10334 DOI: 10.1021/la100390x Langmuir 2010, 26(12), 10334–10340Published on Web 05/12/2010

pubs.acs.org/Langmuir

© 2010 American Chemical Society

Kinetics of Electroless Deposition: The Copper-Dimethylamine

Borane System

Daniela Plana, Andrew I. Campbell, Samson N. Patole, Galyna Shul, and Robert A. W. Dryfe*

School of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom

Received January 27, 2010. Revised Manuscript Received April 30, 2010

A kinetic study of the electroless deposition of copper on gold, using dimethylamine borane (DMAB) as a reducingagent, has been carried out. The copper deposition rate in the electroless bath was determined to be 50 nm min-1,through electrochemical stripping of the copper deposits as well as from direct measurements of the film thickness usingatomic force microscopy (AFM). Comparison with a galvanic cell setup, where the two half-reactions were physicallyseparated, yielded a lower deposition rate of 30 nmmin-1. An important kinetic effect of the surface on the oxidation ofthe reducing agent, and thus on the overall process, was therefore revealed. The efficiency of the process was measuredover time, revealing the contribution of side reactions in the cathodic half-cell, particularly during the initial stages of theelectroless process.

1. Introduction

Electroless deposition is used in a wide range of industrialapplications, ranging from the preparation of electronic circuitryand thermal or electrical conductors to metallurgy and corrosionprevention; the process has also found simple decorative uses.1-4

Electroless deposition of copper, in particular, has found applica-tions in microelectronics and as a metallic starting point forsubsequent metallization by other techniques due to the ease ofcopper deposition on prepatterned circuits, plastics and othernonconducting surfaces, and semiconductors.5-7 It has also beenconsidered as a replacement for aluminum in ultralarge scaleintegration (ULSI) techniques due to its lower resistivity andhigher resistance to stress-induced voiding and electromigration.8,9

Despite the widespread use of electroless deposition, and theapparent simplicity of the process, in which a metallic ion isreduced to its zerovalent state by the presence of a suitablereducing agent and a catalyzing surface, the reaction mechanismsare still not well understood.6 Electroless deposition occurs via acomplex, multistep redox mechanism, involving the diffusion ofthe metal complex and the reducing agent to the catalytic surfaceand their subsequent reaction on it; each of these steps could limitthe reaction.9,10 The evolution of hydrogen, which frequentlyaccompaniesmany electroless processes, further complicates kine-tic and mechanistic studies by producing microconvection which

influences the overall rate of the electroless copper depositionprocess.11 The instability and complexity of typical depositionbaths have made fundamental mechanistic and kinetic studiesdifficult, producing conflicting results.10,12,13

An intrinsic difficulty encountered when investigating the elec-troless deposition process arises from the spontaneous nature ofthe process and the fact that no net current is produced. A numberof alternative techniques have been employed to measure orestimate deposition rates for the specific case of copper electrolessdeposition. Traditionally gravimetric techniques have been used todetermine the weight change during deposition; this is frequentlydone ex-situ so in many cases a limited number of time points areevaluated or average weight gain is determined at a single timepoint.2,7,14-16 In-situ weight measurements can be made via thequartz-crystal microbalance.9,10,17-19 Surface techniques, such asthe use of roughness step testers20 or profilometers,8,21 have beenemployed on a number of occasions tomeasure the thickness of thecopper deposits. Where time-dependent weight gain has beenperformed, constant growth rates have been observed, althoughan initial induction period may be required.

The rate of electroless copper deposition is extremely sensitiveto the bath composition and operating conditions, with thelimiting step for deposition, using formaldehyde as the reducingagent, changing from anodic to cathodic depending on thereagent concentrations.15 A detailed in-situ microbalance studydemonstrated that the deposition process was first order in the

*Corresponding author: e-mail robert.dryfe@manchester.ac.uk; Fax þ44161 275-4734.(1) Balci, S.; Bittner, A.M.; Hahn,K.; Scheu, C.; Knez,M.;Kadri, A.;Wege, C.;

Jeske, H.; Kern, K. Electrochim. Acta 2006, 51, 6251.(2) Jagannathan, R.; Krishnan, M. IBM J. Res. Dev. 1993, 37, 117.(3) Sverdlov, Y.; Bogush, V.; Einati, H.; Shacham-Diamand, Y. J. Electrochem.

Soc. 2005, 152, C631.(4) Lelental, M. J. Catal. 1974, 32, 429.(5) Henry, J. R. Met. Finish. 2002, 100, 409.(6) O’Sullivan, E. J. Fundamental and Practical Aspects of the Electroless

Deposition Reaction. In Advances in Electrochemical Science and Engineering;Alkire, R. C., Kolb, D. M., Eds.; Wiley-VCH: New York, 2002; p 225.(7) Vaskelis, A.; Jaciauskiene, J.; Stalnioniene, I.; Norkus, E. J. Electroanal.

