Ag nanoparticle as a new activator for catalyzing electroless copper bath with 2,2

(1)

ElectrochimicaActa56 (2011) 8880–8883

ContentslistsavailableatScienceDirect

Electrochimica

Acta

jo u r n al h om ep a ge :w w w . e l s e v i e r . c o m / l o c a t e / e l e c t a c t a

Ag

nanoparticle

as

a

new

activator

for

catalyzing

electroless

copper

bath

with

2,2

-bipyridyl

Chien-Liang

Lee

∗

,

Chia-Chieh

Syu

DepartmentofChemicalandMaterialsEngineering,NationalKaohsiungUniversityofAppliedSciences,Kaohsiung807,Taiwan

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received12May2011

Receivedinrevisedform3July2011

Accepted26July2011

Available online 5 August 2011 Keywords:

Activation

Printedcircuitboard

a

b

s

t

r

a

c

t

AgnanoparticlescanbesuccessfullyusedinsteadofPd,whichisanexpensivemetal,asnovel activa-torsforelectrolesscopperdeposition(ECD).Theanalyticalresultsobtainedusingelectrochemicalquartz crystalmicrobalancemeasurementsandmixedpotentialtheoryillustratethattheactivityofAg nanopar-ticlescanbeenhancedbyadding2,2-bipyridyltotheECDbath.TheresultsofapreviousstudyonAg nanoparticlesinasimpleECDbathshowedthatthenanoparticleswereinactive.Incontrast,theresults ofthisstudydemonstratethattheadditionof2,2-bipyridylcanimprovethedepositioncurrentdensity byabout0.004mAcm−2andhelptoachieveadepositionrateof33.9␮gcm−2s−1.

1. Introduction

Electrolesscopperdeposition(ECD)iswidelyusedandis car-riedouttodepositconductiveCuontothesidewallsofthethrough holeinprintedcircuitboardswhilefabricatingelectroniccircuits

[1,2].TheactivationnecessarytotriggeranECDreactionisa cat-alyticreactionthatistriggeredbyactivecolloidsonthesurface ofsubstratesdippedintheECDbath;thiscatalyticreaction pro-ceedsviaHCHOoxidation[3,4].IntheanodicprocessofECD,the bondbetweentheHradicalandthe ˙CHOHO−radicalisﬁrst bro-kenatanenergyabovetheactivationenergyof11.7kcalmol−1[5]. Ontheotherhand,theactivationenergyformetalreductionvia acathodicpathwayis6.5–7.5kcalmol−1[5].Theformed ˙CHOHO− radicalisthenoxidizedtoyieldCH(OH)2O−,releasinganelectron

forCureduction.Simultaneously,theHradicalsproducedinthe ﬁrststepformH2byrecombination[5].Theactivecatalyst

there-foreactsasanelectroncarrierforthetransferofelectronsfrom thereducingagent(HCHO)totheCuions.APd/Sncolloid[4,6–8]

is now usedas a typicalactivatorand catalyst for ECD. Simul-taneously,dispersingpurePdcolloid[9]andnanoparticle[3,10]

havebeenfoundthepotentialforactivatingECDbath.However, Pdisexpensiveandcanresultinahighmaterialscostwhenitis usedforECD.Inaneasierstudy,Ohnoandco-workers[11] suc-cessfullyidentiﬁedtheorderofactivationenergiesfortheanodic oxidationofHCHObyusingvariousbulkelectrodes;theorderof activationwas asfollows: Au<Ag<Pd [11]. Alongwith Pd and Au,Ag maybeusedasanefﬁcientand comparatively inexpen-sivecatalystfortheECDprocessinindustry.Recently,Wanand

∗ Correspondingauthor.Tel.:+886738145265131;fax:+88673830674.

