First-Principle Theory to Understand Autocatalysis in Electroless Cu Deposition
Authors: Zhao Meng , Akolkar Rohan , Anderson Alfred B. 
Published: February 2020

Electroless deposition of a metal takes place without any external applied current. Electroless Cu deposition is used for protective coatings, surface finishing, and advanced nano-scale interconnect fabrication in microprocessors. The commonly used Cu electroless deposition precursor is [Cu(II)EDTA]2- which is reduced to Cu(0) by formaldehyde or glyoxylic acid in basic medium. The measured mixed potential and the deposition rate during electroless deposition are substantially different compared to the theoretical mixed potential and rate (predicted using the mixed potential theory). This indicates the interdependent relationship between the anodic and cathodic half-reactions.1 To understand the catalytic mechanisms involved in [Cu(II)EDTA]2- reduction with formaldehyde or glyoxylic acid, a comprehensive first principle approach was used. Different Cu cluster models were simulated and used in the Gaussian 09 code to calculate the activation energies for carbon-hydrogen bond breaking in hydrolyzed formaldehyde, yielding H(ads) and a formic acid anion, the proposed reducing agent. Low-coordinated surface Cu atoms were found to facilitate H abstraction. Also, we have hypothesized possible steps for anodic and cathodic half-reactions during electroless Cu deposition in base: the Cu(II) complex is reduced Cu(I) and then to Cu(0). The adsorbed H gives rise to evolution of H2(g), as observed. The reversible potentials for electron-transfer steps for reduction were calculated using Interface 1.0, a code that applies density functional theory with dielectric continuum and modified Poisson-Boltzmann theory.2 We chose three-layer thick Cu slab models for the reversible potential calculations, optimizing the structure for the layer on which the adsorption was studied. For electron-transfer steps taking place on the surface, the reversible potentials were assumed to be well-approximated as the standard reversible potentials (U
0) perturbed by adsorption Gibbs energies at the potential of zero charge (pzc). The standard reversible potentials (U
0) are for the corresponding reactions with all species in bulk solution.

In summary, we propose the following mechanism for the [Cu(II)EDTA]2- reduction to Cu(0,ads) using formaldehyde as a model: the formaldehyde reacts with H2O and OH- to give H2C(OH)O-(aq) (methanediol anion) which undergoes C-H bond breaking over low-coordinated Cu atoms to give [HCOOH]-(aq) (formic acid anion). These anions lose their hot electrons3 through space or through the Cu surface to become HCOOH (formic acid) molecules and reduce nearby Cu(II) complexes stepwise to Cu(0,ads). Due to the fact that one H atom is deposited on the surface by every C-H bond broken, and because of the weakness of Cu-H bonds, H2 can form, as is observed during the electroless deposition process. This hypothesized mechanism should also hold when glyoxylic acid is used since both reducing agents contain the aldehyde group, HRC=O , which is the key to the process since it involves breaking C-H bond and putting H on the surface, generating the active anion intermediates.

References

1. L.Yu, L. Guo, R. Preisser, and R. Akolkar, Autocatalysis during Electroless Copper Deposition using Glyoxylic Acid as Reducing Agent, J. Electrochem. Soc. 160, D3004-D3008 (2013).

2. R. Jinnouchi and A.B. Anderson, Electronic Structure Calculations of Liquid-Solid Interfaces: a Combination of Density Functional Theory and Modified Poisson-Boltzmann Theory, Phys. Rev. B., 77, 2454170-24541718 (2008).

3. J.Y. Park, S.M. Kim, H. Lee and I.I. Nedrygailov, Hot-Electron-Mediated Surface Chemistry: Toward Electronic Control of Catalytic Activity. Acc. Chem. Res. 48, 2475-2483 (2015).