biomimetic super-hydrophilic/super-hydrophobicmicropatterned template, Surf. Coat. Technol. 202 (2008) 5535–5538.

24] E. Yu, S.-C. Kim, H.J. Lee, K.H. Oh, M.-W. Moon, Extreme wettability ofnanostructured glass fabricated by non-lithographic anisotropic etching, Sci.Rep. 5 (2015) 9362.

25] S.-J. Cho, S.P. Shrestha, J.-H. Boo, Surface treatment for Cu metallization onpolyimide film by atmospheric pressure dielectric barrier discharge plasmasystem, Curr. Appl. Phys. 11 (2011) S135–S139.

26] J.Y. Meng, Y.Y. Wang, Y.P. Wang, Z.Q. Ding, Effect of cold atmospheric plasmatreatment on hydrophilic properties of fluorosilicone rubber, Surf. InterfaceAnal. 48 (2016) 1429–1435.

27] G.A. Shafeev, P. Hoffmann, Light-enhanced electroless Cu deposition Olaser-treated polyimide surface, Appl. Surf. Sci. 139 (1999) 455–460.

28] P.-C. Chiang, W.-T. Whang, S.-C. Wu, K.-R. Chuang, Effects of titania contentand plasma treatment on the interfacial adhesion mechanism of nanotitania-hybrdized polyimide and copper system, Polymer 45 (2004)4465–4472.

29] S.H. Kim, S.H. Cho, N.-E. Lee, H.M. Kim, Y.W. Nam, Y.-H. Kim, Adhesionproperties of Cu/Cr films on polyimide substrate treated by dielectric barrierdischarge plasma, Surf. Coat. Technol. 193 (2005) 101–106.

30] S.J. Park, T.J. Ko, J. Yon, M.W. Moon, K.H. Oh, J.H. Han, Copper circuit patteringon polymer using selective surface modification and electroless plating, Appl.Surf. Sci. 396 (2017) 1678–1684.

31] C.A. Chang, J. Baglin, A.G. Schrott, K.C. Lin, Enhanced Cu-Teflon adhesion bypresputtering prior to the Cu deposition, Appl. Phys. Lett. 51 (1987) 103–105.

32] K.W. Paik, A.L. Ruoff, Adhesion enhancement of thin copper film on polyimidemodified by oxygen reactive ion beam etching, J. Adhes. Sci. Technol. 4 (1990)465–474.

33] Z. Wang, Z. Li, Y. He, Study of an environmental friendly surface etchingsystem of ABS for improving adhesion of elelctroless Cu film, J. Electrochem.Soc. 11 (2011) D664–D670.

34] W. Zhao, Z. Wang, Adhesion improvement of electroless copper to PCsubstrate by low environmental pollution MnO2-H3PO4-H2SO4-H2O system,Int. J. Adhes. Adhes. 41 (2013) 50–56.

35] W.J. Dressick, C.S. Dulcey, J.H. Georger Jr., G.S. Calabrese, J.M. Calvert, Covalantbinding of Pd catalysts to ligating self-assembled monolayer films for selectiveelectroless metal deposition, J. Electrochem. Soc. 141 (1994) 210–220.

36] W.C. Wang, E.T. Kang, K.G. Neoh, Electroless plating of copper on polyimidefilms modified by plasma graft copolymerization with 4-vinylpyridine, Appl.

37] L. Li, G. Yan, J. Wu, X. Yu, Q. Guo, E. Kang, Electroless plating of copper onpolyimide films modified by surface-initiated atom-transfer radicalpolymerization of 4-vinylpyridine, Appl. Surf. Sci. 254 (2008) 7331–7335.

urface

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[amorphous poly(ethylene terephthalate)(PET) film, J. Mater. Sci. 37 (2002)3675–3683.

