46 The Electrochemical Society Interface • Winter 2004

tate-of-the-art manufactur-ing of semiconductordevices involves the elec-trodeposition of copper foron-chip wiring of integrat-

ed circuits. In the Damascene process,interconnects are fabricated by first pat-terning trenches and vias in a dielectricmedium and then filling by metal elec-trodeposition over the entire wafer sur-face. This is followed by a planarizationstep that leaves an array of wires andinterlevel vias embedded in the dielec-tric matrix. The metallization process,pioneered by IBM, depends on the useof electrolyte additives that affect thelocal metal deposition rate, therebyresulting in superfilling, or bottom-upvoid-free filling of trenches and vias.1,2

An example of this remarkable deposi-tion behavior is given in Fig. 1 wheregrowth as a function of electrodeposi-tion time and feature aspect ratioreveals preferential metal deposition atthe bottom of the trenches followed bybump formation above the filledtrench.3

In the early years of copperDamascene technology an understand-ing of the superfilling process laggedbehind its implementation due to acombination of factors. The first gener-ation of electrolytes contained numer-ous components and general knowl-edge of both the chemistry and pro-cessing conditions was significantlyconstrained by proprietary concerns. Asa result, early modeling studies focusedon traditional leveling theory wherethe location-dependent growth ratederived from diffusion-limited accumu-lation and consumption of an inhibit-ing species on the metal surface.2,4,5 Inthese studies it was necessary to empir-ically modify the area-blockage levelingtheory to describe feature filling.2,4

Despite these modifications, detailedstudies of shape evolution and theobservation of bump formation abovethe filled trenches made it clear thatsuperfilling could not be rationalizedby traditional transport-limited leveling

Superfilling and the CurvatureEnhanced AcceleratorCoverage Mechanism

by T. P. Moffat, D. Wheeler, and D. Josell

S

models.6-8 At the same time, simplifica-tion of electrolyte additive packages totwo, (accelerator, suppressor) and three(accelerator, suppressor, leveler) compo-nents opened the way for detailed stud-ies of the filling process.9-11

Subsequently, a curvature enhancedaccelerator coverage (CEAC) mechanismwas shown to quantitatively describesuperconformal film growth that isresponsible for bottom-up superfilling ofsub-micrometer features in Damasceneprocessing.12-16

The CEAC Filling Mechanism

The essential idea behind the CEACmechanism is that (i) the growth veloci-ty is proportional to the local accelera-tor, or catalyst, surface coverage θcatalystand (ii) the catalyst remains segregatedat the metal/electrolyte interface duringmetal deposition. For growth on non-planar geometries this leads to enrich-ment of the catalyst on advancing con-cave surfaces and dilution on convexsections that, in combination, give riseto distinct bottom-up filling of submi-

crometer features.12,14-16 The enrich-ment and dilution processes becomeincreasingly important at smaller (opti-cal) length scales because the change incatalyst coverage for a given depositionrate is proportional to the area change.In terms of the local curvature κ andgrowth velocity v normal to the surface

[1]

A simulation of CEAC-based bottom-up trench filling as applied to copperelectrodeposition is shown in Fig. 2. Thegrowth contours are colorized to reflectthe local coverage of the catalyst on thegrowth front. Initially, the catalyst is dis-tributed uniformly along the trench pro-file, here a fractional surface coverage of0.054. At first deposition proceeds con-formally except for enrichment of thecatalyst on the bottom concave cornersand the associated formation of 45°inclined surfaces. When the inclinedgrowth fronts meet, further enrichmentof catalyst occurs and accelerated growthleads to a flat bottom profile. As the bot-tom surface advances upward, further

FIG. 1. Bottom-up superfilling of submicrometer trenches by copper deposition from an electrolyte containingPEG-SPS-Cl.3