E-mailaddresses:cl lee@url.com.tw,cllee@kuas.edu.tw(C.-L.Lee).

co-workers[12]reportedexquisiteresearchinwhichtheyused Ag/PdnanoparticlestocatalyseanECDbath.Theyfoundthatthe activityofAg/Pdnanoparticleswithamolarratioof1:1is compa-rabletothatofPdnanoparticles.Theyalsosuccessfullyprepared Ag/Pdnanoparticleswitha Pd-richsurfacetoimproveECD efﬁ-ciency[13].Subsequently,toreducetheamountofPdusedasa catalyst,wepreparedAg/Pdnanoparticles[14],nanoplates[15], andnanorings[16]containingalowamountofPdbyusinga dis-placementmethod.Wesuccessfullyusedthesealloynanoparticles aseffectivecatalysts,particularlythattheinactivedepositionrate catalysedbyPdcolloidswasobservedfrombasingonthesame Pdloading[14,16].Intheaboveexperiments,Agnanoparticlesalso showedaninertactivityinasimpleECDbathwithoutanyadditives

[14,16].Recently,Vaˇskelisandco-workers[17]successfully initi-atedanECDreactionusing5–100-nmAg/SnO2colloidsprepared

viaasynthesissimilartothatusedforPd/Sncolloidsthatare pre-paredusingSn2+_as_a_reducing_agent._{Subsequently,}_Fujiwara_et_al.

[18]usedAg/SnO2 colloidsastestcatalystsinanECDbathwith

added2,2-bipyridylanddepositedconductiveCuontoaglass sub-strate.Thecompound2,2-bipyridylisoftenusedasastabilizerfor ECDbathswhendepositionistriggeredbyaPd/Sncolloid[19].The purposeofthepresentstudyistodemonstratetheuseofpureAg nanoparticlesasnovelECDactivatorsandtoconduct electrochem-icalquartzcrystalmicrobalance(EQCM) measurementsand use mixedpotentialtheory(MPT)toanalysetheeffectof2,2-bipyridyl onthedepositionkinetics.

2. Experimental

2.1. Synthesisofactivator

Initially, approximately 10−3moles of AgNO3

pow-der was added to a beaker containing 100ml of 1.82mM

(2)

C.-L.Lee,C.-C.Syu/ElectrochimicaActa56 (2011) 8880–8883 8883 0.5 0.0 -0.5 -1.0 -1 0 1 2 Current density / mA cm -2 Potential / V vs. SCE Ag with Bipyridine Ag without Bipyridine

Fig.4.Theeffectofthe2,2_-bipyridyl_on_the_polarization_curves_of_formaldehyde

oxidationbyAgnanoparticlesintheanodicbathshowninTable1.

curvesobtainedusingtheTafelequationfor ECDbathswithout orwith2,2-bipyridyl(Table1)catalysedbyAgnanoparticles,Pd nanoparticles,andPd/Sncolloids.Theresultsaresummarizedin

Table2.AccordingtotheMPT,thecoordinatesoftheintersection pointoftheanodicandcathodiccurvesarethedepositioncurrent

(Ideposition)andthemixedpotential(Emp),respectively,atwhich

thedepositionreactionoccursinasteadystate[3,21,22].Forthe activationofthebathswithout2,2-bipyridyl,theEmp valuesfor

Agnanoparticles,Pdnanoparticles,andPd/Sncolloidsareabout −0.46V,−0.25V,and−0.53V,respectively.Forthebathswith2,2 -bipyridyl,thevaluesforAgandPdnanoparticlesare−0.49Vand −0.28V,respectively.TheEmpvalueforPd/Sncolloidsis−0.37V.

