Mechanism of Electroless Copper Deposition from [CuIIEDTA]2

Article pubs.acs.org/JPCC

Mechanism of Electroless Copper Deposition from [CuIIEDTA]2− Complexes Using Aldehyde-Based Reductants Meng Zhao,† Lu Yu,‡ Rohan Akolkar,‡ and Alfred B. Anderson*,† †

Department of Chemistry and ‡Department of Chemical and Biomolecular Engineering, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States ABSTRACT: In this article, we present a mechanism for electroless copper deposition from alkaline solutions containing [CuIIEDTA]2−(aq) and hydrolyzed aldehydes. The mechanism is based on quantum chemical calculations of the activation energies and reversible electron-transfer potentials. Low-coordinated, electroless-deposited surface Cu atoms are found to activate H abstraction from the C−H bond of the aldehyde group, putting H on the surface and releasing a highly reducing electron in the process; this electron reduces [CuIIEDTA]2−(aq) to [CuIEDTA]3−(aq). [CuIEDTA]3−(aq) is subsequently reduced to Cu0(ads) by a second electron generated by the same process, releasing EDTA4−(aq). Calculated reversible potentials for the reduction steps aid in interpretation of the catalytic behavior observed in experimental polarization curves.

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INTRODUCTION Electroless deposition is a technique for depositing thin films of metals, alloys, and compounds.1 Electroless deposition is typically mediated through the use of organic reducing agents, which are oxidized during the process. This, in turn, reduces solution-phase metal ions to solid-phase metal, which is deposited onto a substrate. Metal deposition thus occurs selectively on substrates that effectively catalyze the electroless redox chemistry.2 By nature, electroless deposition does not require an external current supply (as required by electrodeposition1), and thus, electroless deposition is ideally suited for deposition onto insulators, semiconductors, resistive electrodes, and micropatterned surfaces.3,4 Electroless deposition of thin metallic Cu films is gaining interest for the fabrication of nanoscale interconnects in advanced microprocessors and memory devices.4,5 Typically, the electroless Cu bath contains an aqueous solution of Cu2+ stabilized at high pH using a complexant such as deprotonated ethylenediamine tetraacetic acid, EDTA4−(aq). An aldehydebased reducing agent, such as formaldehyde6 (H2CO) or glyoxylic acid7 [HC(O)COOH], enables chemical reduction of dissolved [CuIIEDTA]2−(aq) to metallic Cu on the electrode surface. The aldehyde (RHCO, where R = H for formaldehyde and R = COOH, the carboxylate group, for glyoxylic acid) is believed to be central to the electroless process, because aldehyde oxidation proceeds favorably on catalytic Cu sites during sustained electroless Cu deposition. Despite intensive research, the mechanism of electroless deposition is presently unclear. Specifically, mechanistic insights into the reaction chemistry leading to autocatalytic electroless deposition are unavailable. In this article, we propose a mechanism for autocatalytic electroless deposition that hinges on the activating property of low-coordinated surface copper atoms toward breaking the C−H bond and the powerful reducing ability of the acid anion that is thus formed. This all © XXXX American Chemical Society

takes place on the copper surface immersed in the electrolyte in the absence of external potential control.

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THEORY To gain mechanistic insights, we used a state-of-the-art density functional theory (DFT) code called Interface8,9 for all calculations except the calculations of C−H bond activation energies, for which we used hybrid DFT in the Gaussian 09, revision A.02, package.10 The Interface DFT calculations apply a linear combination of pseudoatomic orbitals (LCPAO)11 with norm-conserving pseudopotentials (NCPP)12 as effective core potentials, a generalized gradient approximation with the revised Perdew−Burke−Ernzerhof (GGA-RPBE)13 exchangecorrelation functional, and a double-ζ polarization (DZP)9 basis set. The modified Poisson−Boltzmann theory is included to give the energy of electrolyte polarization around solvated ions and at surfaces, and a dielectric continuum statistical representation of liquid water is included.14,15 For our work, three-layer Cu(111) slabs were chosen in Interface twodimensional band calculations. The top two layers of the slab were relaxed during the optimization, and the bottom layer was fixed at the calculated lattice constant of 3.73 Å. For small adsorbates such as Cu adatoms, a three-layer slab with a 3 × √3 translational cell containing six Cu atoms per layer was used. A 3 × 6 × 1 k-point mesh for Monkhorst−Pack sampling16 was applied. In contrast, for large adsorbates such as the [CuIEDTA]3− complex, a three-layer slab with a 4 × 3√3 translational cell containing 24 Cu atoms per layer was employed to avoid adsorbate−adsorbate interactions. In this case, a 3 × 2 × 1 k-point mesh was used. To find reversible electron-transfer potentials, Urev, for solution-phase reactions, Received: July 27, 2016 Revised: October 4, 2016