Superfilling and the CurvatureEnhanced AcceleratorCoverage Mechanism

The Electrochemical Society Interface • Winter 2004 47

Table I. ElectrolyteComposition

0.24 mol/L CuSO41.8 mol/L H2SO41.0 mmol/L NaCl88 µmol/L PEG (3400 Mw)x µmol/L SPS (x = 0 to 500)

enrichment on the concave sectionsoccurs as catalyst is transferred fromeliminated sidewall areas. With the cat-alyst coverage on the bottom surfaceapproaching saturation, motion of therapidly advancing bottom surface isaccompanied by almost negligible side-wall motion. In contrast, the growthvelocity on the convex upper corners isattenuated by dilution of the catalystthat accompanies expansion of thegrowth front. Similarly, as the bottomsurface approaches the upper corners,an inversion of curvature occurs andthe growth slows as the highly cat-alyzed growth front dilates and forms abump above the feature.

From a more general perspective,the CEAC mechanism represents anextension of the area change effectsoriginally noted in classical droppingmercury electrode studies of surfactant-based charge transfer inhibition.17 Asimilar analogy to competitive adsorp-tion on the Langmuir-Blodgett troughis also relevant.18 Given a surface satu-rated with two different species, adsor-bate-adsorbate interactions must beevaluated. In the limiting case of onespecies being more strongly bound tothe surface, area reduction accompany-ing motion of a saturated concave sur-face results in expulsion of the moreweakly bound species. In contrast,movement of a convex surface opensup new surface sites for additive adsorp-tion from solution. While specificdetails may vary, the core principlemust remain the correct evaluation ofsurfactant mass conservation.Interestingly, we recently encounteredan earlier assessment of the effects ofarea change on competitive adsorptionat a growing copper electrode in thethesis work of Schulz-Harder.19-21 Thesignificance of his findings has largelygone unnoticed and it disappearedfrom the memory of the plating com-munity for more than threedecades.22,23 In light of the substantialindustrial investment in copper platingtechnology, it is probably one of the

FIG. 2. A simulation of superconformal filling of a SPS-derivatized 0.5 µm wide trench during copper depositionfrom an acid copper sulfate electrolyte containing PEG-Cl. The contours are colorized to reveal the local frac-tional coverage of the adsorbed SPS catalyst with the initial value being 0.054.

most unappreciated electroplating worksof the 20th century.

A PrototypicalSuperfilling Electrolyte

The main characteristic of a superfill-ing electrolyte is competition betweeninhibitors and accelerators for electrodesurface sites.9-16 The impact on themetal deposition rate is quantified bystandard electrochemical methods. Amodel two- component additive packagefor copper superfilling contains a mix-ture of a dilute, i.e., micromolar, sul-fonated disulfide accelerator such as SPS[Na2(SO3(CH2)3S)2] in the presence of apolyether inhibitor, such as PEG (poly-ethylene glycol, Mw = 3400) that is typ-ically an order of magnitude higher inconcentration as indicated in Table I.Effective action of this particularinhibitor and the catalyst requires thepresence of halide as a coadsorbate.

Inhibition by PEG-Cl—In a simplecupric sulfate electrolyte, chloride byitself catalyzes the rate-controllingCu2+/Cu+ reaction while PEG aloneexerts a negligible influence on thedeposition rate.24-27 In contrast, thecombination of PEG-Cl results in nearlytwo orders of magnitude reduction of

the deposition kinetics relative to thatfor an additive-free solution.16,26,27 Theextent of inhibition is quantified interms of the effective activity or coverageof a PEG-Cl film that blocks access ofCu2+ to the electrode surface.16,24-27 Inwhat follows, the voltammetric (η-i) (Fig.3a) and chronoamperometric (i-t) (Fig.3b) behavior observed with the PEG-Clelectrolyte are used to define the activity(i.e., θPEG = 1) of the fully inhibitedsteady-state system and serves as a refer-ence point for subsequent discussion ofthe effects of SPS adsorption.16,27

Catalytic Effect of SPS—SPS additionsto the PEG-Cl electrolyte result in dis-ruption of the PEG-based passivatinglayer and acceleration of the depositionreaction as evident from the hystereticvoltammetric curves shown in Fig. 3a.16

