Stability of iodine on ruthenium during copper electrodeposition and its effects on the nucleation behavior of electrodeposited copper

Cyclic voltammetry, current-time-transient measurements, and X-ray photoelectron spectroscopy (XPS) have been used to study the nucleation behavior of electrochemically deposited Cu films on Ru substrates as a function of Ru pre-treatment. Pre-treatment consisted of cathodic polarization in either 1 M H₂SO₄ or in 1 M H₂SO₄ + 1 mM KI, followed by sample emersion and placement in a 1 M H₂SO₄ + 50 mM CuSO₄ plating bath. XPS measurements confirmed the presence of adsorbed I on the Ru surface following pre-treatment in the KI/H₂SO₄ solution. Cyclic voltammogram (CV) data for electrodes either as-received or pre-reduced in H₂SO₄ and then immersed in the plating solution exhibited a broad peak in the overpotential region consistent with oxide reduction followed by Cu deposition. No underpotential deposition (UPD) feature was observed for these electrodes. In contrast, the sample pre-reduced in I-containing electrolyte exhibited a narrow Cu deposition peak in the overpotential region and a UPD Cu feature centered at 80 mV vs. Ag/AgCl. Current-time-transient (CTT) measurements of Cu deposition on as-received electrodes or electrodes pre-reduced in I-free solution exhibited potential-independent kinetics that are not well described by either progressive or instantaneous nucleation models and which at long times indicate a combination of diffusion and kinetic control. In contrast, CTT measurements of deposition kinetics for samples reduced in I-containing electrolyte exhibited complex, potential-dependent behavior and that at long times indicates diffusion control. XPS results also indicated that the iodine adlayer on Ru reduced in I-containing electrolyte is stable upon polarization to at least −200 mV vs. Ag/AgCl. These data indicate that a protective I adlayer may be deposited on an air-exposed Ru electrode as the oxide surface is electrochemically reduced, and that this layer will inhibit reformation of an oxide during the Cu electroplating process. Therefore, electrochemical pre-treatment in I-containing electrolyte may be of practical utility under industrial conditions for Cu electroplating.     

On the influence of the nucleation overpotential on island growth in electrodeposition

Electrochemical deposition of a metal onto a foreign substrate usually occurs by an island growth mechanism. A key feature of island growth for a material M on a foreign substrate S is that the onset potential for deposition is shifted negative from the equilibrium potential for the metal ion couple. The nucleation overpotential, defined as η_n(M⁺/S) = |U_n(M⁺/S) − U_eq(M⁺/M)|, influences key aspects of deposition of a metal on a foreign substrate. Here we discuss how the nucleation overpotential influences the kinetics of island growth, the implications of the nucleation overpotential on island shape and orientation, and the consequences of the coupling between the island density (applied potential) and the island size at coalescence (grain size). We then discuss the kinetics of island growth in terms of the contributions to vertical and lateral growth. Finally, we present examples of experimental methods to manipulate the nucleation overpotential and overcome some of the limitations imposed by the nucleation overpotential.     

Copper electrodeposition onto the dendrimer-modified native oxide of silicon substrates

Adherent copper films were electrochemically grown onto the native oxide surfaces of Si wafers modified by the adsorption of polyamidoamine (PAMAM) dendrimers. Metallic nuclei of copper grow at isolated nucleation sites, associated with adsorbed dendrimers, and film coalescence can be observed above a metal thickness of about 10,000 monolayers (∼2.5 μm). Film microstructure depends on the deposition mode; higher coverage and better adhesion were obtained by galvanostatic control of the deposition process. It is hypothesized that reduction of Cu²⁺ ions complexed with functional groups of the chemisorbed dendrimer leads to the formation of metallic copper nuclei, and that metal films grow from these nuclei. Further improvement of this process may open the way to the direct integration of metal electrodeposition with silicon microfabrication processes and selective deposition by dendrimer patterning.     