Chem. 2007, 600, 6.(8) Patterson, J. C.; O’Reilly, M.; Crean, G. M.; Barrett, J. Microelectron. Eng.

1997, 33, 65.(9) Dubin, V.M.; Shacham-Diamand, Y.; Zhao, B.; Vasudev, P. K.; Ting, C. H.

J. Electrochem. Soc. 1997, 144, 898.(10) Schumacher, R.; Pesek, J. J.; Melroy, O. R. J. Phys. Chem. 1985, 89, 4338.(11) Donahue, F. M. J. Electrochem. Soc. 1980, 127, 51.

(12) Homma, T.; Tamaki, A.; Nakai, H.; Osaka, T. J. Electroanal. Chem. 2003,559, 131.

(13) Djokic, S. S. Electroless Deposition of Metals and Alloys. In ModernAspects of Electrochemistry; Conway, B. E., White, R. E., Eds.; Kluwer Academic:Dordrecht, 2002; Vol. 35.

(14) Jiang, H. Y.; Liu, Z. J.; Wang, X. W.; Wang, Z. L. Trans. Inst. Met. Finish.2007, 85, 103.

(15) Mishra, K. G.; Paramguru, R. K. J. Electrochem. Soc. 1996, 143, 510.(16) Paunovic, M.; Vitkavage, D. J. Electrochem. Soc. 1979, 126, 2282.(17) Wiese, H.; Weil, K. G. Ber. Bunsen-Ges. Phys. Chem. Chem. Phys. 1987,

91, 619.(18) Hendricks, T. R.; Lee, I. Thin Solid Films 2006, 515, 2347.(19) Feldman, B. J.; Melroy, O. R. J. Electrochem. Soc. 1989, 136, 640.(20) Aithal,R.K.;Yenamandra, S.;Gunasekaran,R.A.;Coane, P.;Varahramyan,

K. Mater. Chem. Phys. 2006, 98, 95.(21) Patterson, J. C.; Dheasuna, C. N.; Barrett, J.; Spalding, T. R.; O’Reilly, M.;

Jiang, X.; Crean, G. M. Appl. Surf. Sci. 1995, 91, 124.

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methylene glycolate concentration (the conjugate base of thehydrated form of formaldehyde) and zeroth order in copper: theobservation of a strong deuterium isotope effect led to theconclusion that the cleavage of the C-H bond in the adsorbedglycolate anionwas the rate-determining step.10 In contrast, earlierkinetic studies of the copper/formaldehyde system have reportedconflicting results, with a fractional order dependence on copperand a low sensitivity to the formaldehyde concentration.22,23 Oneexplanation given for the large differences quoted for reactionkinetics is the variation in reagent concentration between theinterface and bulk solution; reagent orders become more uniformif extrapolated interfacial concentrations are used for kineticstudies.10 A further issue is the possible change in limiting processwith substrate geometry: for deposition into deep or narrowfeatures (important for via filling), mass transport can becomethe limiting factor.12

One recurring question in the kinetic study of electrolessdeposition concerns the interdependence (or otherwise) of thecathodic and anodic processes. If these processes can be treated asindependent, then the mixed potential theory holds.15,24 Thistheory is applied to electrochemical systems where two redoxcouples, with different standard potentials, are in contact andreach equilibriumby reduction of themore electropositive couple,with the simultaneous oxidation of the other couple. The theoryassumes that once this condition is achieved, at a potentialbetween the equilibrium potentials of the two redox systems,the rate of both reactions is equal; hence no net current flows.25,26

Accordingly, physical separation of the cathodic and anodicprocesses (i.e., a galvanic cell, with connection via a salt bridge)would produce a deposition rate identical to that seen in theconventional electroless bath.

There are several limitations to the use of this theory, such aschanges in the rate-determining step throughout the potentialrange covered by each reaction or in the surface kinetics of one orboth reactions, due to adsorption of a bath component or(particularly in the case of an electroless process) due to changesin the structure of the substrate arising from metal deposition.Previous kinetic studies have found a specific interaction betweenthe two couples: for example, the deposition rate of copper (usingformaldehyde) in a galvanic cell was found to be lower than thatseen in the electroless bath.6,15,27,28 As the conditions needed forthe application of the mixed potential theory in the study ofelectroless deposition are not necessarilymet, for example in somecases ofNi-PandCudeposition,more complexmethods, such asrapid solution exchange and bipolar configurations, have beenapplied.17,29,30Weil et al. studied the copper/formaldehyde systemin detail: the solution exchange method (rapid removal of eitherthe copper ions or formaldehyde, while maintaining the substrateat the deposition potential) gave a significant decrease in depositionrate, suggesting that an interdependence of the half-reactionsexisted.17 SubsequentworkbyWeil used capacitancemeasurement,