The metal deposition rate on the nega-tive-going sweep increases with increas-ing SPS concentration. For the voltam-metric conditions employed here, thehysteresis is maximized for an SPS con-centration near 2.59 µmol/L. Furtheradditions of SPS result in progressivelyhigher deposition rates on the negative-going potential sweep, while theresponse on the return scan is effectivelysaturated. Related potential step experi-ments yield rising chronoamperometric

48 The Electrochemical Society Interface • Winter 2004

transients (Fig. 3b) that qualitativelycorrespond to following a vertical tra-jectory across the hysteretic i-η curvesshown in Fig 3a. For a given overpoten-tial the transient rise time decreasesmonotonically with SPS concentration.The films grown in the presence of SPSare much brighter than those producedin its absence despite the significantlygreater cupric ion depletion gradientsthat accompany growth near the limit-ing current; the absence of rougheningindicates that the hysteretic η-i and ris-ing i-t behavior must derive from achange in interfacial chemistry ratherthan surface area. The data in Fig. 3 canbe rationalized in terms of disruption ofthe rapidly formed PEG-Cl blockinglayer by gradual SPS adsorption withthe extent of disruption being a monot-onic function of the catalyst coverage,θSPS.16,27

The important role of the sulfonateend group is demonstrated by examin-ing the rate of copper deposition onvarious catalyst-derivatized electrodesin a catalyst-free PEG-Cl elec-trolyte.16,27 As shown in Fig. 4, thiolsor disulfides with a charged sulfonateterminal group yield significant andsustained catalysis of the metal deposi-tion rate, indicating that they remainon the surface of the deposit and pre-vent formation of the passivating PEG-Cl film, congruent with the tenets ofthe CEAC model. In contrast, electrodesderivatized with molecules containingalternative end groups exhibitincreased inhibition. In -CH3 terminat-ed molecules, the increased inhibitionis sustained for hundreds of secondswhile electrodes modified with -OH or -COOH terminal groups are quicklydeactivated, presumably by incorpora-tion of the molecules into the growingsolid. The decay rate of the respectivecurrent transients may also be used tocharacterize catalyst consumptionkinetics independent of the ambiguitiesassociated with the adsorptionprocess.16 Integration of a completetransient provides an upper bound esti-mate of additive incorporation.

Modeling CompetitiveAdsorption and Its Effect

on Copper Deposition

The central role of the SPS-based cat-alyst is to open channels in the PEG-Clblocking layer, thereby allowing theCu2+/Cu+ reaction to proceed unhin-dered, i.e., activation of a blocked elec-trode. Accelerated copper depositionoccurs in the proximity of SPS adsorp-tion sites with neighboring chlorideeffectively expanding the area cat-

FIG. 3. (a) Hysteretic voltammetry reflects the displacement of inhibiting PEG-Cl species by adsorption of SPS.The bath components are as given in Table I and the scan rate was 1 mV/s for the stationary copper electrode;and (b) Rising chronoamperometry transients characterizing the activation induced by SPS adsorption on sta-tionary PEG-Cl inhibited copper electrodes at –0.25 V.16

(b)

(a)

FIG. 4. Chronoamperometric transients establishing the catalytic behavior of sulfonate terminal groups (red) incontrast to the inhibition provided by the (OH, COOH, CH3) terminal groups.16

alyzed. Electroanalytical and surfaceanalytical measurements are used toquantitatively examine the potentialdependent catalyst adsorption and con-

sumption dynamics. Their potentialdependence is most evident from multi-cycle voltammetry where adsorptionthat is most rapid at large overpotentials

