The influence of 2,2 -dipyridyl on non-formaldehyde electroless copper plating

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1 Eletrohimi At 9 () The influene of, -dipyridyl on non-formldehyde eletroless opper plting Jun Li, Hrley Hyden, Pul A. Kohl Shool of Chemil & Biomoleulr Engineering, Georgi Institute of Tehnology, Atlnt, GA 333-1, USA Reeived 6 Otoer 3; reeived in revised form 1 Deemer 3; epted 1 Deemer 3 Astrt High eletroless opper deposition rtes n e hieved using hypophosphite s the reduing gent. However, the high deposition rte lso results in drk deposits. In the hypophosphite ths, nikel ions (.7 M with Ni + /Cu + mole rtio.1) were used to tlyze hypophosphite oxidtion. In this study, dditives (e.g., -dipyridyl) were investigted to improve the mirostruture nd properties of the opper deposits in the hypophosphite (non-formldehyde) ths. The influene of, -dipyridyl on the deposit omposition, struture, properties, nd the eletrohemil retions of hypophosphite (oxidtion) nd upri ion (redution) hve een investigted. The eletroless deposition rte deresed with the ddition of, -dipyridyl to the plting solution nd the olor of the deposits hnged from drk rown to semi-right with improved uniformity. The deposits lso hd smller rystllite size nd higher (1 1 1) plne orienttion with the use of, -dipyridyl. The resistivity nd nikel ontent of the deposit were not ffeted y, -dipyridyl dditions to the th. The eletrohemil urrent voltge results show tht, -dipyridyl inhiits the tlyti oxidtion of hypophosphite t the tive nikel site. This results in more negtive eletroless deposition potentil nd lower deposition rte. Elsevier Ltd. All rights reserved. Keywords:, -Dipyridyl; Hypophosphite; Eletroless opper plting; Copper 1. Introdution Eletroless opper plting using sodium hypophosphite s the reduing gent in ple of formldehyde is ttrtive euse of its low ph, low ost, nd reltive sfety, ompred with high ph formldehyde-sed solutions [1,]. However, the hypophosphite-sed eletroless opper plting proess is omplited euse opper is not good tlyst for the oxidtion of hypophosphite resulting in little or no plting on pure opper surfe. One pproh to tlyze the oxidtion of the reduing gent is to dd nikel ions (or other metl ions) to the th, resulting in very smll mount of o-deposited nikel in the opper deposit. The nikel serves to tlyze the oxidtion of hypophosphite enling ontinuous opper deposition [ ]. Previous studies hve shown tht elertors, suh s formmidine disulfide, nd nikel re essentil to the eletroless opper plting th to mintin n utotlyti surfe Corresponding uthor. Tel.: ; fx: E-mil ddress: pul.kohl@he.gteh.edu (P.A. Kohl). [6,7]. The utotlyti surfe results in high deposition rte. It ws shown tht eletroless opper plting ourred t low deposition rte in the sene of the elertors in the th, even though dequte nikel(ii) ws in the th. When the Ni + /Cu + mole rtio in the th is low, the deposition rte of the opper plting deresed with time nd finlly stopped euse the surfe tlyti tivity ws not replenished. Thus, it ws neessry to mintin the Ni + /Cu + mole rtio ove ritil vlue in the th to sustin the deposition rte. However, the opper deposit properties were degrded nd the deposit pperne eme drker with inresed Ni + /Cu + mole rtio in the th. Consequently, it is importnt to improve the mirostruture nd properties of the opper deposit while mintining the nikel(ii) onentrtion in the eletroless opper plting th., -Dipyridyl hs een used in formldehyde-sed eletroless opper plting ths to improve the physil properties of the opper deposits [8]. When n optimum mount of, -dipyridyl ws dded into the solution, the opper deposit hd glossy pperne nd good dutility [9]., -Dipyridyl hs lso een used s stilizer in the formldehyde-sed eletroless opper plting /$ see front mtter Elsevier Ltd. All rights reserved. doi:1.116/j.elett.3.1.1