surface-enhanced Raman spectroscopy, and scanning tunnelingmicroscopy to show that a microscopically rough surface wasformed on deposition, leading the authors to conclude that therough surface contained specific copper sites which acted asformaldehyde oxidation catalysts; i.e., the morphology evolvedbecause of the strong interdependence between the cathodic andanodic processes.31-33 By contrast, earlier work on copper deposi-tion with formaldehyde had assumed that the mixed potentialapproach was valid.16,26,34

Similarly for Ni-P alloy deposition via electroless methods,contradictory literature exists which assumes that the mixedpotential theory holds35 or specifically states that it is invalidfor this system.29

This contribution focuses on the mechanism of electrolesscopper deposition: the most widely used reducing agent forcopper is formaldehyde; however, environmental and toxicityconcerns have more recently led to the adoption of alternativereducing agents such asDMAB.2,6,12,36 The aimof this article is toinvestigate the rate of copper deposition using DMAB in theelectroless bath and in the corresponding galvanic cell and toassess the validity of the mixed potential theory in this case. Thecopper deposition rate in both cases was determined by a simplemethod, where the film thickness was determined from theoxidative stripping of the copper; the validity of this approachwas confirmed by comparison with film thickness measurementsusing AFM.

2. Experimental Section

All deposition and stripping experiments were performed inaqueous solution, using water (of 18 MΩ cm resistivity) preparedusing a reverse osmosis unit (Millipore, Watford, UK) coupled toan Elga “Purelab Ultra” purification system (Veolia Water sys-tems, Marlow, UK). Analytical grade copper(II) sulfate, 1,5,8,12-tetraazadodecane, potassium hydroxide, potassium chloride, andagar were obtained from Sigma-Aldrich, DMAB and triethano-lamine were obtained from Alfa-Aesar, and sulfuric acid (95þ%)was purchased from Fisher.

The deposition substrates were gold disks (2 mm diameter)obtained from IJ Cambria Scientific (Burry Port, UK). Beforeeach deposition, the disks were mechanically polished usingdiamond suspensions (1 and 1/4 μm grade, supplied by KemetInternational, Maidstone, UK) and electrochemically cleaned in0.1 M sulfuric acid by repeated cyclic voltammetry in a potentialrange between 0.00 and 1.70 V vs Ag/AgCl.

Two typical electroless baths were used; in these cases all thereactants are present in one solution and the baths differ only inthe concentration of the reducing agent. These are denoted Bath 1and Bath 2 (see Schemes 1 and 2, respectively), where DMAB isthe reducing agent, triethanolamine is added to control the pH,and 1,5,8,12-tetraazadodecane is employed as a complexing agentfor the copper(II) ions to prevent the spontaneous formation ofcopper hydroxide.2,37 The pH was adjusted to 11.6 using potas-sium hydroxide and sulfuric acid as needed, while the tempera-ture of deposition was kept constant at 55.0 ( 0.5 �C, using athermostatic water bath. For baths under hydrodynamic control,an EG&GPARmodel 616 rotator was used to rotate the Au diskat defined frequencies (Oak Ridge, TN). All measurements in the

(22) Molennar, A.; Holdrinet,M. F. E.; van Beek, L. K.H.Plating 1974, 61, 238.(23) Dumesic, J.; Koutsky, J. A.; Chapman, T. W. J. Electrochem. Soc. 1974,

121, 1405.(24) Izumi, O.; Osamu, W.; Shiro, H. J. Electrochem. Soc. 1985, 132, 2323.(25) Wagner, C.; Traud, W. Z. Elektrochem. 1938, 44, 391.(26) Bindra, P.; White, J. R. Fundamental Aspects of Electroless Copper

Plating. In Electroless Plating - Fundamentals and Applications; Mallory, G. O.,Hajdu, J. B., Eds.; William Andrew Publishing: Norwich, NY, 1990; p 289.(27) Spiro, M. A Critique of the Additivity Principle for Mixed Couples. In

Modern Aspects of Electrochemistry; Bockris, J. O. M., Conway, B. E., White, R. E.,Eds.; Plenum Publishers: New York, 2001; Vol. 34, p 1.(28) Mital, C. K.; Shrivastava, P. B.; Dhaneshwar, R. G. Met. Finish. 1987,

85, 87.(29) Abrantes, L. M.; Correia, J. P. J. Electrochem. Soc. 1994, 141, 2356.(30) Plana, D.; Shul, G.; Stephenson, M. J.; Dryfe, R. A. W. Electrochem.

