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Current Applied Physics 11 (2011) S12eS15

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Current Applied Physics

journal homepage: www.elsevier .com/locate/cap

Effect of sputtering parameters on the adhesion force of copper/molybdenummetal on polymer substrate

Sang-Hyuk Lee a, In-Sun Park a, Bo-Hyun Seo a, Jong Hyun Seo a,*, Heehwan Choe b, Jae-Hong Jeon b,Yong Uk Lee c, Munpyo Hong d

aDepartment of Materials Engineering, Korea Aerospace University, Republic of Koreab The School of Electronics, Telecommunication and Computer Engineering, Korea Aerospace University, Republic of Koreac PETEC (The Printable Electronics Technology Centre), CPI Group (Centre for Process Innovation), United KingdomdDepartment of Display and Semiconductor Physics, Korea University, Republic of Korea

a r t i c l e i n f o

Article history:Received 25 July 2010Received in revised form9 June 2011Accepted 20 June 2011Available online 26 June 2011

Keywords:Flexible displayAdhesion forceCopper metallizationMolybdenum

* Corresponding author. Tel.: þ82 2 300 0161; fax:E-mail address: jhseo@kau.ac.kr (J.H. Seo).

1567-1739/$ e see front matter � 2011 Elsevier B.V.doi:10.1016/j.cap.2011.06.019

a b s t r a c t

A pure molybdenum was selected as a copper barrier layer on top of the polyimide substrate because ofits good etchability with copper and high thermal stability. The adhesion forces were measured by usinga micro-scratch tester and an adhesion improvement mechanism is suggested in terms of thin filmmicrostructure and polymer surface modification. The adhesion force of thin copper film on the polymersubstrate is greatly enhanced by variation of deposition conditions such as argon pressure, DC powervoltage oxygen plasma pretreatment and annealing treatment. As the sputter dc voltage increased from500 V to 640 V, the critical adhesion force of the molybdenum on the polyimide film increased from420 mN to 900 mN. As the argon pressure increased from 5 mTorr to 30 mTorr, the adhesion forceslightly decreased from 860 mN to 660 mN. The post-annealing process after the film deposition alsoyielded an enhanced adhesion force.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Until now, flexible displays have been widely developed andresearched. Plastic substrate is an attractive material for thin filmtransistor (TFT) fabrication because of its flexible, light and unbreak-able properties.

However, the commercialization of flexible displays has a numberof problems to solve. One of them is the poor adhesion strengthbetween plastic substrate and thin film. When the film depositionwas conducted at an elevated temperature, thermal residual stresswas built in the thin film due to the difference between thermalexpansion coefficient (CTE)s of the film and the substrate. Such highresidual stress induced on the film directly causes the fatal failuressuch as a film cracking and a film delamination on the peripheries ofthe plastic substrate. The film failures occur in both in-process andin-use environments. This results in the poor quality of the flexibledisplay. Seo reported that both the control of the amorphous siliconfilm thickness and the plasma treatment can greatly reduce theresidual film stress on the plastic substrate and increase the interfaceadhesion force to overcome the film delamination phenomena [1].

þ82 2 3158 3770.

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Since the ductility of the metal line is also a key issue for theflexibility of the display, many researchers agree that copper is thebest candidate for the bus line in the flexible display.

The copper metallization becomes more important especially ina flexible organic emission display with high resolution, whichrequiresmore rapidelectron transfer for a betterdisplayquality. Sincethe copper has a poor adhesion force with the plastic substrate andhigh diffusivity into the substrate, it is essential to deposit a bufferlayer on top of the plastic substrate to prevent external moisture andoxygen gas.

However, little is known about the copper metallization on theflexible substrate and especially on the adhesion property of thecopper. Even though the copper metallization for the flexibledisplay is similar to the copper lines used in the flexible printedcircuit board, the requirements for the metallization are quitedifferent in a pattern size, a film thickness, an etchability in the LCDchemicals and an adhesion strength.

Among the metal candidates, including molybdenum, titanium,nickel and their alloys, we select the molybdenum as a buffer layerof copper film considering it’s etchability in an acid solution. Thecopper/molybdenum was successively patterned by using a phos-phoric acid solution [2]. In this work, the adhesion force of thesputtered molybdenum thin film on polyimide as a function of

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S.-H. Lee et al. / Current Applied Physics 11 (2011) S12eS15 S13

sputtering parameters including argon pressure and applied DCvoltage were investigated. The adhesion strength of the sputteredmolybdenum films on the polyimide substrate was measured usinga micro-scratch tester.