2 179 J. Li et l. / Eletrohimi At 9 () euse it n inhiit the utotlyti deposition proess [1]. In this study, the influene of, -dipyridyl on the hypophosphite-sed eletroless opper proess hs een investigted. The mirostruture nd properties of the opper deposits otined t different, -dipyridyl onentrtions in the th re reported long with voltmetri nlysis of its role.. Experimentl The eletroless opper plting th ontined:. M opper sulfte,.7 M nikel sulfte,.8 M sodium hypophosphite,.1 M sodium itrte,.8 M ori id, ppm elertors (formmidine disulfide) [6,7], nd ppm, -dipyridyl. Deionized wter ws used to prepre the solutions. The ph ws djusted using NOH or H SO to finl vlue of The temperture ws held t 7 ±. C. Plting ws performed in ml eletroless opper solution with ontinuous stirring. Epoxy ords (re: 6m ) or lumin sheets (re: m ) were used s the sustrtes for the eletroless opper plting. The sustrtes were tivted y plldium in the sequene desried in the Shipley proess [6]. Copper deposition rtes were determined y the hnge in weight of the epoxy ords fter 3 min plting ssuming uniform plting nd ulk density. For vi-hole plting, the opper surfe ws tivted in dilute idi PdCl solution for 1 min. The opper thikness ws determined using DekTek profilometer nd vi ross-setions were oserved using optil mirosopy. The resistivity of the s-deposited nd nneled opper were mesured using four-point proe. The opper ws deposited on lumin sheets for nneling t high temperture. The nneling of the opper deposits ws rried out in tue furne with nitrogen mient. The temperture inside the furne ws ontrolled to ±1. C. The rystl struture of the opper deposits ws investigted using X-ry diffrtion (XRD, Cu K rdition nd grphite filter t kv nd 3 ma). Atomi fore mirosopy (AFM) ws used to hrterize the roughness of the deposits. The hemil omposition of the opper deposits t the surfe ws determined y X-ry photoeletron spetrosopy (XPS). The nlysis ws performed fter rief sputter lening. An EG & G PARC model 63A potentiostt ws used for eletrohemil mesurements. The liner sweep voltmmetry (LSV) nd yli voltmmetry (CV) experiments were rried out t 7 C nd t sn rte of 1 mv/s. The working eletrode ws pure opper with. m surfe re; the ounter eletrode ws pltinum wire, nd the referene eletrode ws sturted lomel eletrode (SCE). Prior to eh test, the eletrolyte ws deerted with nitrogen gs nd the eletrodes were immersed in the eletrolyte until stedy open iruit potentil (OCP) ws rehed. 3. Results nd disussion The effet of, -dipyridyl on the deposition rte nd opper properties were investigted. The hnge in deposition rte of the eletroless opper plting s funtion of, -dipyridyl onentrtion is shown in Fig. 1. The nikel ion onentrtion in the th ws.7 M nd the Ni + /Cu + mole rtio ws.1. The deposition rte of the eletroless plting in the sene of, -dipyridyl ws high nd the deposits were drk., -Dipyridyl hd signifint ffet on the opper deposition rte. The deposition rte deresed from 18 to 7 m/h (s mesured fter 3 min plting) s the onentrtion of, -dipyridyl ws inresed from to ppm. The derese in deposition rte with further dditions of, -dipyridyl ourred more grdully. A similr effet on the deposition rte with, -dipyridyl ddition to formldehyde-sed eletroless opper plting th ws oserved previsously [8]. It is possile tht the redution in the deposition rte with the ddition of, -dipyridyl results from its dsorption on the eletrode surfe using lower rte of eletron trnsfer or nuletion. The effet slows with further dded, -dipyridyl s the surfe dsorption rehes sturtion. The olor of the deposits hnged from drk rown to semi-right with the ddition of, -dipyridyl to the plting th. AFM imges nd root men squre (RMS) roughness of the deposits versus, -dipyridyl onentrtion re shown in Figs. nd 3, respetively. The topogrphy of the opper deposit in the sene of, -dipyridyl in the th ws rough. The deposits eme smooth, nd the RMS roughness of the deposits dropped from 398 to nm with the ddition of, -dipyridyl to the th. A orresponding improvement in olor of the deposit lso ourred. The right pperne of the deposit usully indites etter mehnil nd physil properties. The XRD ptterns of the opper deposits otined t different, -dipyridyl onentrtions re shown in Fig.. The deposits exhiited the hrteristi peks orresponding to (1 1 1) orienttion. The opper oxide phse ws not deteted in the deposits. The effetive rystllite size of the opper deposits n e estimted from the rodening of the diffrtion pek from the (1 1 1) plnes y the use of Sherrer s eqution [11,1]. D =.89λ (1) β 1/ os θ Deposition rte (um/hr) ,'- dipyridyl onentrtion (ppm) Fig. 1. The hnge in deposition rte of the eletroless opper plting s funtion of, -dipyridyl onentrtion.

