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Electrochimica Acta 50 (2005) 1849–1861 Effect of plating mode, thiourea and chloride on the morphology of copper deposits produced in acidic sulphate solutions Nisit Tantavichet 1 , Mark D. Pritzker ∗ Department of Chemical Engineering, University of Waterloo, Waterloo, Ont., Canada N2L 3G1 Received 14 May 2004; received in revised form 30 July 2004; accepted 14 August 2004 Abstract The influence of plating mode, chloride and thiourea (TU) on morphology of copper deposits has been studied. All
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  Electrochimica Acta 50 (2005) 1849–1861 Effect of plating mode, thiourea and chloride on the morphology of copper deposits produced in acidic sulphate solutions Nisit Tantavichet 1 , Mark D. Pritzker ∗  Department of Chemical Engineering, University of Waterloo, Waterloo, Ont., Canada N2L 3G1 Received 14 May 2004; received in revised form 30 July 2004; accepted 14 August 2004 Abstract The influence of plating mode, chloride and thiourea (TU) on morphology of copper deposits has been studied. All experiments wereconducted on disc electrodes rotating at 500rpm and an average current density of 4Adm − 2 to produce 10  m thick deposits. In additive-freesolutions,theuseofpulsedcurrent(PC)improveddepositmorphologyandbrightnessoverDCplating.Inthepresenceofthiourea(noCl − ),thedepositsobtainedbyDCandPCplatingweresimilarundermostplatingconditions.Thepresenceofthioureagenerallyimproveddepositqualityoverthatobtainedinadditive-freesolutions,butcausedtheformationofmicroscopicnodulesandthedepositstoappearslightlycloudy,resultingin lower reflectances than that of a polished uncoated copper surface. The addition of Cl − to thiourea-containing solutions strongly influenceddeposit morphology at both microscopic and macroscopic scales depending on chloride concentration and pulse conditions. It preventednodule formation and created microscopically bright and reflective deposits, but caused extreme macroscopic roughness. Nevertheless, PCplating at 50Hz in solutions containing appropriate amounts of thiourea and Cl − was found to yield macroscopically and microscopicallysmooth deposits with reflectance similar to that of a polished uncoated copper substrate.© 2004 Elsevier Ltd. All rights reserved. Keywords:  Pulse plating; Copper; Thiourea; Chloride; Morphology 1. Introduction Typically,dullcopperdepositsareproducedbyDCplatingin sulphate-plating baths. Two ways to improve their qualityaretheuseofpulseplatingandtheinclusionofadditivesintheplating bath. Pulse current (PC) plating is known to improvethe morphology and properties of deposits in the absenceof additives [1–5] due to its positive effects on mass trans-port[6–8],electrodekinetics[9]andthenucleationofgrowth centres[1,10].Pulseplatingalsooffersalargernumberofpa- rameters (i.e., on-time, off-time, cathodic- and anodic-pulsecurrent density and frequency) than does DC plating to im-prove deposit quality. ∗ Corresponding author. Tel.: +1 519 888 4567x2542;fax: +1 519 746 4979.  E-mail address:  pritzker@uwaterloo.ca (M.D. Pritzker). 1 Present address: Department of Chemical Technology, Faculty of Sci-ence, Chulalongkorn University, Bangkok, Thailand. Deposit properties (i.e., brightness, smoothness and mi-crohardness) can also be improved through the inclusionof additives in a sulphate-plating bath. Both electrocrystal-lization and deposition kinetics are strongly sensitive to thepresence of additives in the plating bath at very low concen-trations. Typically, they influence deposit morphology andstructure by adsorbing on the cathode and inhibiting variousprocesses during electrodeposition.Although numerous studies have focused on the influenceof either pulse plating or additives on copper electrodeposi-tion, relatively few have considered their combined effects[11–16]. In most of these reported studies, either the addi-tives were unidentified or interest was focused more on thethickness distribution or throwing power than deposit mor-phology.Two additives commonly used during copper electrode-position are chloride and thiourea (TU). The presence of small concentrations of chloride as the sole additive in 0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.electacta.2004.08.045  1850  N. Tantavichet, M.D. Pritzker / Electrochimica Acta 50 (2005) 1849–1861 sulphate-plating baths has been reported to improve cop-per deposit properties by some researchers [17,18], but yield poorer coatings by others [19,20]. Chloride has also been included in sulphate baths for copper deposition along withother additives [11]. Thiourea is commonly used as a bright- eningagentduringcopperdeposition[21–29].Itfunctionsby interactingstronglywiththeelectrodesurfaceandalteringthemode of