Commun. 2009, 11, 61.

(31) Bittner, A.; Wanner, M.; Weil, K. G. Ber. Bunsen-Ges. Phys. Chem. Chem.Phys. 1992, 96, 647.

(32) Wanner, M.; Wiese, H.; Weil, K. G. Ber. Bunsen-Ges. Phys. Chem. Chem.Phys. 1988, 92, 736.

(33) Weber, C. J.; Pickering, H. W.; Weil, K. G. J. Electrochem. Soc. 1997, 144,2364.

(34) Elraghy, S. M.; Abosalama, A. A. J. Electrochem. Soc. 1979, 126, 171.(35) Kim, Y. S.; Sohn, H. J. J. Electrochem. Soc. 1996, 143, 505.(36) Sargent, A.; Sadik, O. A.; Luis, J. M. J. Electrochem. Soc. 2001, 148, C257.(37) Masahiro, S.; Hideyuki, T.; Makoto, S.; Masayuki, K. Electroless gold

plating solution. European Patent Office, 1987.

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electroless bath were performed six times, and the average valuesare presented here. Note that in many applications of electrolessdeposition baths contain other components (levelers, brighteners,etc.), which are typically surface-active organic molecules addedto alter the morphology of the final deposit. Such additives weredeliberately excluded from the baths studied here to simplifykinetic analysis.

In order to study the interdependence of the two half-reactionswhich constitute the electroless deposition process, a galvanic cellcorresponding to Bath 1 was used. The galvanic cell consisted oftwo glass cells connected by a salt bridge (glass U-tube filled withagar gel containing 1.0 M KCl), as shown in Figure 1. The goldelectrodes were connected externally through a switch. To followthe course of the reaction, a National Instruments (Austin, TX)PCI-4065 digital multimeter card was used; a custom-writtenLabView program allowed the open circuit potential (OCP)before deposition to be recorded, alongwith the current transientsproduced during deposition. At its highest accuracy setting of22 bits, the PCI-4065 card can probe the reaction at a maximumrate of 10 Hz, with an accuracy of (0.15 μA and (26 μV whenmeasuring dc current andvoltage, respectively. Experiments usingthis setup were performed at least three times, and average valueswere taken.

Two different chemical compositions were used in the galvanicsetup described above and are denoted as Cell 1 and Cell 2 (see

Schemes 3 and 4, respectively). Temperature and pH were con-trolled as stated for the electroless baths.

Given that the composition of the active surface changes duringthe electroless process, a copper electrode was introduced to theDMAB-containing half-cell to study the effect of the coppersurface on the deposition rate. The Cu electrode was preparedby prior electroless deposition on gold, using Bath 1 for 10 min.The corresponding galvanic cell is denotedasCell 3 (see Scheme5).

Note that reference electrodes were not present during thedeposition experiments (in either the galvanic cell or electrolessbath configurations). Possible chloride contamination of thedeposit from the salt bridge was minimized by the distancebetween the bridge and the deposition substrate (2-3 cm): neitherelemental analysis nor X-ray diffraction (see below) suggestedCuCl formation, nor was any precipitate observed in the deposi-tion solution.

Stripping measurements were performed using anAutolab 100potentiostat (Eco-chemie, Utrecht, TheNetherlands) with anAg/AgClsat reference electrode made in-house and a platinum gauzeas the counter electrode. The copper-covered electrodes wereimmersed in 0.1 M H2SO4, and a potential of 0.45 V vs Ag/AgClwas applied for 180 s (60 s was sufficient for samples where thedeposition timewas less than 1min) to remove all the copper fromthe surface.38 Representative chronoamperometric responses,obtained during anodic stripping of the copper deposits, can beseen in Figure 2. The charge transferred was used to calculate thenumber of moles of copper deposited, using the Faraday equa-tion, assuming that a two-electron oxidation occurs. This impli-citly assumes that ambient oxidation of the copper deposit isminimal: the validity of this assumption is discussed below. Thethickness of the deposits was obtained assuming homogeneousdeposits with the properties of metallic copper.39

All AFM measurements were performed in contact mode, inair, with a Quesant 250 AFM supplied by Windsor ScientificLimited (Slough, UK). The samples for AFM were prepared on1 cm2 gold foils, with thicknesses of either 0.25 or 0.50 mm(Goodfellow, Huntingdon, UK), which were cleaned by submer-sion in dilute nitric acid andmechanical polishing as stated for thegold disks. For copper film thickness measurements, the copperwas electrolessly deposited, as described above, on selected partsof the gold substrates. Nail varnish was applied on half of eachgold foil to achieve partial copper deposition: the viscosity of thevarnish produces a well-defined edge (to within a micrometer) ondrying. The varnishwas then removedwith acetone prior toAFM

Scheme 1. Bath 1: Typical Electroless Bath Used

Scheme 2. Bath 2: Electroless Bath with Lowered DMAB

Concentration (with Respect to Bath 1)

Figure 1. Schematic of the galvanic cell.