3 J. Li et l. / Eletrohimi At 9 () Fig.. AFM imges of opper deposits from different, -dipyridyl onentrtion th: () ppm; ppm; () 1 ppm; (d) ppm. RMS roughness (nm) ,'- dipyridyl onentrtion (ppm) Fig. 3. Dependene of RMS roughness of opper deposits on, -dipyridyl onentrtion. where λ is the wvelength of Cu K, β 1/ the hlf width of diffrtion line, nd θ the diffrtion ngle. The lulted effetive rystllite size of the opper deposits nd reltive intensities of the diffrtion peks from the (1 1 1), ( ), nd ( ) plnes re listed in Tle 1. It n e seen tht the opper deposit hs deresed rystllite size nd intensified (1 1 1) plne orienttion with the ddition of, -dipyridyl to the plting th. The onentrtion of impurities t the surfe of the opper deposits s funtion of, -dipyridyl onentrtion in the th ws investigted using XPS. The deposit surfe onsisted minly of opper with tre of nikel s given in Tle. The nikel onentrtion ws 7 t.% nd ws unffeted y, -dipyridyl in the plting th. Intensity (111) thet Fig.. XRD ptterns of opper deposits from different, -dipyridyl onentrtion th: () ppm; ppm; () ppm. () () Tle 1 Chrteristi pek reltive intensities nd rystllite size of the opper deposits from the th with different, -dipyridyl onentrtions Copper deposits, -Dipyridyl onentrtion (ppm) Crystllite size I (111) I () I ()

4 179 J. Li et l. / Eletrohimi At 9 () Tle The nikel ontent in the opper deposits from the th with different, -dipyridyl onentrtions Copper deposits, -Dipyridyl onentrtion (ppm) Ni (t.%) Nikel ions re required in the hypophosphite-sed eletroless opper plting th to mintin the deposition rte. Nikel tlytilly prtiiptes in the oxidtion of hypophosphite. The opper deposition rte inresed with nikel ion onentrtion [7]. At.7 M nikel ion onentrtion in the th without, -dipyridyl, the opper deposition rte ws very high resulting in lrge rystllite with poor mirostruture due to the rpid nuletion nd growth [13]. This poor mirostruture is onsistent with the high RMS roughness of the opper deposit oserved when, -dipyridyl ws sent from the th. In ddition, nikel toms in the opper lttie inrese the rystl defets with the opper deposit. Consequently, the poor qulity opper deposits produed in the sene of, -dipyridyl in the th hve higher resistivity. The resistivity of the s-plted opper deposits versus, -dipyridyl onentrtion is shown in Fig.. It n e seen tht the resistivity of the opper deposits were higher thn those deposited from formldehyde-sed eletroless opper plting ths (. to m) [1]. The resistivity of the opper deposits did not hnge with the ddition of, -dipyridyl Resistivity (1-6 ohm.m) () Resistivity (1-6 ohm.m) ,'- dipyridyl onentrtion (ppm) Anneling temperture ( C) Fig.. Resistivity of the opper deposits: () dependene of the resistivity of opper deposits on, -dipyridyl onentrtion; dependene of opper deposit resistivity ( ppm, -dipyridyl) on nneling temperture. to the plting th, even though the deposit mirostruture improved with ddition of, -dipyridyl. Normlly, the deposit resistivity inreses with deresing rystllite size due to eletron sttering from grin oundries. The resistivity of the opper deposits ( ppm, -dipyridyl) nneled t different tempertures for 1. h in nitrogen mient is shown in Fig.. The redution in the resistivity ourred with 3 C nnel nd the resistivity rehed minimum ( m) with C nnel. The redution in the resistivity fter the 3 C nnel is used y rerystlliztion in the deposit, whih is ompnied y rystllite growth, nd rystl defet nd miro-void nnihiltion. Aording to the mixed potentil theory of eletroless plting [1,16], the overll retion of the eletroless proess is determined y the two hlf-retions on the sme eletrode, i.e. the redution of upri ions nd the oxidtion of the reduing gent. Consequently, studying the effets of, -dipyridyl on the hlf-retions n help understnd the funtion of, -dipyridyl in the eletroless opper plting th. Fig. 6 shows the LSV (from OCP positively to.1 V) for hypophosphite oxidtion on the opper eletrode in n eletrolyte ontining.8 M sodium hypophosphite,.1 M sodium itrte,.8 M ori id with nd without.7 M nikel ions. It n e seen tht the ddition I ( ua/m ) () I (ua/m ) ppm dipyridyl 1 ppm dipyridyl ppm dipyridyl ppm dipyridyl ppm dipyridyl 1 ppm dipyridyl ppm dipyridyl ppm dipyridyl Fig. 6. Current potentil urves for the oxidtion of hypophosphite with different, -dipyridyl onentrtions in the eletrolyte ontining.8 M sodium hypophosphite,.1 M sodium itrte,.8 M ori id: () without nikel ions; with.7 M nikel ions.