[55] V. Tanrattanakul, W.G. Perkins, F.L. Massey, A. Moet, A. Hiltner, E. Baer,Fracture mechanisms of poly(ethylene terephthalate) and blends withstyrene-butadiene-styrene elastomers, J. Mater. Sci. 32 (1997) 4749–4758.

S.J. Park et al. / Applied S

38] J. Cao, Z. Wu, J. Yang, S. Li, H. Tang, G. Xie, Site-selective electroless plating ofcopper on poly(ethylene terephthalate) surface modified with aself-assembled monolayer, Colloids Surf. A 415 (2012) 374–379.

39] K.J. Min, S.C. Park, K.H. Lee, Y. Jeong, Y.B. Park, Interfacial adhesioncharacteristics between electroless-plated Ni and polyimide films modifiedby alkali surface pretreatment, J. Electron. Mater. 38 (2009) 2455–2460.

40] V.M. Marx, C. Kirchlechner, I. Zizak, M.J. Cordill, G. Dehm, Adhesionmeasurement of buried Cr interlayer, Philos. Mag. 95 (2015) 1982–1991.

41] M.J. Cordill, A. Taylor, J. Schalko, G. Dehm, Microstructure and adhesion ofas-deposited and annealed Cu/Ti films on polyimide, Int. J. Mater. Res. 102(2011) 729–734.

42] W.J. Lee, Y.B. Kim, Adhesion and interfacial characteristics of metal/PIcomposite film modified by O-2 ion beam, Thin Solid Films 517 (2008)1191–1194.

43] W.K. Choi, D.H. Park, Polyimide surface modification by linear stationaryplasma thruster, Surf. Coat. Technol. 203 (2009) 2739–2742.

44] Y.-I. Lee, Y.-H. Choa, Adhesion enhancement of ink-jet printed conductivecopper patterns on a flexible substrate, J. Mater. Chem. 22 (2012)12517–12522.

45] Z.J. Yu, E.T. Kang, K.G. Neoh, Electroless plating of copper on polyimide filmsmodified by surface grafting of tertiary and quaternary amines polymers,Polymer 43 (2002) 4137–4146.

46] J. Jang, T. Earmme, Interfacial study of polyimide/copper system using

silane-modified polyvinylimidazoles as adhesion promoters, Polymer 42(2001) 2871–2876.

47] Y. Lu, Improvement of copper plating adhesion on silane modified PET film byultrasonic-assisted electroless deposition, Appl. Surf. Sci. 256 (2010)3554–3558.

Science 427 (2018) 1–9 9

48] T.-J. Ko, K.H. Oh, M.-W. Moon, Plasma-induced hetero-nanostructures on apolymer with selective metal co-deposition, Adv. Mater. Interfaces 2 (2015)1400431.

49] A. Chiolerio, P. Rivolo, S. Porro, S. Stassi, S. Ricciardi, P. Mandracci, G.Canavese, K. Bejtkaa, C.F. Pirri, Inkjet-printed PEDOT:PSS electrodes onplasma-modified PDMS nanocomposites: quantifying plasma treatmenthardness, RSC Adv. 4 (2014) 51477–51485.

50] A. Vesel, M. Mozetic, A. Zalar, XPS study of oxygen plasma activated PET,Vacuum 82 (2008) 248–251.

51] S.-J. Park, T.-J. Ko, J. Yoon, M.-W. Moon, J.H. Han, Etchless fabrication of Cucircuits using wettability modification and electroless plating, Korean J.Mater. Res. 25 (2015) 622–629.

52] C.Y. Hui, A. Ruina, C. Creton, E.J. Kramer, Micromechanics of crack growth intoa craze in a polymer glass, Macromolecules 25 (1992) 3948–3955.

53] R.A.C. Deblieck, D.J.M. van Beek, K. Remerie, I.M. Ward, Failure mechanisms inpolyolefines: the role of crazing, shear yielding and entanglement network,Polymer 52 (2011) 2979–2990.

54] A. Arkhireyeva, S. Hashemi, Effect of temperature on fracture properties of an