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Acousticsignal

Load (N)

Lc1

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2. Experimental details

Plasma deposition was performed by using a DC magnetronsputter system. Copper (3000Å)/Molybdenum (300Å) wasdeposited on the polyimide substrate and the depositiontemperature was room temperature. Deposition pressure was5e30 mTorr at a flow rate of 5 sccm. Prior to the deposition, thepolyimide substrates were cleaned for 5 min in an ultrasonic bathand rinsed with water. Plasma pretreatment was conducted for0e90 s on the PI substrate. The adhesion test of the coatings wasstudied with a micro-scratch test instrument (CSM, Neuchatel,Switzerland) using a Rockwell diamond indenter with tip radius of50 mm. During the scratch test, the applied load was linearlyincreased (0.03e15 N), with the scan speed (5 mm/min), scanlength (7 mm) and loading rate (10.69 N/min).

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Acousticsignal

Load (N)

Lc1

Fig. 1. Acoustic signal versus the applied load for (a) polyimide substrate and (b) glasssubstrate.

3. Results and discussion

Even though themicro-scratch test is awell-establishedmethodto evaluate the adhesion force of the thin film quantitatively on thestiff glass substrate, little research has been done on the research onthe films on the soft plastic substrate. In case of the plasticsubstrate, the acoustic emission signal which indicates the criticalload for the adhesion of the film exhibits a quite different type ofresponse from the glass substrate [3,4]. The measured acousticemission signals from Cu/Mo films both on the glass substrate andplastic substrate were compared in Fig. 1.

The large difference in the adhesion force between the glass andthe plastic substrate is originated from not only the poor metal/polymer interface but also the highly induced thermal residualstress on the film. The thermal residual stress stems from thedifference in the thermal expansion coefficient (CTE) between thefilm and the substrate. Coatings on plastic substrate of thermalstress can be calculated by the following equation [5].

sf ¼�as � af

�DTYf

1� vf(1)

Where as � af is the thermal expansion coefficient of substrate andfilm, af is the thermal stress in the film and DT is the depositiontemperature of film and cooling temperature of film which issubtracted by the deposition temperature and the coolingtemperature of film. Yf is Young’smodulus of film and nf is Poisson’sratio of film. The thermal expansion coefficient of glass is 3.25 ppm/K, polyimide is 50 ppm/K, and molybdenum is 4.8 ppm/K. From theabove equation, calculated thermal stresses of the films both on theglass and the polyimide are 370 MPa and 1.1 GPa, respectively. Thethermal stress imposed on the film and on the plastic substrate issufficient for the detachment of the film from the substrate.

Cu/Mo thin film on the polyimide substrate showed an abruptlyincreased peak at 580 mN (Fig. 1a). This peak is designated asa adhesion force or a critical load (Lc1) corresponding to the cohesivefailure of the film. It was reported that the ion bombardment,cleaning of the surface contaminants, and knocked-in phenomenonwere responsible for the adhesion of metal/polymer [6].

Fig. 2 shows the whole SEM images of the scratch track on Cu/Mo thin film on polyimide. The arrow indicates the scratchdirection of indenter movement. Semicircular arc-like cracksexhibited in Fig. 2a. The first crack on the PI substrate was observed

and it propagated toward the scratch edges. This indicated thecohesive failure between film and the substrate interface. The highmagnification image shows the area between cracks and thesemicircular arc that rectangular patterns of small cells werespread out in cracks. During the scratch test on the polyimidesubstrate, the scratch tip plowed the thin film ahead of it andfragments were piled up sideways of the indenter. Moreover, thefragments created during the test, induced the resistance againstthe tip movement. Under the higher load, the high penetrationdepth of the tip caused the compressive stress at the front of theindenter. At a higher applied load, the film delamination appeared(Fig. 2(c)) and many cracks were connected and rose from the filminterface [7].

In contrary to the flexible printed circuit board, there exista repeated thermal cycles after the film deposition process in thethin film transistor manufacturing process. Therefore, the barrierlayer for the copper metallization should have an excellent thermaldurability after repeated thermal cycles. In case of polyimidesubstrate, the manufacturing process temperature lies within200 �C below its glass transition temperature.

The adhesion force of Cu/Mo films on polyimidewas observed toincrease slightly with even a slight high post-annealing tempera-ture, Fig. 3. The Cu/Mo films shows the enhanced adhesion forcescompared to aluminum on polyimide over the entire post-annealing temperature ranges except 250 �C. The adhesion force

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Stress (Mpa)

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500

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Stress(Mpa)

a

Fig. 2. SEM image of scratch track obtained from the scratch measurement of Cu/Mo thin films deposited onto the polyimide substrate with the thickness of 200 nm.

S.-H. Lee et al. / Current Applied Physics 11 (2011) S12eS15S14

had a peak value at 130 �C and a catastrophic degradation occurredat 250 �C.

When the elastic strain energy in the film caused by both thethermal stress and the intrinsic stress exceeds the adhesion forcebetween the metal and polyimide, the failure occurs.