5 J. Li et l. / Eletrohimi At 9 () ppm dipyridyl. 1 ppm dipyridyl. ppm dipyridyl d. ppm dipyridyl I ( ma/m ) 3 d () E (mv SCE). ppm dipyridyl. 1 ppm dipyridyl. ppm dipyridyl d. ppm dipyridyl I ( ma/m ) 3 1 d E (mv SCE) Fig. 7. Current potentil urves for the redution of upri ion with different, -dipyridyl onentrtions in the eletrolyte ontining. M opper sulfte,.1 M sodium itrte,.8 M ori id: () without nikel ions; with.7 M nikel ions. of, -dipyridyl to the eletrolyte without nikel ions hd little effet on the oxidtion of hypophosphite (Fig. 6). The presene of.7 M nikel ions in the eletrolyte filitted the oxidtion of hypophosphite in the sene of, -dipyridyl (Fig. 6). The oxidtion of hypophosphite on the opper eletrode ws lowered y the ddition of, -dipyridyl resulting in derese in the oxidtion urrent with, -dipyridyl. Thus,, -dipyridyl effets the oxidtion of hypophosphite in the presene of nikel ions in the plting th. The redution of upri ions on the opper eletrode ws lso investigted versus nikel ion nd, -dipyridyl onentrtion y LSV in n eletrolyte ontining. M CuSO H O,.1 M sodium itrte,.8 M ori id, s shown in Fig. 7. The ddition of 1 ppm (or greter), -dipyridyl to the eletrolyte used derese in the mgnitude of the thodi urrent t the more negtive potentils. There ws little differene in the thodi urrent t higher, -dipyridyl onentrtions. This shows tht, -dipyridyl inreses the overpotentil for the redution of upri ions or hydrogen evolution. The presene of nikel ions in the eletrolyte hd no effet on the upri ion redution nd the effet of, -dipyridyl on the upri ion redution ws not dependent on the presene of nikel ion in the eletrolyte. The yli voltmmogrms of opper eletrode in the eletroless opper plting th ontining, -dipyridyl re shown in Fig. 8. The yli voltmmetry ws performed strting from V nd going negtive to 1. V. For omprison, the yli voltmmetry of the eletroless opper plting th without nikel ions or, -dipyridyl is lso shown in Fig. 8. In the th ontining.7 M nikel ions (Fig. 8), n oxidtion pek t.9 V ws oserved on the positive-going sn. It is interesting tht the oxidtion pek t.9 V ourred t more negtive potentil thn the redution peks t. nd.9 V, respetively. This indites tht some of the nikel ions were redued nd o-deposited with opper on the eletrode during the negtive-going sweep, nd the deposited nikel then tlyzed the oxidtion of hypophosphite t.9 V. The pek urrent density for the oxidtion of hypophosphite on the o-deposited nikel ws higher in the sene of, -dipyridyl nd deresed with, -dipyridyl onentrtion. Fig. 9 shows the urrent versus time on opper eletrode t.9 V in the eletroless opper plting th s

6 179 J. Li et l. / Eletrohimi At 9 () ppm dipyridyl. 1 ppm dipyridyl. ppm dipyridyl I (ma/m ) () I (ma/m ) Fig. 8. Cyli voltmmogrms for opper eletrode in the eletroless opper plting: () with different, -dipyridyl onentrtions nd.7 M nikel ions; without, -dipyridyl nd nikel ions. funtion of, -dipyridyl onentrtion. The net urrent in the sene of, -dipyridyl ws nodi, inditing the oxidtion rte of hypophosphite ws higher thn the redution rte of upri nd nikel ions. When 1 ppm, -dipyridyl ws dded to the th, the oxidtion rte of hypophosphite ws lower nd the net urrent eme thodi. The thodi urrent inresed slightly with, -dipyridyl onentrtion. The dsorption of, -dipyridyl on the eletrode inhiited the oxidtion of hypophosphite presumly y deresing the tlyti effet of nikel. The derese in the oxidtion of hypophosphite y, -dipyridyl ws lrger thn the orresponding