Polyimide Surface Modi“cation by Using Microwave Plasma for …home.skku.edu/~jhboo/publications/paper/2011/2011-10.pdf · 2011-07-15 · polyimide (PI) lms as a treatment to enhance

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Copyright copy 2011 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol 11 5328ndash5333 2011

Polyimide Surface Modification by UsingMicrowave Plasma for Adhesion

Enhancement of Cu Electroless Plating

Sang-Jin Cho Trieu Nguyen and Jin-Hyo Boolowast

Department of Chemistry and Institute of Basic Science SungKyunKwan University Suwon 440-746 Republic of Korea

Microwave (MW) plasma was applied to the surface of polyimide (PI) films as a treatment to enhancethe adhesion between copper deposition layer and PI surface for electroless plating The influencesof nitrogen MW plasma treatment on chemical composition of the PI surface were investigatedby using X-Ray photoelectron spectroscopy (XPS) The wettability was also investigated by watercontact angle measurement The surface morphologies of PI films before and after treatment werecharacterized with atomic force microscopy (AFM) The contact angle results show that was dra-matically decreased to 161 at the optimal treatment condition from 721 (untreated PI) Howeverthe root mean square (RMS) roughness of treated PI film was almost unchanged The AFM rough-ness was stayed from 10 to 12 withwithout plasma treatment XPS data show a nitrogen increasewhen PI films exposed to N2 MW plasma Electroless copper depositions were carried out with thefree-formaldehyde method using glyoxylic acid as the reducing reagent and mixture palladium chlo-ride tin chloride as activation solution Adhesion property between polyimide surface and copperlayer was investigated by tape test

Keywords Polyimide Adhesion Copper Electroless Plating Microwave Plasma SurfaceModification

1 INTRODUCTION

A PCB is the platform upon which electronic compo-nents such as integrated circuit chips and capacitors aremounted It can be classified into rigid and flexible typesA flexible printed circuit board (FPCB) has many advan-tages compared to rigid boards because of their flexibilityespecially in applications demanding more integrated elec-tronic devices FPCB which is used for slide type cellularphones consists of flexible copper clad laminate (FCCL)and cover layer Electric signals are conveyed via FCCL ofFPCB between keypad and display panel Polyimide (PI)films coated copper is used as FCCL in electronic indus-try However in spite of the extensive usage as well asthe detailed characterization of the PIs applications of thematerials are limited by their hydrophobic surfaces whichresult in poor wettability and adhesion The adhesion of ametal to the PI film is generally poor unless the PI surfaceis pretreated prior to metallizationPretreatment of polymers with the purpose to change

adhesion properties can be classified in two main meth-ods chemical surface modifications and physical surface

lowastAuthor to whom correspondence should be addressed

modifications Wet chemical processes are highly efficientbut result in the disadvantage of a very strong substraterougheningAmong those modification methods plasma and ion-

beam treatments are being used to an increasing extentThe effects caused on the polymer surface by these tech-niques are the incorporation of functional groups changesin the surface morphology and alteration in the chainstructureMost previous plasma works on polyimide or other

polymers focused on radio frequency (RF) plasmatreatment1ndash3 While RF plasmas are currently used as thestandard production for polymer modification interest inmicrowave (MW) plasma has multiplied in the last fewyears Characterized by their high density of active parti-cles are capable for etching the same material at the rateat least ten times greater than RF plasmas because of highion density by MW plasma4

In this work MW plasma was applied to the surface ofpolyimide (PI) films as a treatment to enhance the adhe-sion of electroless copper deposition layers The influ-ences of nitrogen MW plasma treatment on characteristicsof electroless copper coatings as well as on the chemi-cal composition of the PI film surface were investigated at

5328 J Nanosci Nanotechnol 2011 Vol 11 No 6 1533-48802011115328006 doi101166jnn20113793


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Cho et al PI Surface Modification by Using MW Plasma for Adhesion Enhancement of Cu Electroless Plating

various operating parameters such as time gas flow rateand plasma discharge power Furthermore effect of theMW plasma treatment on peel strength of Cu layer and PIfilms was examined