Copper electrodeposition on pyrolytic graphite electrodes: Effect of the copper salt on the electrodeposition process

The electrodeposition of copper on pyrolytic graphite from CuSO₄ or Cu(NO₃)₂ in a 1.8 M H₂SO₄ aqueous solution was investigated. The Cu deposits were formed potentiostatically and characterized by electrochemical methods, scanning electron microscopy, energy dispersive X-ray and X-ray photoelectron spectroscopy. It was found that the deposition of copper in the presence of CuSO₄ induced the codeposition of sulfate anions. In addition, electrochemical quartz crystal microbalance revealed that the increase of the Cu mass was higher than expected from Faraday's law with the CuSO₄/H₂SO₄ solution. These results confirmed the specific adsorption of anions during the Cu deposition. On the other hand, the use of Cu(NO₃)₂ resulted in a non-contaminated surface with different surface morphologies. The Cu nuclei size, the population density and the surface coverage were monitored as a function of the deposition potential. From the analysis of the chronoamperometric curves, the nucleation kinetics was studied by using various theoretical models. Independently of the Cu source, the nucleation mechanism follows a three-dimensional (3D) process. Copper nucleates according to an instantaneous mode when the deposition potential is more negative than −300 mV versus Ag/AgCl, while the nucleation was interpreted in terms of a progressive mode at −150 mV. The nuclei population densities were also determined by using two common fitting models for 3D nucleation and growth (Scharifker-Mostany and Mirkin-Nilov-Heerman-Tarallo). Their values are reported here as a function of the deposition potential.     

The oxide electrochemistry of ruthenium and its relevance to trench liner applications in damascene copper plating

There is considerable interest in the use of ruthenium as an ultrathin trench liner in damascene copper plating used to fabricate on-chip interconnects. The problem is that when freshly deposited ruthenium films are exposed to air, their surfaces tend to undergo spontaneous oxidation, and such deposits (as demonstrated here) are reluctant to undergo reduction. Copper deposition in an acid plating bath occurs readily on the oxidized ruthenium, but the presence of oxide is known to have a detrimental effect both on the copper superfilling process and copper adhesion at the Ru/Cu interface.     

Effects of anodization and electrodeposition conditions on the growth of copper and cobalt nanostructures in aluminum oxide films

Anodic aluminum oxide (AAO) films were prepared by alternative current (ac) oxidation in sulfuric acid and phosphoric acid solution. The porous structure of the AAO templates was probed by ac electrodeposition of copper. AAO templates grown using an applied square waveform signal in cold sulfuric acid solution exhibit a greater pore density and a more homogeneous barrier layer. UV–vis–NIR reflectance spectra of the Cu/AAO assemblies exhibit a plasmon absorption peak centered at 580 nm, consistent with the formation of Cu nanostructures slightly larger than 10 nm in diameter. Spectroscopic data also indicate that there is little or no oxide layer surrounding the Cu nanostructures grown by ac electrodeposition. The effect of pH of the cobalt plating solution on the magnetic properties of the Co/AAO assemblies was also investigated. Co nanowire arrays electrodeposited at pH 5.5 in H₂SO₄-grown AAO templates exhibit a fair coercivity of 1325 Oe, a magnetization squarness of about 72%, and a significant effective anisotropy. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Electrochemical deposition of copper and ruthenium on titanium

Copper electrochemical deposition on titanium with a ruthenium seed layer was investigated. The chemicals for the acid-bath ruthenium electrochemical deposition were ruthenium(III) chloride hydrate (RuCl₃·3H₂O), hydrochloric acid (HCl), sulfamic acid (NH₂SO₃H), and polyethylene glycol. The chemicals for the acid-bath copper electrochemical depositions were copper(II) sulfate hydrate (CuSO₄·5H₂O), sulfuric acid (H₂SO₄), and polyethylene glycol. Results were analyzed by field-emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Rutherford backscattering spectrometry (RBS). Ruthenium thin film of ∼30 nm thickness, with equiaxial grains <10 nm, was deposited, on a blanket Ti with a root mean square roughness of 8.3 nm, at 2 V for 90 s. XPS and RBS analyses showed the presence of metallic Ru. The Ti substrate was found stable with respect to ECD of Ru but the Ru/Ti bilayer was not found stable in the Cu acid bath, resulting in the diffusion of Ti into Ru film. The depth profiling studies indicates that Ru film thickness ca. 1.4 nm and deposition time of 10 s can act as a good seed layer.     