deposition to produce coatings that are microscopi-cally smooth and so appear bright. There is strong evidencethatthioureaformsabondonthecoppersurfaceviaitsunsat-uratedsulfuratom,therebyinhibitingthesurfacediffusionof the copper adatoms and promoting a smaller-grained struc-ture[26,30–34].Insituatomicforcemicroscopystudieshaverevealed that the early stage of electrocrystallization is dom-inated by the formation of flat copper islands that grow andeventually merge [35,36]. The edges of these islands appearto act as nuclei for further growth.The combined effect of thiourea and chloride as additiveshas also been studied. Chloride has been shown to enhancethe adsorption and levelling effect of thiourea during cop-per deposition [37,38]. Investigation by in situ surface en-hanced Raman spectroscopy has revealed that the adsorptionof thiourea onto a copper electrode from a sulphate solution(containingnoCu 2+ )increasesinthepresenceofCl − [30,32].In this study, we investigate the effects of plating modeand the additives chloride and thiourea on the morphologyof copper electrodeposits. The use of PC and pulse reverse(PR) plating over a wide range of frequencies from 50 to50,000Hz is compared to that of DC plating. Deposit qualityischaracterizedbyscanningelectronmicroscopy(SEM),op-tical microscopy, interference microscopy and scatterometryand related to cyclic voltammograms for Cu 2+ reduction andelectrode potentials monitored during plating. 2. Experimental The electrodeposition experiments were conducted usingarotatingdiscelectrodeimmersedinacylindricalelectrolyticcellcontaininga50cm 3 solution.Theworkingelectrodewasa 0.635cm diameter (0.317cm 2 area) copper disc polishedwith SiC-type abrasive paper (600-grade) and with 0.3 and0.05  maluminapowdertoamirrorfinishusingamotorizedpolisher (Buehler Metaserv Motopol 8). It was then mountedto the end of a teflon shaft of a rotating disc assembly (PineInstruments). The counter electrode was a 3.8cm-diametercopper disc placed at the bottom of the cell located about2cm from the working electrode.Pulse plating was generally carried out using a two-electrode system with the working and counter electrodesconnectedtoamodelPARAM4pulse-platingrectifier(LWDScientific). Whenever the working electrode potential wasmonitored, a standard three-electrode system was used andthe electrode response was monitored on a digital oscil-loscope (Agilent 54624A). DC plating and voltammetryexperiments were carried out using an Autolab PGSTAT 10potentiostat(EcoChemie)andaconventionalthree-electrodesystem. A mercury/mercurous sulphate electrode (MSE, Ra-diometer Analytical) was used as the reference electrode,although the electrode potentials reported herein were con-verted to the SHE scale.An acidic sulphate-plating bath consisting of 0.1MCuSO 4  and1MH 2 SO 4  wasusedforallexperimentsandpre-paredfromdoublydistilledwater.Tostudytheeffectsofaddi-tives on copper deposition, various amounts of hydrochloricacid(AldrichChemical)orthiourea(AldrichChemical)wereused. Stock solutions of thiourea were prepared freshly be-fore the plating experiments. The presence of these additivesintheconcentrationrangesusedinthisworkhadonlyanegli-gible effect on the open circuit potential (i.e., 0.275 ± 0.01VSHE).All plating experiments were conducted for 12min at anaverage current density of 4Adm − 2 and rotational speed of 500rpm. At this current density and plating time, coatingswith an average thickness of 10.6  m were produced. Thechange in electrode mass over the course of each plating ex-periment was measured. In all cases, this value correspondedto that expected if Cu 2+ reduction to metallic copper was theonly electrode reaction.Scanning electron microscopy (LEO fuel-emission 1530scanning electron microscope), scatterometry (SMS   ScanSystem, Schmitt Industries Inc.), optical microscopy and in-terference microscopy (Veeco, Wyko NT3300) were usedto characterize deposit morphology. Scatterometry (incidentvisible light with a 1300nm wavelength and 25 ◦ incidentangle) allowed quantitative measurement of surface bright-ness and RMS microroughness. Polished uncoated copperdiscswitha ∼ 93%specularreflectanceand100–200 ˚ARMSroughness were used as a standard to assess the quality of thecopper electrodeposits. 