2 EXPERIMENTAL DETAILS

21 Materials

Kaptonreg film (PI film DuPonttrade) was prepared as flexi-ble substrate and cut in 60times15 mm2 Polyimide has twodifference surfaces The lsquotoprsquo surface is referred to as thelsquobrightrsquo or lsquoshinyrdquo side the lsquobottomrsquo side is lsquodullrsquo andpurposely roughened in the manufacture of the film toimprove handling characteristics Most adhesives bond bet-ter to the dull side5 In our experiments we always choosethe roughened side of the PI as the modification side thisside also has a slightly convex curvature

22 Surface Modification

The experiments were performed in a MW plasma systemmodel MPS-60R produced by Woosin CryoVac companyoperating at 245 GHz The aluminum process chamber480times 480times 500 mm3 is equipped with a grounded alu-minum substrate holder and a fused silica window Thepump speed was held constant The process parametersin the MW plasma treatment (ie power time flow rate)were varied in order to achieve the optimal result

23 Electroless Plating

Following the microwave plasma treatment the sampleswere activated with the SnPd catalyst in 4 min at roomtemperature Then electroless copper was deposited byfree formaldehyde method using glyoxylic acid as a reduc-ing agent for 40 min at 40 C Chemical solutions foractivation step6 and electroless deposition were shown inTable I


The wettability of the surface of PI films was character-ized by the contact angle meter model SEO 300A by Sur-face and Electro-Optics Deionized (DI) water was used as

Table I Composition and operating conditions of electroless Cudeposition

Activating solutions (gdm3) Electroless solutions (gdm3)

PdCl2 04 CuCl2 middot2H2O 3SnCl2 middot2H2O 25 K-Na tartrate 168NaCl 200 NaOH 12HCl 40 Glyoxylic acid 10

bull Activation time 4 minutes bull Deposition time 40 minutesbull Sn compounds were removed from bull Temperature 40 Cthe surface by using HBF4 acid

a liquid in the measurement A pendent water drop thatwas formed at the tip of the syringe and the specimenwas raised until it touched the bottom of the drop Afterthe DI water drop was dropped onto the surface of theplasma-treated PI substrate the advancing contact angleswere measured immediately by a sessile drop methodIt was found that the shape of the drop on the surfacewas not changed significantly during the measurement Allmeasurements were carried out at 25plusmn01 C Contactmode atomic force microscopy (AFM THERMOMICRO-SCOPEStrade AP-0100) was used in order to characterize themorphological change of microwave plasma-treated PI sur-faces Images were obtained using Thermo Microscopeswith silicon nitride probe mounted on a cantilever Imagingwas performed in contact mode The X-ray photoelectronspectroscopy (XPS) measurement of PI surfaces was per-formed using MgK light source (12536 eV) and concen-tric hemispherical analyzer (CHA) SEM images (PhillipsXL30-FEG JEOL JSM-890) and XRD spectra (HR-XRDBruker D8 Discover system) were used to study Cu surfacestructure Practical adhesion of the electroless Cu films onPI substrate was evaluated using the standard Scotchreg tapetest performed with the 3 M Scotchreg tape

3 RESULTS AND DISCUSSION

31 Wettability of Polyimide Film Surface

Figure 1 shows contact angle of N2 plasma treated PIwith different MW power and N2 flow rate Water con-tact angle was decreased with N2 plasma treatment Espe-cially increasing N2 flow rate led to wettable surfacemore than untreated PI surface (until 100 sccm) Howeverafter 100 sccm of N2 water contact angle was increasednamely 100 sccm of N2 flow rate is turning point of inthis experiment Also at this point contact angles of 400500 and 600 W samples were stayed in the same errorrange with almost the same value Thus we determined

Fig 1 Contact angles of MW plasma-treated PI films with various MWplasma RF power and N2 gas flow rate

J Nanosci Nanotechnol 11 5328ndash5333 2011 5329


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PI Surface Modification by Using MW Plasma for Adhesion Enhancement of Cu Electroless Plating Cho et al

the optimal condition that is 400 W of MW power with100 sccm of N2 flow rate In plasma modification thechanges of surface wettability depend on surface freeenergy of materials While surface free energy is deter-mined by discharge energy Discharge energy equals to theproduct of discharge power and treatment time Generallydifferent discharge system can produce different dischargeenergy Thus treatment efficiency can be correlated withthe parameter ldquodegree of treatmentrdquo W (Jcm2 defined asthe discharge energy delivered per unit area of the treatedsample surface7 It is expressed as follows