Electrodeposition of copper on Ru(0 0 0 1) in sulfuric acid solution: Growth kinetics and nucleation behavior

The electrodeposition of Cu on Ru(0 0 0 1) from 0.1 M CuSO₄/0.5 M H₂SO₄ solution has been studied by cyclic voltammetry, current-time transient measurements, and by in situ electrochemical atomic force microscopy (EC-AFM). Cyclic voltammetry measurements show that the as-prepared Ru(0 0 0 1) electrode exhibits a UPD peak, while EC-AFM data indicate a broadly terraced surface with step heights of atomic dimensions. Kinetic data show that the electrodeposition/nucleation process is not well described by 3D or 2D nucleation models. The EC-AFM data show that at potentials near the OPD/UPD threshold, Cu crystallites exhibit pronounced growth anisotropy, with lateral dimensions greatly exceeding vertical dimensions. AFM data also show that deposition at more cathodic potentials result in smaller crystallites.     

Filling of mesoporous silicon with copper by electrodeposition from an aqueous solution

Copper filling into mesopores formed in highly doped p-type silicon was investigated. When the copper electrodeposition was carried out at a very small constant current density (−6.4 μA cm⁻²), the mesopores with 4 μm depth were filled with copper continuously from the bottom to the opening. When the electrodeposition current was set at an absolute value twice as large as in the above condition, the isolated particles were electrodeposited in the mesopores. The depth also affected the filling behavior. The pores with 8 μm in depth were not continuously filled with copper even in the condition at which the pores of 4 μm in length were completely filled. Electrodeposition behavior in mesopores was also simulated using a simple model. The numerical simulation suggested that the diffusion-limited electrodeposition could be achieved in mesopores at a very small current, at which the diffusion-limited condition had never been realized on a planar electrode.     

Stress behavior of electrodeposited copper films as mechanical supporters for light emitting diodes

Thick copper films electrodeposited on vertically structured GaN-based LEDs play a critical role in supporting thin GaN multi-layers mechanically after laser lift-off (LLO) of the sapphire substrate. The intrinsic stress of electrodeposited copper was measured from the change in the substrate's curvature using a modified Stoney equation. 150-μm thick copper supporters showed very low intrinsic stress of 4-28 MPa (tensile stress), which developed during copper electrodeposition on Au seed. The stress generation was attributed to nuclei coalescence and formation of grain boundaries of which volume increased with the applied current density. The preferential texture plane of thick copper supporters was (2 2 0) at high current densities, while copper films thinner than 1 μm was strongly oriented along (1 1 1) plane. Various seed metals such as Cu, Ni, Au, Ag, and Al were employed to observe the influence of seed material on the stress of copper supporters. The effect of wetting agent on the stress of copper supporters was also monitored in the concentration range of 0.1-1.5 g/L of sodium lauryl sulfate (SLS).     

Nucleation and growth of copper under combined charge transfer and diffusion limitations—Part II

Progressive nucleation of copper on glassy carbon is examined by means of combined “current-time”, i(t), and “number of nuclei-time”, N(t), experiments performed at constant overpotentials. Information is obtained on the most important nucleation and growth parameters: the nucleation rate of copper clusters, the charge transfer coefficient, the exchange current density at the “copper-solution” interface boundary and the diffusion coefficient of copper ions. The copper ion's discharge is considered as a two-stage electrochemical reaction and the growth current of the supercritical copper crystals is interpreted in terms of the theory of progressive nucleation and growth under combined charged transfer and diffusion limitations.     