The Electrochemical Society Interface • Winter 2004 49

The potential dependence of theadsorption rate constant is most clearlyrevealed by changes in the switchingpotential. The high metal depositionrate that characterizes the return sweepfor a switching potential of -0.45 V isnot accessible if the switching potentialis smaller than –0.20 V.Voltammograms at intermediate valuesare consistent with this trend. Theseresults demonstrate that the rate of dis-placement of the inhibiting PEG-Cllayer by SPS adsorption is an increasingfunction of overpotential; voltammet-ric cycling to larger overpotentials per-mits a larger increase in the catalyst sur-face coverage that manifests itself ashigher currents on the return sweep.Simulations based on competitive PEGvs. SPS adsorption are able to effective-ly capture the essential characteristicsof this system.16

Feature Filling:Experiment and Simulation

The most dramatic and unambiguousdemonstration of the CEAC mechanismis provided by superfilling of submi-crometer features in a two step processthat involves derivatization of a pat-terned copper seed layer with a sub-monolayer quantity of catalyst, followedby copper plating in a PEG-Cl electrolytethat is free of catalyst. A comparisonbetween the trench filling experi-ments16,28 and simulation16 reveals sev-eral distinct and important phenomenathat are summarized in Fig. 6.

Experiment—Catalyst derivatizationinvolved a 30 s immersion in a stagnantsulfuric acid solution containing 0.5, 5,50, 500, or 1000 µmol/L of the catalystprecursor, either the disulfide, SPS, orthiol, MPS. The acceleration of thedeposition rate provided by the disul-fide (or thiolate) catalyst is evidentfrom the decrease in the feature fillingtime from ≈ 200 s for specimens deriva-tized in 0.5 µmol/L SPS to ≈ 40 s forderivatization in 500 mmol/L SPS. Forthe specimens derivatized in 0.5µmol/L SPS, deposition proceeds con-formally and eventually results in voidformation when the rough sidewallsimpinge. For the 5, 50, and 500 µmol/LSPS derivatizations, the initial incre-ment of growth is generally conformalexcept for the onset of acceleratedgrowth at the bottom, concave corners.This yields the V-shaped bottom profilevisible in the specimens plated for 70,40, and 20 s, respectively. Subsequently,catalyst enrichment at the vertex of theV-shaped bottom leads to further accel-eration and conversion to a flat bottom

FIG. 5. Variation of the voltmmetric switching potential [(a) –0.45, (b) –0.3, (c) –0.2 V] reveals the strongpotential dependence of the catalyst adsorption and consumption. The sweep rate was 1 mV/s and total scantime 2400 s in each experiment.

(a)

(b)

(c)

can be convolved to different degreeswith consumption that is most severe atlow overpotentials.

Multicycle Voltammetry—As seen inFig. 5, the second and subsequent nega-

tive going sweeps in repetitive potentialcycling do not entirely follow the pathof the preceding return sweep. The pointof departure corresponds to the onset ofsignificant catalyst deactivation.16,27

50 The Electrochemical Society Interface • Winter 2004

profile. Between 80 and100 s the 5 µmol/L derivatized speci-mens exhibit rapid bottom-up filling,the hallmark of the superfilling process.However, by 130 s the sidewallsimpinge just before the rapidly advanc-ing trench bottom reaches the top ofthe feature. The 50 mmol/L specimensexhibit near optimum superfillingbehavior with rapid bottom-up fillingoccurring between 50 and 70 s withnegligible sidewall motion. By 70 s thegrowth front curvature has becomeconvex, and by 100 s a large bump isseen above the trench. For the 500µmol/L SPS and 1 mmol/L MPS speci-mens, void formation is clearly evident,this being more severe in the latter case.These experiments provide thestrongest evidence that, in accord withthe CEAC mechanism, superconformalfilling of submicrometer featuresderives chiefly from the evolution of asubmonolayer quantity of a surface-confined catalyst, rather than throughtransport or chemistry within the elec-trolyte.