Scheme 3. Cell 1: Galvanic Cell with Similar Chemical Composition

to Bath 1

Scheme 4. Cell 2: Galvanic Cell with Lower Concentration of Copper

Ions (with Respect to Cell 1)

Scheme 5. Cell 3: Galvanic Cell with Similar Chemical Composition

to Bath 1

Figure 2. Anodic stripping chronoamperomograms for depositsmade from immersion in Bath 1 for 2, 5, and 10 min (dashed,dotted, and solid lines, respectively).

(38) Herranen, M.; Carlsson, J. O. Corros. Sci. 2001, 43, 365.(39) CRC Handbook of Chemistry and Physics, 76th ed.; Lide, D. R., Ed.; CRC

Press, Inc.: Boca Raton, FL, 1995.

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measurement to reveal the edge at the copper/gold interface (seeFigure 3). Film thicknesses weremeasured across the gold/copperinterface on at least five points of each sample, and an averagevalue is given. A three-point tilt removing filter was used in allcases.An example of anAFM image, obtained for a 2min depositperformed as described, is shown in Figure 3.

AFM has been used to study the morphology of electrolessdeposits on a number of previous occasions.8,20,40 In this study,the roughness values were obtained from the root-mean-squarevalues of surface height measurements, using the image analysissoftware supplied with the instrument. The roughness was mea-sured over an area of 3 μm� 3 μm, and all themeasurements weretaken from the central areas of the image to avoid edge effects.The rms roughness values reported are averages of at least sixmeasurements on different areas across the surface.X-ray diffrac-tion (XRD) was also used to provide structural information onthe deposit: an X-pert (Philips) diffractometer was used for thispurpose. Samples for diffraction were prepared on the same goldfilms as used forAFMmeasurements. The elemental compositionof the deposits was determined by inductively coupled plasmaoptical emission spectroscopy (Fisons Horizon).

3. Results and Discussion

Electroless copper deposition was carried out, for varyingamounts of time, using Bath 1 in order to determine the depositionrate. Film thickness determinationwas performed through anodicstripping and AFM measurements as described in the Experi-mental Section; the results are presented in Figure 4. Using linearfits as shown in the figure, a steady deposition rate of 52 ( 3 nmmin-1 was obtained from stripping measurements and a value of49( 7 nmmin-1 was found fromAFM, for a range of depositiontimes from 1 min to half an hour. The agreement between the twomeasurementmethods indicated that electrochemical stripping is asuitably simple, yet effective, technique to evaluate the kinetics ofelectroless deposition; it was consequently used throughout therest of this work.

The good agreement between the two methods, particularly atlong times, implies that the fraction occupied by void space(which, if present, would increase the microscopic thicknessmeasurement) in the copper deposit is quite low. The level ofagreement also implies that the contribution due to ambientoxidation of the copper surface (which, if present, would decreasethe effective thickness determined electrochemically) is low. XRD

analysis of the deposit only revealed diffraction peaks attributableto metallic copper and the gold substrate (data not shown). Thecopper is deposited under strongly reducing conditions; i.e., thepresence of the DMAB should minimize oxide formation duringthe deposition process. On removal from the deposition system,either to the ambient or to the acid solution for strippingmeasurement, oxidation of the copper surface may occur. Ana-lysis of the elemental composition of the deposits indicated thatthey contained >97.5 wt % Cu and 0.2% C. The remainingsample weight is therefore attributed to oxygen, which corres-ponds to an oxygen content by atom of 9%; this value is expectedto be considerably higher than that of the films followingexposure to the ambient, as the elemental analysis was performedon pulverized electroless copper deposits, with surface to volumeratios a great deal higher than the as-deposited films. No traces ofboron were found, in agreement with earlier reports on theelectroless deposition of copper using boron-based reducingagents.6 Literature reports state that ambient oxidation of copperis restricted to a thin (<5nm) layer, following 48 h of atmosphericexposure.41 Similarly, copper surfaces are reported to be stable inacidic solutions in the absence of an oxidative potential.42