A

DOI: 10.1021/acs.jpcc.6b07546 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C we calculated the reaction energies, G, of the reactant and product molecules, both in the aqueous phase, using the equation Urev = −Δreact G /(nF )

where 4.454 V comes from the calculated 4.454 eV thermodynamic workfunction for the standard hydrogen electrode. The energies of the electrode surface before and after Cu atom deposition in eq 2 also depend on q. Equation 2 provides an alternative and more accurate prediction of the reversible potential for this step.

(1)

where ΔreactG is the calculated Gibbs energy of reaction and is quoted in electronvolts (eV). The components of the Gibbs free energy of neutral species, G, include the kinetic energies, exchange-correlation energies, electrostatic energies, free energies from nonelectrostatic solute−solvent and ion-solute interactions, entropies of electrons and ions, and a thermal contribution term. The thermal contribution term is calculated from standard statistical thermodynamic models: For adsorbed species, it is calculated from vibration frequencies using the harmonic vibration model, and the frequencies are obtained from the Hessian matrix; for isolated molecules, translational, rotational, and vibration modes are included. The Gibbs energy of a charged species includes an additional mass-conservation term calculated from Fermi level and the number of electrons, anions, and cations.9 All potentials quoted in this work are on the standard hydrogen electrode (SHE) scale. The precursor to metallic copper deposition in this work is the water-soluble complex [CuIIEDTA]2−(aq). It has a high formation constant, log Kf = 18.8, and its standard reduction reversible potential is −0.213 V.6 The reduction of the molecular complex adds a Cu atom to the copper surface and the released EDTA4−(aq) remains in the electrolyte, where it can react with Cu2+(aq) to form a new [CuIIEDTA]2−(aq) complex. When seeking reversible potentials for reactions in which the intermediate [CuIEDTA]3−(ads) is adsorbed, ΔG was calculated at the potential of zero charge (PZC), meaning that no net electron charge from external sources was added to or subtracted from the surface translational cell in our twodimensional Interface DFT band calculations. Reversible potentials determined in this way are more approximate than those determined by the method that uses calculated electrodepotential-dependent reactant and product Gibbs energies and determines the reversible potential as their crossing point.8,9,17 Because of computational slowness for such large systems, we used the simpler approach for [CuIIEDTA]2−(aq) reduction to [CuIEDTA]3−(ads) and for [CuIEDTA]3−(ads) reduction to Cu0(ads) + EDTA4−(aq). The errors in reversible potential predictions are occasionally up to 0.3 V.18 Contributions to the errors include (i) the inexactness of energies calculated by the Interface computational code we used, (ii) the use of the PZC instead of the correct electrode potential in the calculation of adsorption energies, and (iii) the partial modeling of solvation. For calculating reversible potentials for reactions in which the intermediate is [CuIEDTA]3−(aq), the surface systems are small enough that we could use the condition of equilibrium8,9,18 in which the Gibbs energies of the reactants and products in the reaction