TheEmpvaluesforPdandAgnanoparticlesweremorenegativeby

about30mVwhenthe2,2-bipyridylwaspresent.Thisdeviation indicatesthatthedrivingforcefortriggeringECDincreases. How-ever,forPd/Sncolloids,abarrierisobservedbecauseEmpincreases

by160mVwhen2,2-bipyridylispresent.Thisindicatesthat2,2 -bipyridylcouldsuppresstheinherentactivityofPd/Sncolloidsand actonlyasastabilizer.NotethatinthecaseofAgandPd nanoparti-cles,Idepositionisenhancedwhen2,2-bipyridylispresent.Asshown

inTable2,thedifferenceinIdeposition(Ideposition)iscalculatedusing

measurementsfortheECDbathswithandwithout2,2-bipyridyl.

Itcanbeseenthat Ideposition decreasesin thefollowingorder:

Agnanoparticles>Pdnanoparticles>Pd/Sncolloid.Typically,the depositionrateisproportionaltothemagnitudeofIdeposition[21].

ThisrelationshipthereforesuggeststhattheactivityofAg nanopar-ticlescanbeimprovedwiththeadditionof2,2-bipyridyl.

Typically, the cathodic elementary step for ECD is that Cu(II)–EDTAcomplexwasreducedtoCu(I),andthentoCu0_._If_the

signiﬁcantcontentofCu(I)waspresent,Cu2Oﬁnepowerswere

formedandfurthergrownthroughthereactionasfollows: Cu2O+H2O→Cu+Cu2++2OH− (1)

ThisoftencausestheinstabilityofECDbath.Theadditionof2,2 -bipyridylcaninactivate theseCu2Opowers bytheiradsorption

ontheCu2Osurfaceandstabilizetheelectrolyte.Accompanying

decreasein depositionratecanhappen [23]. Inthis study, this suppressiveeffectof2,2-bipyridylfortheIdeposition ofPd/Sn

col-loidswasobservedbased ontheMPTanalyses.Additionally,as showninFig.4,thepolarizationcurvesofformaldehyde oxida-tionrevealedthereasonoftheenhancementeffectof2,2-bipyridyl uponAgnanoparticles.Comparedtothecurrentdensitywithout 2,2-bipyridyl,thecurrentdensityofoxidizingformaldehydewith additional2,2-bipyridylisgreater,indicatingthatthepowerofAg

nanoparticlesforoxidizingformaldehydeisimproved.Thiscauses thattheinactiveECDreactionisfurtheractivated.

AnothersurprisingﬁndingisthatastheAgnanoparticlesare sufﬁcientlyactive,thedepositionrateofCucanbedetermined.The depositionratesachievedusingdifferentactivatorscanbe calcu-latedbyconsideringthechangesintheelectrodefrequency(Fig.2) usingtheSauerbreyequation[24].Theoretically,theamountofCu depositedissensitivetofrequencychangesandisexpressedbythe Sauerbreyequation:[24]

m= FA √ −2F2

0

where m (g) is the increased mass after deposition, F is the frequency change, F0 is the starting frequency, A is the

areaoftheAusubstrate(0.159cm2_),_is_the_shear_modulus_of

quartz (2.947×1011_g_cm−1_s−2_), _and _is _the _density _of _quartz

(2.648gcm−3). The results of these calculations are listed in

Table2.Themeandepositionrates achievedusingAg nanopar-ticles,Pdnanoparticles,and Pd/Sncolloidsare33.9_␮gcm−2s−1, 42␮gcm−2s−1,and88.1␮gcm−2s−1,respectively.Themeasured depositionratesindicatethatAgnanoparticlescanbeusedasnovel activatorsforECDandthattheyserveasanalternativeforreducing thematerialscostinadvancedelectronicprocesses.

4. Conclusion

PureAgnanoparticlesweresuccessfullyusedasnovel activa-torsfortheECDprocess.EQCMandMPTanalysesshowedthatthe activityofAgnanoparticlescanbeenhancedbyusing2,2-bipyridyl asanadditiveintheECDbath.

Acknowledgement

TheauthorswouldliketothanktheNationalScienceCouncil oftheRepublicofChina,Taiwan,forﬁnanciallysupportingthis researchunderContractNo.NSC98-2221-E-151-033-MY2.

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