The steady deposition rate displayed in Figure 4 indicates thatthe reaction is surface limited rather than under mass transportcontrol of one of the reagents. The deposition reaction must havean autocatalytic function; i.e., the Cu deposit must (like Au)41

function as a DMAB oxidation catalyst. When the reaction wasperformed with the concentration of all the reactants in Bath 1doubled, but maintaining identical plating conditions (pH, tem-perature, and deposition times), the rate of reaction doubledrelative to that shown in Figure 4, indicating a clear influence ofsolution concentration on the kinetics of the system. This effectcould also reflect surface control over the deposition kinetics viaan adsorptive process, which would be sensitive to the concentra-tion of the adsorbate. Surface adsorption control of electrolessdeposition processes has been suggested in a recent study whichuses DMAB as a reducing agent.42

The steady deposition rate could also be partly due to localconvection created by the formation of gas bubbles during the

Figure 3. AFMimage of a copper deposit (left) on a gold substrate(right), using Bath 1 for 2min. Plan view (upper) and cross-sectionview (lower) are shown.

Figure 4. Thickness of the copper deposits as a function of deposi-tion time in Bath 1, measured using electrochemical strippingmethods (circle) andAFMmeasurements (triangle). Least-squareslines of best fit of each data set are shown.

(40) Sverdlov, Y.; Shacham-Diamand, Y. Microelectron. Eng. 2003, 70, 512.

(41) Finkelstein, D. A.; Da Mota, N.; Cohen, J. L.; Abruna, H. D. J. Phys.Chem. C 2009, 113, 19700.

(42) Shacham-Diamand, Y.; Sverdlov, Y.; Bogush, V.; Ofek-Almog, R. J. SolidState Electrochem. 2007, 11, 929.

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deposition process and their evolution from the surface. To studythe effect ofmass transport, and particularly convection, a platingsurface was employed in the rotating disk configuration. Theresulting deposit thickness, as a function of rotation frequency foran immersion time of 5 min, can be seen in Figure 5. The graphsuggests that the increase in mass transport due to rotation of theplating surface has little or no influence on the deposition kinetics.This again indicates that the limiting step occurs at the surface.

The electroless bath (defined as Bath 1) has a large stoichio-metric excess of the reducing agent in comparison to the coppercomplex (∼6-fold), as the DMAB concentration is double that ofthe copper, and each molecule of DMAB donates six electrons,i.e., 3 times the number needed to reduceCu(II).43,44 If the processwere simply dependent on the mass transport of the two species,the stoichiometry would lead to Cu ions being the limitingreactant, consequently halving the DMAB concentration shouldhave little effect on the overall rate. Doing the latter, however,lowered the rate of deposition slightly (by ∼20%), as can be seenin Figure 6, where Bath 1 and Bath 2 are compared. The mass

transport of DMAB does not appear to be the main contributorto the system’s kinetics (as proven by the rotating disk experi-ments): the data suggest that DMAB oxidation is limited by itssurface reaction (adsorption) or that there is an overall mixedtransport and surface control on the deposition kinetics.

Deposition at shorter times was less reproducible than that atlonger times: an induction period of ca. 30 s is apparent in Figure7, which shows the number ofmoles of copper (nCu) deposited in atime frame from 10 s to 2 min. The data in Figure 7 are notexpressed as thickness because a homogeneous coverage cannotbe assumed. There is an obvious break in the graph between 25and 30 s, after which the rate tends to that observed in Figure 4.This indicates that an initially slow and random nucleation ofcopper takes place on the gold surface,while the steadydepositionrate is obtained once the main process is the autocatalytic growthof copper on copper.

The galvanic cell described in Figure 1 was used to probe theinterdependence of the two half-reactions (the oxidation ofDMAB and the reduction of copper), as well as the effect eachprocess has on the overall kinetics. Figure 8 presents a comparisonbetween the deposition rates obtained in the electroless bath andthe galvanic cell via the resultant copper deposit thickness. In thelatter, a steady rate of 31 ( 2 nm min-1 was observed over thetime frame studied; i.e. deposition in the electroless bath is

Figure 5. Deposit thicknesses obtained in the electroless bath as afunction of rotation speed, for a deposition time of 5 min. Thedotted lines represent the spread of the deposits obtained understatic conditions, after 5 min.

Figure 6. Thickness of the copper deposits as a function of deposi-tion time in Bath 1 (circles) and Bath 2 (inverted triangles).

Figure 7. Moles of copper deposited in Bath 1 over short deposi-tion times.

Figure 8. Thickness of the copper deposits as a function of time,comparing Bath 1 (circle) and Cell 1 (triangle).

(43) Burke, L. D.; Lee, B. H. J. Appl. Electrochem. 1992, 22, 48.(44) Nagle, L. C.; Rohan, J. F. Electrochem. Solid-State Lett. 2005, 8, C77.