3. Results and discussion 3.1. Polarization curves Before discussing the effect of additives on depositmorphology, it will be useful to present their effect on theelectroderesponseduringCu 2+ reduction.Fig.1showspolar-ization curves for copper deposition obtained in the absenceof additives and in the presence of 274  M HCl (10ppm),20  M TU and 20  M TU+274  M HCl. In the presenceof HCl alone, the electrode potential is depolarized towardmore positive potentials, similar to that reported by others[11,20,34,39]. This enhancement in the reaction rate hasbeen attributed to the formation of a bridge between Cu 2+ and Cl − at the electrode surface with a shorter spacing thanthat of Cu 2+ H 2 O metal bridges in chloride-free systems[39,40].On the other hand, the voltammograms obtained in thesolutions containing thiourea alone are polarized towardmore negative potential relative to that of an additive-free   N. Tantavichet, M.D. Pritzker / Electrochimica Acta 50 (2005) 1849–1861  1851Fig. 1. Voltammograms for Cu 2+ reduction in: (a) 274  M HCl, (b) 20  MTU and (c) 20  M TU+274  M HCl solutions at 500rpm and 10mVs − 1 scan rate. Voltammogram for Cu 2+ reduction in solution containing no ad-ditive is also included for comparison. solution, even at thiourea concentrations as low as 20  M.The voltammogram in Fig. 1 shows almost complete sup-pression of current until the electrode decreases below about − 0.05V SHE. A similar effect has been reported by Farndonet al. [25] and Muresan et al. [29] although for different elec- trolyte compositions. This suppression is presumably due tothe complete coverage by a film that inhibits Cu 2+ reduction.Whenthepotentialbecomesmorenegativethan − 0.05V,thefilm may be reduced to form copper or break down allowingthe reduction of Cu 2+ from the solution to occur. Once cur-rent begins to flow, it rises very sharply to about 3.5Adm − 2 whereupon it begins to increase more slowly. The limitingcurrent plateau is much less well defined than in the case of an additive-free solution, as observed previously [25].When the electrolyte contains both thiourea and Cl − ,Cu 2+ reduction is suppressed, as in the presence of thioureaalone. However, some difference in the polarization curvesis observed. Once Cu 2+ reduction begins in the presenceof the two additives, the current density rises rapidly allthe way to the limiting current density. A well-defined lim-iting current density plateau is restored. A second differ-ence arises in the appearance of an additional peak cen-tered at  ∼ 0.2V. Although not shown here, the height of this peak increases if the amount of Cl − added to the elec-trolyte is increased. The process associated with this peak may involve the formation of a thiourea–Cu(I)–chloridefilm.ItisworthnotingfromFig.1thatthelimitingcurrentden- sityremainsessentiallythesameregardlessoftheelectrolytecomposition. This indicates that Cu 2+ is likely the predomi-nant form of soluble copper involved in copper deposition inall cases. The concentration of any soluble copper–thioureacomplexthatwouldformislimitedbytheamountofthioureain solution and so the limiting current density based on thisform of soluble copper would be far less than that observedin Fig. 1. 3.2. DC plating TheSEMimagesofthedepositsobtainedbyDCplatingintheabsenceandpresenceofadditivesareshowninFig.2.The% reflectance and microroughness of each deposit obtainedbyscatterometryareindicatedontheimages.Thedepositob-tained by DC plating in the absence of additives appears dull(38%reflectance)withasalmon-redcoloursurface(Fig.2a).In the presence of 274  M HCl alone, the resulting deposit Fig. 2. SEM images of deposits produced by DC plating at 4Adm − 2 in 0.1M CuSO 4 –1M H 2 SO 4  solutions containing (a) no additive, (b) 274  M HCl, (c)20  M TU and (d) 20  M TU+274  M HCl ( × 5000 magnification).  1852  N. Tantavichet, M.D. Pritzker / Electrochimica Acta 50 (2005) 1849–1861 has even poorer quality, as shown in Fig. 2b. It is very dulland non-metallic ( ∼ 2–3% reflectance), also with a strongsalmon-red colour and microroughness exceeding 2283 ˚A. Asimilar effect of chloride on deposit roughness has also beenobserved previously via AFM examination [20].As expected, the addition of 20  M TU to the 0.1MCuSO 4 –1M H 2 SO 4  solution causes the polarization of theelectrode potential during plating at a current density of 4Adm − 2 to increase from  − 0.063V SHE to  ∼ − 0.185V.The SEM image of the deposit obtained under these condi-tions is shown in Fig. 2c. The role of thiourea as a brightener is clearly evident in that a compact and reflective coating(83% reflectance) is produced. The coating appearance issimilar to what is expected based on the observations andmechanism proposed by Schmidt et al. [36]. The improve-ment in surface reflectance is consistent with their findingthat thiourea tends to promote two-dimensional growth of flat copper islands during electrocrystallization. The smallergrain size is consistent with previous findings that nucleationduring electrodeposition changes from instantaneous to pro-gressive in the presence of thiourea, with the formation of numerous new nucleation sites [35,41]. However, it shouldbe noted that excessive amounts of thiourea (500  M andabove) are detrimental to deposit morphology by producingswollen, flaky