Nucleation and growth of copper under combined charge transfer and diffusion limitations: Part I

The nucleation and growth of copper crystals on a glassy carbon electrode are studied under potentiostatic conditions. Current transients are recorded at different overpotentials and are interpreted in terms of the theory of progressive nucleation and growth under combined charged transfer and diffusion limitations. Data are obtained on the overpotential dependence of the stationary nucleation rate and on the size of the critical copper nuclei. The influence of ion transfer and electron transfer electrochemical reactions taking place prior to and simultaneously with the process of nucleus formation is discussed and their contribution to the total current is accounted for.     

Electrochemical nucleation and growth of Co and CoFe alloys on Pt/Si substrates

Electrochemical nucleation and growth of Co and CoFe alloys on Pt/Si(1 0 0) surface from watts (mixed chloride sulfate) baths were studied by voltammetric, chronoamperometric and AFM measurements. The CoFe alloys were deposited from solution with molar ratio of (1/1) and (10/1). The Scharifker and Hills model was employed to analyse the current transients. For both Co and CoFe (10/1) alloys the nucleation was a good agreement with the instantaneous model followed by 3D diffusion-limited growth. Inversely, for CoFe (1/1) alloy the nucleation was an agreement with the progressive model. It is evident that the compositions of the electrolyte influence greatly the type of nucleation. The atomic force microscopy (AFM) images revealed a compact and a granular structure of the electrodeposited Co layers and CoFe alloys.     

Nucleation on resistive substrates: Analysis of effect on film uniformity

When plating onto resistive substrates, potential drop in the electrode can cause non-uniformities in the current density distribution. Multiple studies of this so-called “terminal effect” have been conducted, assuming a layer-by-layer growth mechanism, as would apply, for example, to Cu deposition onto a thin Cu seed layer. However, when depositing Cu onto a material such as Ta or Ru, electrodeposition may occur by three-dimensional nucleation followed by growth. In such cases, a reduction in substrate resistance may not be realized prior to coalescence of the deposited film. Simulations show that nucleation can have a very significant impact on film-thickness uniformity. Results show a linear and significant increase in non-uniformity with the coalescence thickness of the depositing film. Simulations are extended to account for spatial variations of coalescence length. Implications of the model to wafer scale plating of Cu for interconnect applications are discussed.     

The influence of stresses on ageing kinetics of 3Y- and 4Y- stabilized zirconia

Hydrothermal ageing is one of the most important limiting factors for the use of yttria-stabilized zirconia ceramics in contact with water-containing environments. It consists in the transformation of tetragonal phase to monoclinic phase, initiates on the surface of zirconia in the presence of water, and leads to roughening and potentially to micro-cracking and loss of integrity. The present work seeks to explore the influence of applied and residual mechanical stresses on the ageing kinetics of 3Y- and 4Y-TZP. Residual stresses were obtained by rough polishing. A subsequent Annealing step was employed for the preparation of samples free of residual stresses. All samples were submitted to in situ 3-points bending tests in water vapour atmosphere inside an autoclave at 134 °C, allowing surfaces with a mechanical stress gradient to be exposed to hydrothermal ageing. The evolution of the monoclinic fraction with time and stress was then analyzed using Mehl-Avrami-Johnson equation.     

Influence of the substrate's surface structure on the mechanism and kinetics of the electrochemical UPD formation of a copper monolayer on gold

The copper electrodeposition process was studied onto different gold substrates, single crystal (1 1 1) and polycrystalline, using electrochemical techniques. It was found, from the analysis of the experimental current density transients, that the potentiostatic formation of a full copper monolayer onto the gold electrode under UPD conditions follows the same mechanism, regardless of the crystallinity of the substrate. The mechanism involved the simultaneous presence of an adsorption process and of two 2D nucleation processes, progressive and instantaneous, respectively.     