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approximately 1.7 times faster than in the configuration withphysically separate half-cells.

Because of the reaction stoichiometry discussed above, itwouldbe reasonable to assume that Cu(II) is the limiting reactant, thushaving an important effect on the kinetics of the system. To probethis, the concentration of the copper salt and the complexingagent were halved in the copper-containing half-cell, while keep-ing the DMAB half-cell unchanged (Cell 2); Figure 9 shows thethickness of the resultant deposits. Although the rate of reactiondecreases, it does so by∼35%, revealing that the rate is related tothe Cu(II) concentration by a fractional order, in common withprevious observations on electroless systems.9,26,34

Amajor differencebetween the electroless bath andgalvanic cell,beyond the physical separation of the two half-reactions in thelatter case, is that the surface on which deposition occurs in theformer case changes over time. In the conventional electroless bath,the DMAB oxidation initially takes place on the gold surface, butwithin a minute (see Figure 6) the reaction takes place on a coppersubstrate. By contrast, in the galvanic cell, this net compositionalchange only occurs in one of the half-cells. To investigate the effectof the copper substrate on the anodic reaction, a copper electrodewas used as the deposition substrate in the DMAB-containingcompartment of the galvanic cell. The resultant effect on thedeposition rate can be seen in Figure 9; in this case the rate ishigher than that previously obtained in Cell 1, indicating thatDMAB oxidation proceeds at a faster rate on a copper surface, asthe copper half-cell was unchanged. This finding is consistent withour interpretation of Figure 7 and with previous work, which hadsuggested that DMAB oxidation may be faster on Cu than onAu.45 It should be noted that previous studies of copper electrolessdeposition with formaldehyde have demonstrated that the freshcopper deposit is an efficient catalyst for formaldehydeoxidation.17

To a first approximation, the electroless bath rate seems to beregained in Cell 3; however, a closer look reveals that the initialdeposition rate is actually higher (Figure 10) in the copperelectrode case. The subsequent deposition rate drops below thatof the electroless bath, implying that, although the anodic reac-tion substrate plays amajor role on the kinetics of the process, it isnot the only mode of interaction between the two half-reactions.

As well as offering a direct means of comparison with theelectroless bath, a further advantage of the galvanic cell is that the

open-circuit potentials and the deposition currents can be mea-sured. TheOCP is simply the potential difference between the twohalf-cells and provides information about the driving force of theprocess. Table 1 shows the OCP values for the three galvanic cellsinvestigated; although the accuracy of the multimeter is high, thevalues measured varied slightly between experiments under iden-tical conditions, so average values of all tests performed undersimilar conditions (no less than 18 per cell) are presented. It can beseen that changing the concentration of the copper complex haslittle effect (compare Cells 1 and 2), while substituting a copperelectrode for the gold electrode in the DMAB-containing half-cell, increases the potential difference by ∼250 mV. This addi-tional driving force is consistent with the increased rate observed.

Examples of the current transients observed for the galvanic cellconfiguration can be seen in Figure 11, where absolute currentmagnitudes are shown. These currents are a direct measure of theelectrons producedby theDMABoxidationand transferred to thecopper-containing half-cell. The magnitude of the current in-creases when a copper electrode is used, when compared to a goldelectrode, again showing that copper is a more efficient electro-catalyst forDMABoxidation than gold.The transient produced isalso noisier, which implies gas evolution accompanies the DMABoxidation reaction. Gas evolution could be attributed to a numberof processes in the anodic half-cell: hydrogen produced by hydro-lysis of DMAB catalyzed by the copper substrate, hydrogenevolved as a side product of the DMAB oxidation, and/or theevolution of the dimethylamine from DMAB (a gas at theconditions used),39 once thenitrogen-boronbondof the borane-amine adduct is broken.4,44 If the gas is derived from a cathodicreaction, hydrogen evolution is the most likely process.

Integration of current transients such as those in Figure 11yields the overall charge transferred during deposition; the ratioof the stripping charge (Qs) to the overall deposition charge (Qod)gives a measure of the faradaic efficiency of the depositionprocess, as it is a direct measure of the charge produced in theDMAB compartment that is used to reduce copper. Figure 12

Figure 9. Thickness of the copper deposits as a function of deposi-tion time in Cell 1 (triangle), Cell 2 (open circle), and Cell 3(diamond).

Figure 10. Thickness of the copper deposit at short times inBath 1(filled circle) and in Cell 3 (open diamond).

Table 1. Open-Circuit Potentials Measured for the Three Galvanic

Cells Studied

ΔE/V

cell 1 0.69cell 2 0.71cell 3 0.95

(45) Iacovangelo, C. D. J. Electrochem. Soc. 1991, 138, 976.