and peeling deposits.Althoughthedepositisrelativelysmoothandbrightwhenthe TU level is 20  M, it exhibits a relatively high micror-oughness (>2283 ˚A) and is not as reflective as the polisheduncoated copper substrate due to the appearance of a lightfilm. This light film likely arises due to the formation of mi-croscopic nodules (>1  m diameter, as estimated from SEMand interference microscopy) over the surface (Fig. 2c), al- though the deposit elsewhere is smooth and fined-grained.This combination of surface features leads to a bright surfacewitharelativelyhighmicroroughness.Noduleformationdur-ing copper deposition in the presence of thiourea has beenreported previously [27,36,42]. Schmidt et al. [36] proposed that once the flat copper islands that tend to make the depositvery reflective begin to merge, three-dimensional growth oc-curs where these islands intersect. These intersection spotsreceivehighercurrentthanotherareasandultimatelydevelopinto microscopic roughness. Although nodules appear in theSEM images, the deposits are still very smooth macroscopi-cally.Someofthepreviouslyreportedobservationsoftheforma-tion of flat copper islands and nodules and the occurrence of electrocrystallization via progressive nucleation were madeduring studies that focused only on the initial stages of de-position on substrates (i.e., glassy carbon and Au) differentfromthatusedinthecurrentstudy.Similarityofthemorphol-ogy observed in Fig. 2c to that of the earlier studies indicates that these phenomena continue to occur at least up to coat-ing thicknesses of 10  m. Furthermore, the similarities alsosuggest that these phenomena do not depend critically on thenature of the substrate, but more so on interactions betweenthiourea and copper adatoms.To determine if a combination of the two additives couldhave a synergetic effect on copper deposition, the use of 0.1M CuSO 4 –1M H 2 SO 4  solutions containing 20  M TUand 274  M HCl was studied. The SEM image of the de-posit obtained by DC plating (Fig. 2d) shows significant im- provementindepositmicrostructureoverthatproducedinthepresenceof20  MTUalone(Fig.2c)orHClalone(Fig.2b). Nodulesarenolongerformedandafine-andsmooth-grainedstructure over the nanometer to micron range is produced.However, the surface becomes so rough on a macroscopicscale, that the % reflectance and RMS roughness could notbe accurately measured. No distinct pattern to the roughnessisevidentatthisHClconcentration.Roughnon-uniformcop-perdepositshavealsobeenreportedduringDCplatinginsul-phate solutions containing chloride and gelatin or bindarine[17]. 3.3. Pulsed current (PC) plating To compare the effectiveness of PC plating to that of DCplating, a series of experiments were conducted to determinethe effect of pulse frequency and duty cycle on coating mor-phology. In order to make a fair comparison, these experi-ments were carried out at a time-averaged current density of 4Adm − 2 insolutionswiththesamecompositionsasconsid-eredintheprevioussection.TheSEMimagesoftheresultingdeposits are shown in Fig. 3.In the absence of additives, PC plating leads to a morecompact and smaller-grained structure than does the DCmode, yielding a brighter, smoother and more metallic sur-face (Fig. 3a). These trends agree with the known effects of  pulse plating to enhance metal nucleation [43] and to reduce mass transport limitations [1,8,43,44]. Another contributing factor in improving morphology is the likely effect of plat-ing mode on the deposition mechanism. Electrocrystalliza-tion during DC plating of copper in the absence of additivestends to occur by an instantaneous nucleation mechanism[35,36,41]. With the step change in electrode potential in thenegative direction at the beginning of each pulse cycle, PCplatinglikelyshiftsthemechanismtowardthatofprogressivenucleation where nuclei form continually throughout depo-sition leading to a finer and smaller-grained structure.Two other noteworthy trends in Fig. 3a are the effectsof pulse frequency and duty cycle during PC plating. For agiven duty cycle, % reflectance and deposit microsmooth-ness improve as the pulse frequency is increased from 50 to500Hz, but then begin to deteriorate at higher frequencies.This trend likely reflects the competing effects of pulse dura-tion and amplitude. With an increase in frequency from 50 to500Hz,thelargernumberofpulsesoutweighsthesimultane-ous diminution of their amplitudes and produces a smaller-grained and brighter surface. This idea is supported by theresults in Fig. 4a and b that show the measured electrode po- tentialsduringtheon-timearesimilarat50and500Hz.How-ever, as the frequency is increased above 500Hz, the smalleramplitudeoftheelectrodepotentialpulseduetodoublelayer

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