derese in the redution of upri ions. The eletroless deposition rte thus deresed signifintly with the ddition of, -dipyridyl in the solution nd the opper rystllites eme finer grin. I (ma/m ) d. ppm dipyridyl.1 ppm dipyridyl. ppm dipyridyl d. ppm dipyridyl Time (s) Fig. 9. Current time urves on the opper eletrode t.9 V in the eletroless opper plting with different, -dipyridyl onentrtions. Fig. 1. Cross-setion of vi: () efore plting; fter plting with totl of 1 m opper deposit. The eletroless opper plting th with ppm, - dipyridyl ws used in vi-hole plting on printed iruit ord. Formldehyde-sed eletroless opper plting ths re often operted t ph vlue ove 11 nd re not omptile with some photoimgile mterils used in the frition of high density printed iruit ords. Sine the eletroless opper plting solution using hypophosphite s reduing gent with ppm, -dipyridyl n e operted t ph of 9, nd hs high deposition rte, it n e used for vi-hole plting. Fig. 1 shows ross-setion of vi-hole efore nd fter plting. The thikness of the plted opper ws 1 m. It n e seen tht the photoimgile mteril ws stle during plting. The opper deposit in the vi is uniform nd ompt. The verge deposition rte ws 3 m/h.. Conlusions, -Dipyridyl improves the mirostruture nd properties of opper deposits otined from eletroless opper plting using hypophosphite s reduing gent. With the ddition of, -dipyridyl to the plting th ontining.7 M nikel ions, the olor of the deposit hnged from drk rown to semi-right nd the deposits eme uniform nd ompt. The deposits hd smller rystllite size nd higher (1 1 1) plne orienttion with the ddition of, -dipyridyl.

7 J. Li et l. / Eletrohimi At 9 () The eletroless deposition rte deresed signifintly with the ddition of, -dipyridyl to the th. Eletrohemil mesurements showed tht, -dipyridyl inhiited the tlyti oxidtion of hypophosphite t the tive site of nikel on the deposit surfe nd inresed the overpotentil for redution retion (opper deposition or hydrogen evolution) y dsorption on the eletrode, thus reduing the deposition rte nd mking the deposit finer in struture. The resistivity nd nikel ontent ( 7 t.%) of the deposit ws not effeted y the ddition of, -dipyridyl in the plting th. The resistivity of the s-plted deposit ws higher thn tht of the deposits from formldehyde-sed eletroless opper plting due to the presene of nikel. The resistivity of the deposit from ths with.7 M nikel n e lowered to m y nneling t 3 C. Referenes [1] D.H. Cheng, W.Y. Xu, Z.Y. Zhng, Z.H. Yio, Met. Finish. 9 (1) (1997) 3. [] A. Hung, K.M. Chen, J. Eletrohem. So. 136 (1) (1989) 7. [3] S.Z. Chu, M. Skiri, H. Tkhshi, J. Eletrohem. So. 17 () () 13. [] H. Honm, T. Fujinmi, Y. Tershim, S. Hyhsi, S. Shimizu, US Ptent 6,193,789 (1). [] A. Hung, Plt. Surf. Finish. 7 (1) (1988) 6. [6] J. Li, P.A. Kohl, J. Eletrohem. So. 19 (1) () C631. [7] J. Li, P.A. Kohl, J. Eletrohem. So. 1 (8) (3) C8. [8] M. Oit, M. Mtsuok, C. Iwkur, Eletrohim. At (9) (1997) 13. [9] K. Kondo, S. Amkus, K. Murkw, K. Kojim, N. Ishid, J. Ishikw, F. Ishikw, US Ptent,96,11 (1999). [1] P. Andrios, S.H. Boetther, F.R. MFeely, M. Punovi, US Ptent 6,16,81 (). [11] M. Mtsuok, J. Muri, C. Iwkur, J. Eletrohem. So. 139 (9) (199) 66. [1] B.D. Cullity, Elements of X-ry Diffrtion, Addison-Wesley, London (1978). [13] W.T. Tseng, C.H. Lo, S.C. Lee, J. Eletrohem. So. 18 () (1) C333. [1] J.C. Ptterson, M. O Reilly, G.M. Cren, J. Brrett, Miroeletron. Eng. 33 (1997) 6. [1] M. Punovi, Plting (1968) [16] I. Ohno, O. Wkyshi, S. Hruym, J. Eletrohem. So. 13 (198) 33.