10340 DOI: 10.1021/la100390x Langmuir 2010, 26(12), 10334–10340

Article Plana et al.

presents the charge ratio as a functionof time for the three galvaniccells studied. In all three cases the efficiency increases with time,almost reaching unity. This demonstrates that a side reaction (orreactions), such as hydrogen evolution as mentioned above, musttakeplace in the copper-containinghalf-cell to consumepart of thecharge produced by the anodic reaction. This “parasitic” process ismore important during the initial stages of the deposition, possiblybecause the copper reduction is less prominent at first, as the initialnucleation process is less favorable than hydrogen evolution. Itseffect is also more pronounced when the copper concentration islowered, as the amount of copper reacting decreases, so more ofthe charge goes toward these side reactions.

The morphology of the copper deposits obtained was alsocompared; Table 2 shows the results obtained through AFMmeasurements, where the roughness (on this length scale) of theunderlying gold substrate was 4 nm. The roughness increases withdeposition time, and thus with deposit thickness, in both the celland bath cases. However, the roughness of the galvanic deposit issubstantially lower than the corresponding electroless bath. Thisobservation is not simply an effect of the thinner deposits in theformer case when similar times are directly compared: a 30 mindeposit from Cell 1 is ∼3 times thicker than a 5 min deposit fromBath 1; however, the roughness of the former is less than half that

of the bath. The increase in roughness seen on physical colocationof the anodic and cathodic processes (i.e., in the bath) suggests thatDMABoxidation in the bath case is directly associatedwith copperreduction. The change in morphology observed on separation ofthe reactions implies that a coupling exists between two processes,in agreement with the kinetic data presented earlier. The mostlogical coupling is that the oxidation and reduction processes occurat the same site, with one reaction catalyzing the other.

4. Conclusions

The electroless deposition rate for the Cu(II)/DMAB systeminvestigated here was steady at 50 nm min-1, over a time framefrom 1 to 30min, with an induction time of∼30 s. The depositionrate was determined using electrochemical stripping and AFMthickness measurements; good agreement was obtained betweenthe techniques, confirming that the stripping method is a simpleyet effective technique for the study of films deposited via theelectroless method. Separating the two half-reactions, using thegalvanic setup described inFigure 1, decreased the rate by 40%, to30 nm min-1. The roughness of the deposits also decreased onphysical separationof the reagents, using the galvanic configuration.

By introducing a copper electrode to the DMAB-containinghalf-cell, the rate increased to a value close to the electroless bathdeposition rate, demonstrating that the oxidation of the reducingagent is the limiting step in the system’s kinetics, even though thereducing agent is in stoichiometric excess compared to the metalcomplex. These experiments also showed that copper is a bettercatalyst for DMAB oxidation than gold, the initial substrate. Thefaradaic efficiency of the process is above 80% after the first fewminutes, although it never reaches 100%, indicating that sidereactions occur in the cathodic half-cell, which are more promi-nent during the initial stages of the deposition process.

The galvanic cell has proved to be a simple and effective way ofprobing the electroless deposition process, providing informationabout its kinetics and efficiency, as well as the interaction betweenthe two half-reactions. This setup can also be used to probeconditions where the electroless bath would be unstable, enablingthe study of a wider range of concentrations and temperatures; thiswould promote a better understanding of the electroless depositionprocess, by allowing a more complete kinetic and mechanisticanalysis. Of wider significance, the kinetic data reported here showthat theoxidationand reductionprocesses arenotadditively coupledfor the specific case of the electroless deposition of copper byDMAB. In this case, the fact that the rate of DMAB oxidationdiffers on Cu and Au and the overall rates decrease when the twohalf-reactions are separated means that the mixed potential theorydoes not hold for this system. Ongoing studies in this laboratory areinvestigating the electrooxidationmechanismofDMAB,noting thatthere is somedebate over this process,41which is considered tobe thelimiting factor in the overall electroless deposition investigated here.

Acknowledgment. We thank EPSRC for financial support(grant ref: EP/D04717X/1).

Figure 11. Current transients for 5 min deposition in the galvanicsystemusing a gold electrode (dashed) and a copper electrode (solid)in the DMAB-containing half-cell; absolute values are shown.

Table 2. Roughness of the Copper Deposits Obtained in the

Electroless Bath and the Galvanic System

Bath 1 Cell 1

time/minroughness/nm(3 μm � 3 μm) time/min

roughness/nm(3 μm � 3 μm)

1 12 2 72 27 5 75 90 15 2630 135 30 39

Figure 12. Ratio of the stripping and overall deposition charges asa functionofdeposition time forCell 1 (triangle),Cell 2 (opencircle),and Cell 3 (filled diamond).