W = P times t

A(1)

Here t is the treatment time (second) P is the plasmapower (watt) A is the sample area (cm2 exposed to thedischarge In general the changes of wettability are depen-dent on the energy transmitted to material surface by reac-tive species in plasma As shown in Eq (1) degree oftreatment W belongs to the plasma power P and the treat-ment time t if area A is constantFrom the Figure 1 400 W of MW power and 100 sccm

of N2 flow rate were selected the optimal condition Alsofrom the Eq (1) we could use treatment time depen-dence for PI surface modification Table II shows contactangle with increasing treatment time It reveals that thewater contact angles were decreased when treatment timesincreased Contact angle was decreased with increasinglsquoW rsquo as shown in Figure 2When lsquoW rsquo increases this usually leads to increasing

energetic species or plasma heating effect The energeticspecies collide with material surface and transmit theenergy to the surface This leads to increasing the sur-face free energy (or surface energy) and improvementof wettability The reactive sites on PI were created bythe MW plasma treatment such as active nitrogen species(N2 plasma) PI surface was exposed with highly reactiveMW plasma treatment After MW plasma treatment thewettability is improved

32 Morphology of Polyimide Film Surface

Morphological changes of PI surface by nitrogen MWplasma were investigated by contact mode AFM as shownin Figure 3 RMS roughness seems to be decreased byMW plasma treatment Plasma bombardment was notaffected to surface If plasma heating effect is applied

Table II Contact angle by increasing MW plasma treatment time with400 W of MW plasma and 100 sccm of N2 flow rate

Treatment time (s) Contact angle (degree)

20 176plusmn08760 176plusmn074120 175plusmn065180 161plusmn071

Fig 2 Contact angle with degree of treatment

to this phenomenon contact angle must be increased bysmooth surface However contact angle was decreased byMW plasma treatment It caused by increasing the surfaceenergy by nitrogen doping

33 XPS Spectra of Polyimide Film Surface

To fully understand the effect of MW plasma treatmentson the surface chemical modifications chemical bondingstates on the MW plasma-treated PI surfaces were ana-lyzed by XPS The elemental percents (relative percent ofelements on the surface within the XPS region) of the PIsurface after irradiation by nitrogen MW plasma were cal-culated from the integral intensities of the peaks of eachelement and are shown in Table IIIIt is evidence with N2 MW plasma treated PI that the

atomic percents of carbon were reduced in comparison withnone treated PI Moreover the atomic percents of nitrogenand oxygen were increased The results indicated that when

(a)

(b)

Fig 3 AFM images of (a) none treated PI and (b) N2 MW treated PI

5330 J Nanosci Nanotechnol 11 5328ndash5333 2011


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Table III The Atomic percents on PI surface withwithout N2 MWplasma treatment

Atomic percent (at)

Sample O N C

None treatment 201 43 756Nitrogen 291 66 644

nitrogen MW plasmas were applied on the PI films the PIfilms were modified by activated species that broke chemi-cal bond of CndashC on the PI surface It resulted in the decreas-ing of the relative surface composition of carbon on PIfilms The reason of the increasing nitrogen content is thatnitrogen containing functional group was introduced on thePI surface during N2 MW plasma treatmentIn order to understand the changes in the chemical bond-

ing states after MW plasma the core-level XPS spec-tra were analyzed in detail It was reported that most ofnoticeable changes in XPS spectra after plasma or ion-beam treatments can be identified from C 1s spectra28ndash10

Figure 4(a) shows the C 1s core level XPS spectra of nonetreated PI The component peak 1 at 2846 eV in the C 1speak is attributed to CndashC bonding in two benzene rings ofthe oxydianiline (ODA) structure8 The component peak 2at 2857 eV should be separated into 2 different peaks ifthe peak separation is carried out more strictly Howeversince the positions of these peaks are too close to be sepa-rated these peaks were considered as one summed peak8