Electrodeposition of CdTe thin films onto n-Si(1 0 0): nucleation and growth mechanisms

The mechanisms related to the initial stages of the nucleation and growth of cadmium telluride (CdTe) thin films on the rough face side of a (1 0 0) monocrystalline n-type silicon have been studied as a function of different potential steps that varied from an initial value of −0.200 V to values comprised between −0.515 and −0.600 V versus saturated calomel electrode (SCE). The analysis of the corresponding potentiostatic j/t transients suggests that the main phenomena involved at short times is the formation of a Te-Cd bi-layer (BL). For potentials below −0.540 V, the formation of this bi-layer can be considered independent of potential. At greater times, the mechanisms is controlled by two process: (i) progressive nucleation three dimensional charge transfer controlled growth (PN-3D)_ct and (ii) progressive nucleation three dimensional diffusion controlled growth (PN-3D)_diff, both giving account for the formation of conical and hemispherical nuclei, respectively. Ex situ AFM images of the surface seem to support these assumptions.     

Nucleation and growth of tin on low carbon steel

The nucleation and growth of tin from stannous sulfate and sulfuric acid with an organic additive of gelatin was studied on low carbon steel. Investigations were conducted by linear sweep voltammetry and current step methods. The agitation effect on the morphology of tin deposits was also studied. From the results, a mechanism of tin deposition on a steel substrate is suggested. Initially, 3D tin crystallites are formed preferentially on step edges, and this is followed by fast covering of neighboring sites with much smaller and densely packed crystallites of about 150-180 nm in size. Without gelatin, the coverage was poor and tri-modal structures were dominating. It is discovered that the synergy between hydrogen co-evolution and gelatin contributes to fast and complete coverage of the steel substrate and the bi-modal grain size distribution is observed. The diffusion coefficient of tin ion was determined according to the Sand equation.     

Synthesis and kinetics of growth of metal nanoparticles inside ion-exchange polymers

Copper and silver nanoparticles have been obtained by means of saturation of a sulfostyrene-divinylbenzene cation-exchange polymer with metal ions and their subsequent chemical reduction. This procedure was repeated several times up to formation of a long-range conducting network (percolating cluster). Another system under study was an ensemble of Ag nanoparticles of various sizes on the silver electrode surface obtained by reduction of anodically formed layers of silver oxide. Recrystallization of deposited metal crystals inside the polymer matrix in contact with metal-ion containing solution is very slow for electrically separated particles. Formation of the electric network results in an enormous acceleration of this process via electron-ion mechanism, with growth of the average particle size, so that their potential will approach that of the compact metal with time. The initial period of the particle growth is well described by the parabolic law (Burke and Turnbull). The values of the particle-growth coefficient in this law, k, are drastically different for particles inside the matrix and on the electrode surface. Particle-to-particle electron transfer is impeded by insulating areas inside the polymer matrix. Besides, ionogenic centers of the matrix restrict the mobility of metal cations, thus slowing down the ion transfer within the recrystallization circuit. These observations have allowed us to establish the conditions resulting in long-term stabilization of metal nanoparticles inside the ion-exchange matrix with respect to their recrystallization.     

Electrocrystallization of Au nanoparticles on glassy carbon from HClO4 solution containing [AuCl4]

The mechanism and kinetics of electrocrystallization of Au nanoparticles on glassy carbon (GC) were investigated in the system GC/1 mM KAuCl₄ + 0.1 M HClO₄. Experimental results show that the gold electrodeposition follows the so-called Volmer-Weber growth mechanism involving formation and growth of 3D Au nanoparticles on an unmodified GC substrate. The analysis of current transients shows that at relatively positive electrode potentials (E ≥ 0.84 V) the deposition kinetics corresponds to the theoretical model for progressive nucleation and diffusion-controlled 3D growth of Au nanoparticles. The potential dependence of the nucleation rate extracted from the current transients is in agreement with the atomistic theory of nucleation. At sufficiently negative electrode potentials (E ≤ 0.64 V) the nucleation frequency becomes very high and the nucleation occurs instantaneously. Based on this behaviour is applied a potentiostatic double-pulse routine, which allows controlled electrodeposition of Au nanoparticles with a relatively narrow size distribution.