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RESULTS AND DISCUSSION The well-studied reducing agents participating in the electroless deposition of copper, namely, formaldehyde and glyoxylic acid, contain aldehyde groups, −CHO, which, when hydrolyzed in acidic electrolyte, become diols, −CH(OH)2. Because electroless deposition is performed in alkaline electrolyte, the diol takes the anionic form, −CH(OH)O−. During electroless deposition, these ions are known to be oxidized to H2 and formic acid6 or oxalic acid.7 To confirm that these diol anions are good reducing agents, we calculated the reaction Gibbs energies for oxidizing them using the Interface code and then used eq 1 to calculate reversible potentials for oxidizing the anions. This was done for the reactions involving formaldehyde or glyoxylic acid as the reductant H 2C(OH)O−(aq) ⇌ HCOOH(aq) +

(4)

and (−OOC)(H)C(OH)O−(aq) ⇌ (−OOC)COOH(aq) +

(2)

are equal. Here, Ef is the energy of an electron at the Fermi level. It is dependent on the presence of adsorbates and on the net electronic charge q that is added to or removed from the surface translational cell. The electrode potential for the system depends on q according to the expression U (q) = −4.454 V − Ef (q)F

1 H 2(g) + e−(Ef ) 2

(5)

by calculating the Gibbs energies of the reactant and product molecules and adjusting Ef to the equilibrium conditions and then using eq 3 to find the reversible potential with q = 0. The resulting reversible potentials are −1.272 and −1.217 V, respectively. These potentials mean that both anions have the thermodynamic capability to reduce [CuIIEDTA]2−(aq) by a mechanism that generates H2(g) and carboxylic acid. In these reactions, to arrive at the acid products, H is abstracted from the aldehyde C atom. In neutral closed-shell molecules, such as methane, H abstraction requires an energy input of several electronvolts, which would prevent the reaction, but for these anions, the 1− charge is expected to weaken the C−H bond by delocalizing charge into the empty σ* orbital as it drops in energy during bond stretching. While the methyl diol is depositing a hydrogen atom on the copper surface and becoming a formic acid molecule, an electron is released. In the basic electrolyte, the formic acid product reacts with OH−(aq) to form a formate anion and a water molecule. Two of these reactions are needed to deposit a Cu atom. When glyoxylic acid is used as the reducing agent, the loss of C−H bonds during copper reduction leads to oxylate anions as the solution-phase product. It is known that rough electroless-deposited copper surfaces are more active than smooth Cu films toward glyoxylic acid oxidation in base.19 One piece of supporting evidence from our Gaussian calculation results is the greater adsorption bond strength of methyl diol anion on the rough Cu cluster (0.503 eV) than on the smooth Cu cluster (0.134 eV). Structures of the adsorbed anions are shown in Figure 1. Two cluster models of the Cu(111) surface were used: For the smooth surface, a hexagonal 19-atom cluster was employed, and for the rough surface, a Cu adatom was placed on a 3-fold site, forming a 20-

[Cu IEDTA]3 −(aq) + Cu(surface)(q) + e−[Ef (q)] ⇌ Cu 0(ads)(q) + EDTA4 −(aq)

1 H 2(g) + e−(Ef ) 2

(3) B

DOI: 10.1021/acs.jpcc.6b07546 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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individual cathodic and anodic half-reactions, the electroless deposition rate and the theoretical mixed potential can be predicted. However, during actual electroless deposition, the deposition rate measured by weight gain was found to be about 6 times larger than that predicted by applying the mixed potential theory to the partial polarization curves. Also, the mixed potential deviated significantly from the predicted mixed potential by about 120 mV. This implies that both halfreactions are accelerated in an actual electroless deposition process. The above observations suggest interactions between the two half-reactions and a more complex reaction pathway. Similar observations were made in formaldehyde-based electroless copper systems by Wiese and Weil.21 In the present work, we performed experiments analogous to those in ref 19 but under more controlled conditions. Figure 2

Figure 1. Hydrogen-atom-abstraction energy curves from the methyl diol anion calculated using B3LYP hybrid density functional theory as implemented in Gaussian 09 with the 6-31++G** basis set for C, H and O and the LANL2DZ basis set for Cu.