One part of this peak is attributed to CndashC bonding of car-bon atoms in the benzene rings of the pyromellitic dian-hydride (PDMA) structure another part contains a peakrelated to NndashC bond Peak 3 at 2867 eV corresponds to aCndashO bond11 Peak 4 at 2886 is attributed to C O doublebond After exposing to N2 MW plasma the decrease ofcomponent peak 1 was observed Furthermore the compo-nent peak 2 was increased as shown in Figure 4(b) Thisindicates that during N2 MW plasma treatment nitrogenwas introduced onto the surface of PI film in the form ofNndashC These resulted in the increase of wettability of thePI surface and low water contact angles

34 Electroless Copper Deposition

Electroless plating bases on the reduction that takes placeonly at a solidliquid catalytic interface The palladiumchloride is reduced to palladium metal then acts as acatalyst to initiate the deposition of electroless copperThe reaction of palladium chloride in the activation solu-tion is12

Pd2++Sn2+ minusrarr Pd0+Sn4+lowastThe overall reaction of glyoxylic acid and copper in plat-ing bath is13

Cu2++2CHOCOOH+4OHminus

minusrarr Cu0+2HC2Ominus4 +2H2O+H2uarr lowastlowast

Fig 4 C 1s core-level XPS spectra of polyimide films (a) none treatedPI and (b) N2 MW plasma

SEM images revealed that copper layer deposited onpolyimide films was uniform in the case of N2 MWplasma treatment in comparison with none treated sample(Fig 5(a)) The X-ray diffraction patterns of Cu layers onPI substrates were shown in Figure 5(b) It can be seenthat the copper films had three main crystal faces (111)(200) and (220) of which itrsquos clear face was the (111)plane This agrees with the standard taken from JCPDSmdashinternational centre for diffraction data PCPDFWIN v21From these results the intensity of the peaks (111) in thecase of N2 MW plasma pretreatment was higher than nonetreated sample Moreover the volume resistance of elec-troless Cu layer was 10 middot cm with N2 MW plasmatreatment while it was 28 middotcm in none treated PI filmN2 Mw plasma treatment effect was explained achieve-

ment of highly uniform Cu layer and high XRD peakintensity because electroless plating bases on the reduc-tion that takes place only at a solidliquid catalytic inter-face The palladium chloride is reduced to palladium metalthen acts as a catalyst to initiate the deposition of electro-less copper Then Pd the catalyst of the redox reaction



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(a)

(b)

Fig 5 (a) SEM images of Cu electroless deposition layer on PI substrates with (i) N2 MW plasma treatment and (ii) none treated PI and (b) X-raydiffraction patterns of Cu electroless deposition layer on PI substrates

cannot be chemisorbed directly on polymer surfaces car-rying only oxygenated species This can be interpretedby strong chemical affinity of Sn towards oxygen and bythe non-affinity of Pd towards this same element On theother hand Pd can be directly chemisorbed well on thepolymer surfaces carrying nitrogenated species This canbe explained by the strong chemical affinity of palladiumtowards nitrogen14ndash16 As proven when we analyzed XPSspectra in Figure 4 that the N2 MW plasma did inser-tion of nitrogen on the PI surface Thus N2 MW plasmatreated PI surface has more Pd atoms than none treated PIAs a result large amount of copper atom was depositedon PI surface by N2 MW plasma treatment because morePd atoms were adsorbed on PI surfaceXP spectra of activated PI substrate by PdSn solution

were also investigated to make sure N2 MW plasma treat-ment effect (shown in Fig 6) In reaction (lowast) Sn has bothoxidation states of Sn2+ and Sn4+ in the activation solu-tion because Sn was used in a large amount (25 g in100 ml solution) in comparison with Pd (004 g in 100 mlsolution) it was still on the PI surface even the sampleswere washed by HBF4 acid As proven by XPS spectra inpart 33 after N2 MW plasma treatment the oxygen con-tent on the PI surface was increased in comparison with

non-treatment sample this led to increase Sn adsorbatemore than none treated PI These are clearly shown inFigure 6(a) in which the intensity of both Sn2+ (4875 eV)and Sn4+ (4885 eV) were increased after N2 MW plasmatreatmentFigure 6(b) shows that Pd has two bonding states