Simulation—The initial catalyst cov-erages for the respective simulations areindicated in Fig. 6 where the contourlines are colorized to reflect the localcatalyst coverage ranging, in order,from orange (θSPS = 0) through yellow,green, blue, and red (θSPS = 1).Favorable comparison with experimentis readily apparent.16 For initial catalystcoverage of 0.00054, deposition is con-formal as there is so little catalyst thatthe effects of geometrical enrichmentare negligible. When the initial catalystcoverage is increased to 0.0054 the firstindications of superfilling are evident;catalyst enrichment at the bottom cor-ners leads to significant acceleration ofthe copper deposition rate and forma-tion of inclined growth fronts. Furtherenrichment accompanies the shapetransition from a V-shaped bottom to aflat bottom when the two inclined sur-faces meet. The higher catalyst coverageleads to a marked increase in theupward velocity of the bottom surface.Further catalyst enrichment continuesthrough accumulation from the side-wall area being eliminated by the rapidupward motion of the bottom surface.The narrowed bottom surface is pre-dicted to escape the trench barelyavoiding impingement of the sidewalls.Increasing the initial catalyst coverageto 0.054 results in robust, near optimalsuperfilling behavior. As in the previoussimulation, the advancing bottom sur-face accelerates as it collects catalystfrom the eliminated sidewall areas,eventually approaching saturation cov-

erage as the bottom surface reaches thetop of the trench. From this point on,the expanding surface area associatedwith the advancing convex section leadsto progressive dilution of the local cata-lyst coverage. In this near optimum caseof superfilling the geometrically differ-entiated surface reactivity predicted bythe CEAC model dominates feature fill-ing even in the presence of the substan-tial metal ion concentration gradientthat accompanies the rapid metal depo-sition.

Increasing the initial catalyst cover-age to 0.44 or 0.88 is predicted to inducea reversion to failure to superfill thetrench. In the first case a V-shaped bot-tom is rapidly established, in agreementwith the experiment on the 500 µmol/L- 30 s derivatized SPS electrodes.However, with near-saturation coverageattained on the inclined surfaces, thecoverage can rise no further when theyimpinge, such that no additional accel-eration nor the associated flat bottomshape transition can occur at that site.

FIG. 6. Superfilling of trenches that were pretreated with catalyst prior to copper plating in a PEG-Cl electrolyteat –0.25 V. The conditions used for electrode derivatization are indicated above the features. Simulations of fea-ture filling for the respective derivatization conditions are shown to the right of the experiments. The contourlines are colorized to reflect the local catalyst coverage. The filling times corresponding to the last simulatedgrowth contour listed in order of increasing adsorbate coverage θ are 177, 113, 85, 39, and 24 s.

θ

θ

θ

θθ

The Electrochemical Society Interface • Winter 2004 51

FIG. 8. Seedless superfill is demonstrated by a sequence of images of direct copper deposition on trenches coatedwith a ruthenium barrier.3

FIG. 7. (a) A time sequence of images showing bottom-up copper superfilling of a via. (b) Tracking the interface midheight position during via filling provides a compari-son between the simulation and experiment. The inset shows a θSPS colorized contour plot of simulated bottom-up film growth.16

(a) (b)

With further increases in the differentialdeposition rate being so constrained, thesidewalls impinge before the bottom sur-face is able to escape the trench. For ini-tial catalyst coverage of 0.88, the deposi-tion rate is predicted to be effectively sat-urated at the start of metal depositionwith geometrically driven changes incatalyst coverage on the concave sur-faces being minimal. Depletion effects

are considered only across the hydrody-namic boundary in these simulations.However, for the higher deposition ratesassociated with higher catalyst cover-ages, significant depletion of the Cu2+

ion occurs within the trench, resultingin faster deposition toward the top of thefeature. This leads to an earlier impinge-ment at this location and void forma-tion as observed in two experimental

specimens (θ = 0.44 and 0.88) ratherthan the predicted seams. Such deple-tion effects have been fully modeledelsewhere.15