atom cluster. These calculations also included the polarizable continuum model (PCM) for approximate hydration stabilization. Calculated activation energies are consistent with expectations. On the hexagonal 19-atom cluster, the H-atom abstraction over a 1-fold site had an activation energy of 0.26 eV, and when this was done over an adatom placed on a 3-fold site, the activation energy was reduced to 0.06 eV. Reactant, transition-state, and product structures for the methyl diol anion are shown in Figure 1. To calculate reversible potentials for oxidizing the anions in solution, we turned again to the Interface code. For the formic acid anion oxidation reaction HCOOH−(aq) ⇌ HCOOH(aq) + e−(Ef )

Figure 2. Polarization curves obtained on a Cu RDE (200 rpm) from a complete electroless bath (red curve), a “glyoxylic acid only” bath (blue curve) and a “Cu only” bath (black curve). These polarization data clearly show the discrepancy between the mixed potential predicted by the mixed potential theory and that measured experimentally in the “complete electroless bath”.

shows polarization behavior for a Cu rotating disk electrode immersed in an electroless deposition bath at 60 °C containing 0.036 M copper sulfate, 0.24 M EDTA4−, and 0.19 M glyoxylic acid adjusted with sodium hydroxide to pH 12.5. Polarization behaviors for an electrolyte without CuII (glyoxylic acid only bath) and for an electrolyte without reductant (Cu only bath) are also shown. Polarization of the complete electroless bath provides a measured mixed potential of −0.43 V, whereas the polarization behaviors of the two half-baths provide (through application of the mixed potential theory) a theoretical mixed potential of −0.29 V. Keeping in mind the negative calculated oxidation potential for glyoxylic acid, the small measured current density below −0.2 V for the glyoxylic acid only bath appears consistent with a high activation energy barrier for H abstraction from the diol anion over highly coordinated Cu atoms on the smooth Cu surface. For the Cu only bath, the small current density from CuII reduction shown from −0.1 V to about −0.5 V corresponds to a kinetic limitation in reducing the complex and opening it to deposit Cu atoms on the smooth Cu surface. We propose that H abstraction from the C atom of the diol anion reducing agent is activated over a low-coordinate surface Cu atom. During breaking of the C−H bond and formation of the Cu−H bond, the electron is ejected from the anion. The

(6)

we obtained Urev −2.267 V. For the oxalic acid anion oxidation reaction HOOCCOOH−(aq) ⇌ HOOCCOOH(aq) + e−(Ef )

(7)

we obtained Urev −1.032 V. These acid anions are therefore strong reducing agents in aqueous media. Both calculated reversible potentials are negative of the −0.213 V standard reversible potential for [CuIIEDTA]2−(aq) reduction to metallic copper.6 In ref 19, the onset potential for the reduction reaction in an electrolyte containing 0.036 M copper sulfate and 0.24 M EDTA4− at 60 °C was measured to be about −0.2 V. This is close to the −0.188 V value for these conditions from the Nernst equation. In the aforementioned article, the authors attempted to apply the mixed potential theory20 to study the electroless copper deposition system with glyoxylic acid as the reducing agent. The mixed potential theory postulates that the cathodic (i.e., metal reduction) and anodic (i.e., reducing agent oxidation) half-reactions during electroless deposition proceed independently, establishing a surface mixed potential at which the cathodic and anodic partial currents are balanced. Consequently, by measuring the polarization behavior of the C

DOI: 10.1021/acs.jpcc.6b07546 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C electron can either jump through space to [CuIIEDTA]2−(aq) or go into the copper conduction band and transfer to [CuIIEDTA]2−(aq) from the surface. The mean free path for a bulk electron in copper at room temperature is 39.9 nm,22 so the surface-reaction cross section for reduction of a [CuIIEDTA]2−(aq) ion by an excited metallic electron before it decays to the Fermi level is large. Our Interface calculations showed the octahedrally coordinated Cu I I in the [CuIIEDTA]2−(aq) complex partially opens up when reduced to [CuIEDTA]3−(aq), with CuI coordination decreasing from 6 to 4, and further opens up to coordination of 2 when CuI is reduced to Cu0 by a second electron. This is shown in Figure 3.

Figure 4. Proposed stepwise mechanism for electroless Cu deposition in the case of the methyl diol anion as the reducing agent.