3377 eV corresponding to Pd0 and 3385 eV correspond-ing to Pd2+ for none treated PI Only a small shoulderof the peak 3385 was observed in the N2 MW samplewhile the peak 3377 was much higher than none treatedPI because the lone pair electrons on the nitrogen and oxy-gen on the PI surface are the interaction sites the lowshift in the Pd binding energy was explained formation ofnew the chemical bonds between palladium and nitrogenwhich has lower electronegativity than oxygen This showsthat nitrogen was easily adsored on PI surface with N2

MW plasma treatment From these results N2 MW plasmatreatment sample contained lots of Pd adsorbate more thannone treated PIThe adhesion strength between copper layer and PI sub-

strate was also checked by using Scotchreg tape test17 Theresult showed that copper layers were easily removed fromnone treated PI substrate Whereas in the case of N2 MWplasma treatment the copper layers were stable



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Fig 6 Core-level XP spectra of (a) Sn and (b) Pd on PI surface

4 CONCLUSION

Polyimide films were treated by N2 microwave (MW)plasma Surface properties and chemical composition ofthe plasma treated polyimide films were investigated bycontact angle AFM images and XPS spectra The copperelectroless deposition layer properties were investigated byusing SEM XRD spectra The electrical volume resistivityof copper layer on polyimide substrate was measured byfour-point probe method Tape test was applied to examineadhesion strength The results showed that N2 MW plasmawas successfully used for achievement of chemical mod-ification on the PI surface The N2 MW plasma produces

chemical change to the PI surface through nitrogen inser-tion on the PI surface A direct correlation between theN concentration and Cu coverage was observed Uniformsurface and low volume resistance of copper layer wereachieved by electroless plating with N2 MW plasma treat-ment In this work the adhesion improvement processbetween copper layer and PI substrate was suggested byusing N2 MW plasma treatment instead of NH3 plasmaUsing N2 gas process is an environmental friendly process

Acknowledgment Support of this research by the BK21 project of the Ministry of Education Korea is grate-fully acknowledged This work was also supported by theNational Research Foundation of Korea (2009 009 4025and 2009 007 7534)

References and Notes

1 J M Burkstrand J Vac Sci Technol 15 223 (1978)2 F D Egitto L J Matienzo K J Blackwell and A R Knoll

J Adhes Sci Technol 8 411 (1994)3 N Inagaki S Tasaka and K Hibi J Adhes Sci Technol 8 395

(1994)4 N Chou J Paraszczak E Babich Y Chaug and R Goldblatt

Microelectron Eng 5 375 (1986)5 J B Ma J Dragon W Van Derveer A Entenberg V Lindberg

M Anschel D Y Shih and P Lauro J Adhes Sci Technol 9 487(1995)

6 G Rozovskis J Vinkevicius and J Jaciauskiene J Adhes SciTechnol 10 399 (1996)

7 S Sapieha J Cerny J Klemberg-Sapieha and L Martinu J Adhes42 91 (1993)

8 S H Kim S W Na N E Lee Y W Nam and Y-H Kim SurfCoat Technol 200 2072 (2005)

9 R Flitsch and D Y Shih J Vac Sci Technol A 8 2376 (1990)10 A D Katnani A Knoll and M A Mycek J Adhes Sci Technol

3 441 (1989)11 A M Ektessabi and S Hakamata Thin Solid Films 377ndash378 621

(2000)12 G Krulik Platinum Met Rev 26 58 (1982)13 H Honma and T Kobayashi J Electrochem Soc 141 730 (1994)14 M Charbonnier M Alami and M Romand J Electrochem Soc

143 472 (1996)15 D Bhusari H Hayden R Tanikella S Allen and P Kohl J Elec-

trochem Soc 152 F162 (2005)16 M Charbonnier and M Romand Int J Adhes Adhes 23 277

(2003)17 M Charbonnier M Romand Y Goepfert D Leonard F Bessueille

and M Bouadi Thin Solid Films 515 1623 (2006)

Received 31 July 2009 Accepted 1 March 2010



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various operating parameters such as time gas flow rateand plasma discharge power Furthermore effect of theMW plasma treatment on peel strength of Cu layer and PIfilms was examined