Via Filling Experiments andSimulations—Deposition in vias, unliketrenches, is a three-dimensional prob-lem by virtue of the nonzero curvatureof the cylindrical sidewalls. Cross sec-tions detailing the filling of cylindricalvias with a 4.5° sidewall slope areshown in 10 s increments in Fig. 7.16 Inthis experiment the catalyst is adsorbedsimultaneously with copper plating inaccord with conventional practice.Wafer fragments were immersed in theSPS-PEG-Cl electrolyte, containing 6.4µmol/L SPS, with a -0.25 V (Cu/Cu2+)growth potential already applied.Growth is essentially conformal for thefirst 60 s followed by the onset of bot-tom-up superfilling at ≈70 s. The fillingprocess is summarized by tracking theheight of the deposit along the center-line of the via, as a function of deposi-tion time. Favorable agreement withthe CEAC simulation is evident and thecorresponding simulated growth con-tours are given in the inset.16

Seedless Superfill

Current metallization technologyemploys three layers, a barrier metal,

52 The Electrochemical Society Interface • Winter 2004

typically tantalum, a physical vapordeposited (PVD) copper seed, and elec-trodeposited copper. As feature sizes con-tinue to shrink, the resistive barrier mate-rials may account for an increasing por-tion of the cross-sectional area and thusnegatively impact electrical perfor-mance. These difficulties have driven asearch for alternative barrier materialsand processes. Ruthenium is a particular-ly attractive candidate because its electri-cal and thermal conductivities areapproximately twice those of conven-tional tantalum barriers and rutheniumand copper are immiscible.29,30 Asshown in Fig. 8,3 direct copper electro-plating on PVD ruthenium-seed layersresults in rapid coalescence of the copperlayer followed by void-free bottom-upsuperfilling of 70 nm wide trenches in≈10 s. Such substitution of a noble metalbarrier/seed layer for the current Ta/Cutechnology promises to offer the com-bined advantages of process simplifica-tion and enhanced performance.

Summary and Outlook

The CEAC mechanism has beenshown to explain the bottom-up super-filling used for producing electrodeposit-ed copper interconnects. A quantitativeconnection between standard electroan-alytical measurements on planar elec-trodes and superfilling of submicrometerfeatures has been developed. Thisenables inexpensive electroanalyticalmeasurements to be used to screen forpotential superfilling electrolytes andoffers new insights to process controlmeasurements.31-33 The CEAC mecha-nism also provides a natural explanationfor brightening because the impact ofarea change on adsorbate coverage for afixed increment of growth scales inverse-ly with feature size making it naturallyrelevant for the length scales associatedwith optical scattering.34-35 Finally, thegenerality of the CEAC has been estab-lished by the successful development ofsuperfilling silver36-37 and gold38 elec-trodeposition and copper chemical vapordeposition39 processes for the filling ofsubmicrometer features. This processtechnology is likely to also be importantto 3D integration of circuits by interchipvias, as well as the fabrication of micro-electromechanical systems (MEMS)devices, and other evolving technologiesand materials.40 �

References

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4. H. Deligianni, J. O. Dukovic, P. C.Andricacos, and E.G. Walton, inElectrochemcial Processing in ULSI Fabricationand Semiconductor/Metal Deposition II, P. C.Andricacos, J. L. Stickney and G. M. Olezak,Editors, PV 99-9, p. 52, The ElectrochemicalSociety Proceedings Series, Pennington, NJ(2000).

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About the Authors

T. P. MOFFAT is with theElectrochemical Processing Group inthe Metallurgy Division at theNational Institute for Standards andtechnology (NIST). His work focuseson the application of electrochemicalmethods toward understanding thin-film deposition processes. He can bereached by e-mail at thomas.moffat@nist.gov.

D. WHEELER is a member of theCenter for Computational andTheoretical Material Science at NISTwhere he uses numerical methods tosolve interesting materials scienceproblems. He can be reached by e-mail at daniel.wheeler@nist.gov.

D. JOSELL works in the MaterialsStructure and Characterization Groupof the Metallurgy Division at NIST.His effort centers on the study of thin-film growth phenomena and its effecton the resulting physical properties.He can be reached by e-mail atdaniel.josell@nist.gov.