Figure 3. Opening of the octahedral coordination of the [CuIIEDTA]2−(aq) complex upon reduction.

produce the stable close-shell structure HCOOH(aq), which reacts in the basic solution to form formate anion and water. The released electron has high energy and is therefore strongly reducing, and it reduces the neighboring [CuIIEDTA]2−(aq) to [CuIEDTA]3−(aq). The process repeats, depositing another H(ads) on the electrode surface. Based on our calculations, the most preferable adsorption site of H(ads) on the Cu(111) surface is the 3-fold hcp site. Adsorption on neighboring fcc sites is ∼0.01 eV less stable. This indicates a high mobility of H(ads) on the Cu(111) electrode surface. Therefore, two H(ads) can combine, resulting in the evolution of H2(g), observed as bubble formation from the electrode surface. The second electron-transfer process reduces [CuIEDTA]3−(aq) complex to Cu0(ads). This Cu0 provides an additional adsorption site for methyl diol anion activation. As more Cu0 adatoms are added to the surface, more low-coordinate Cu sites become available for methyl diol anion oxidation and [Cu II EDTA] 2− (aq) reduction. This completes the autocatalytic mechanism during electroless Cu deposition in base solution.

The reversible potential calculated for [CuIIEDTA]2−(aq) reduction to [CuIEDTA]3−(aq) is −0.571 V. This value differs significantly from the standard value for Cu2+(aq) reduction to Cu+(aq) of 0.15 V because of the coordination by EDTA4−(aq). Using the PZC computational method, the calculated reversible potential for [CuIEDTA]3−(aq) reduction to Cu0(ads) + EDTA4−(aq) is 0.241 V, and when the equilibrium conditions in eq 2 are used, the predicted value is −0.047 V. These values are less than the standard value for Cu+(aq) reduction (0.52 V) because of the complexation of CuI, and they differ from one another because of the different methods of calculation. It is important that the reversible potential for the second reduction step is much higher than that for the first reduction. This strongly suggests that the second reduction is instantaneous following the first and that H abstraction from the diol anion is rate-determining. The other route considered has the aqueous CuI complex adsorbed on a 3-fold site on the smooth Cu surface. The calculated adsorption bond strength is 1.187 eV, and using the PZC method, the calculated reversible potential for reducing [CuIEDTA]3−(ads) to Cu(ads) + EDTA4−(aq) is −0.946 V. Although the reversible potentials suggest diol anion oxidation during H-atom abstraction can perform all three of the above reduction steps, the reduction of [CuIEDTA]3−(ads) is predicted to take place at a potential that is too negative of the approximately −0.5 V onset of high current density in the Cu only bath shown in Figure 2. This leaves [CuIEDTA]3−(aq) as the likely intermediate in the two-electron reduction.

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CONCLUSIONS Self-consistent quantum chemical calculations strongly suggest that low-coordinated Cu atoms at the Cu surface activate C−H bond scission in the reducing agents, forming H(ads) and releasing an electron that is able to reduce in two steps the CuII complex to Cu0. During reduction, the folded [CuIIEDTA]2− complex opens up and deposits its central Cu atom on the surface. This creates a new low-coordinate Cu site, and copper deposition continues in this autocatalytic process, forming a thickening roughened film.

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SUMMARY Based on the above discussion, the stepwise mechanism for electroless Cu deposition in alkaline electrolyte using formaldehyde as the reducing agent shown in Figure 4 is proposed. Formaldehyde first reacts with OH−(aq) to produce the methyl diol anion, H2C(OH)O−(aq). The methyl diol anion is rapidly dehydrogenated on a low-coordinated Cu adatom, forming the formic acid anion, HCOOH−(aq), and H(ads). This lowcoordinated Cu adatom facilities the C−H bond-breaking process in H2C(OH)O−(aq). During the transfer of the H atom from C to Cu, the formic acid anion loses its electron to

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 216-368-5044. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.Z. thanks the Chemistry Department of Case Western Reserve University for financial support. D

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DOI: 10.1021/acs.jpcc.6b07546 J. Phys. Chem. C XXXX, XXX, XXX−XXX