2 EXPERIMENTAL DETAILS

21 Materials

Kaptonreg film (PI film DuPonttrade) was prepared as flexi-ble substrate and cut in 60times15 mm2 Polyimide has twodifference surfaces The lsquotoprsquo surface is referred to as thelsquobrightrsquo or lsquoshinyrdquo side the lsquobottomrsquo side is lsquodullrsquo andpurposely roughened in the manufacture of the film toimprove handling characteristics Most adhesives bond bet-ter to the dull side5 In our experiments we always choosethe roughened side of the PI as the modification side thisside also has a slightly convex curvature

22 Surface Modification

The experiments were performed in a MW plasma systemmodel MPS-60R produced by Woosin CryoVac companyoperating at 245 GHz The aluminum process chamber480times 480times 500 mm3 is equipped with a grounded alu-minum substrate holder and a fused silica window Thepump speed was held constant The process parametersin the MW plasma treatment (ie power time flow rate)were varied in order to achieve the optimal result


Following the microwave plasma treatment the sampleswere activated with the SnPd catalyst in 4 min at roomtemperature Then electroless copper was deposited byfree formaldehyde method using glyoxylic acid as a reduc-ing agent for 40 min at 40 C Chemical solutions foractivation step6 and electroless deposition were shown inTable I


The wettability of the surface of PI films was character-ized by the contact angle meter model SEO 300A by Sur-face and Electro-Optics Deionized (DI) water was used as

Table I Composition and operating conditions of electroless Cudeposition

Activating solutions (gdm3) Electroless solutions (gdm3)

PdCl2 04 CuCl2 middot2H2O 3SnCl2 middot2H2O 25 K-Na tartrate 168NaCl 200 NaOH 12HCl 40 Glyoxylic acid 10

bull Activation time 4 minutes bull Deposition time 40 minutesbull Sn compounds were removed from bull Temperature 40 Cthe surface by using HBF4 acid

a liquid in the measurement A pendent water drop thatwas formed at the tip of the syringe and the specimenwas raised until it touched the bottom of the drop Afterthe DI water drop was dropped onto the surface of theplasma-treated PI substrate the advancing contact angleswere measured immediately by a sessile drop methodIt was found that the shape of the drop on the surfacewas not changed significantly during the measurement Allmeasurements were carried out at 25plusmn01 C Contactmode atomic force microscopy (AFM THERMOMICRO-SCOPEStrade AP-0100) was used in order to characterize themorphological change of microwave plasma-treated PI sur-faces Images were obtained using Thermo Microscopeswith silicon nitride probe mounted on a cantilever Imagingwas performed in contact mode The X-ray photoelectronspectroscopy (XPS) measurement of PI surfaces was per-formed using MgK light source (12536 eV) and concen-tric hemispherical analyzer (CHA) SEM images (PhillipsXL30-FEG JEOL JSM-890) and XRD spectra (HR-XRDBruker D8 Discover system) were used to study Cu surfacestructure Practical adhesion of the electroless Cu films onPI substrate was evaluated using the standard Scotchreg tapetest performed with the 3 M Scotchreg tape

3 RESULTS AND DISCUSSION

31 Wettability of Polyimide Film Surface

Figure 1 shows contact angle of N2 plasma treated PIwith different MW power and N2 flow rate Water con-tact angle was decreased with N2 plasma treatment Espe-cially increasing N2 flow rate led to wettable surfacemore than untreated PI surface (until 100 sccm) Howeverafter 100 sccm of N2 water contact angle was increasednamely 100 sccm of N2 flow rate is turning point of inthis experiment Also at this point contact angles of 400500 and 600 W samples were stayed in the same errorrange with almost the same value Thus we determined

Fig 1 Contact angles of MW plasma-treated PI films with various MWplasma RF power and N2 gas flow rate



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W = P times t

A(1)














(a)

(b)




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Atomic percent (at)

Sample O N C
















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(a)

(b)










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4 CONCLUSION























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W = P times t

A(1)














(a)

(b)




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Atomic percent (at)

Sample O N C
















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(a)

(b)










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4 CONCLUSION























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Atomic percent (at)

Sample O N C
















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(a)

(b)










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4 CONCLUSION























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(a)

(b)










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4 CONCLUSION























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Documents

Polyimide Surface Modi“cation by Using Microwave Plasma for …home.skku.edu/~jhboo/publications/paper/2011/2011-10.pdf · 2011-07-15 · polyimide (PI) lms as a treatment to enhance