As the post-annealing temperature increases, the residual stressin the film can be relaxed. It resulted in the enhancement in theadhesion force. In case of aluminum, the easy formation of thealuminum oxide layer at the metal/polyimide interface may lead tomore degradation in the adhesion at higher temperature comparedto the molybdenum, which has high thermal stability and littleaffinity to oxygen. The oxide formation reaction with incorporatedwater in polyimide film may cause the deterioration in the adhesionforce [8]. The standard enthalpies of formation for aluminum andmolybdenum are �1676 kJ/mol and �745.1 kJ/mol, respectively [9].

Fig. 4 presents the deposition rate and residual stress changedby argon pressure and DC bias voltage. The higher the sputteringvoltage, the faster the deposition rate was and the stress moved tocompressive. The high energetic ion bombardments at high DCvoltage yield the greater adhesion in Fig. 5.

In contrary to the bias voltage, as the argon pressure wasincreased, the deposition ratewas decreased and the residual stressof the film shifted to more tensile stress. It resulted from thescattering process of sputtered ions with other gas species inchamber.

Adhesion force of Cu/Mo thin films on polyimide was measuredas functions of sputter process parameters. The forcewas observed to

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Criticalload(mN)

Mo/Al/PI

Cu/Mo/PI

Fig. 3. Adhesion force of Cu/Mo and Mo/Al on polyimide as a function of post-annealing temperature.

increase linearly with a high deposition power and degrade witha high argon pressure, Fig. 5. The adhesion force is correlated withthe energetic ion bombardment during the deposition process. It wasreported that the high dose of radio-frequency Ar plasma surfacemodification yielded the greater adhesion force of Cr/polyimide byforming Cr carbide layer at the metal/polymer interface [8].

In addition, the high tensile thermal residual stress in the filmwhich was induced by the elevated substrate temperature duringthe sputtering was perhaps compensated by the compressiveintrinsic stress at high DC voltage and low argon pressure.

The effects of oxygen plasma pretreatments on the adhesionstrength of copper film are shown in Fig. 6. According to Kinloch’s

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Ar pressure (mTorr)

-100

500 510 520 530 540 550 560 570 580 590 600 610 620 630 640

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DC power (V)

-150

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Fig. 4. Changes of deposition rate and residual stress as a function of (a) argon pres-sure and (b) DC power voltage.

500 510 520 530 540 550 560 570 580 590 600 610 620 630 640

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Fig. 5. Dependency of critical loads on (a) DC power and (b) argon pressure.

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Cu/Mo/PI

Cu/Mo/Glass

Fig. 6. Effect of oxygen plasma pretreatment on the variation in the adhesion forces ofpolyimide substrate and glass substrate.

S.-H. Lee et al. / Current Applied Physics 11 (2011) S12eS15 S15

theory removed the subsistence of grease-like pollutants on thesubstrate surface, eliminated the weak boundary layer andincreased the contact area. Such surface cleaning effects by oxygenplasma treatment may enhance the adhesion between the thin filmand the plastic substrate. The similar behavior by oxygen plasmatreatment was also observed in the coating films on the PIsubstrate.

During the exposure of the oxygen plasma, the hydrophobicplastic substrate changed into the hydrophilic surface by oxygenradical interaction with surface polymers of the substrate. Suchchange is coincident with improvement in the measured values ofcontact angle from 80� without oxygen pretreatment to 10�. Thesurface roughness of the polymer film dramatically decreased afterexcessive oxygen plasma exposure of 60 s. In the case of copper/molybdenum multi-layer shown in Fig. 6, the peak value of adhe-sion force between Cu/Mo and PI substrate of 1340 mN wereobtained after 60 s oxygen plasma treatments. At the prolongedexposure time of 90 s, the adhesion force dropped to 650 mN. This

implies that prolonged plasma pretreatment might deteriorate theadhesion strength and decreased the surface roughness.

4. Conclusion

In the present work, the effects of the deposition parametersincluding argon pressure, dc bias voltage, and oxygen plasmapretreatment on the adhesion force of DC sputtered Cu/Mo thinfilms on the polyimide substrates were investigated by a micro-scratch tester. The films grown at a high DC power and in a lowargon pressure show an excellent adhesion force. Results werecorrelated with ion bombardment and gas scattering effect duringthe sputtering process. The adhesion force of Cu/Mo/PI showsa peak value at 130 �C and catastrophic degrade at 250 �C in post-annealing treatment after the film deposition. The oxygen plasmapretreatment changed the hydrophobic plastic surface to a hydro-philic surface by ions and radicals.

Acknowledgments

This work was supported by the IT Research and Developmentprogram of MKE/KEIT (KI002182, TFT backplane technology fornext generation display).

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