ENFOQUE DE HABILIDADES DE MAX WEBER

Both MPS and BuSH adsorb onto Cu through the thiol moiety. However, the presence of the sulfonate moiety in MPS significantly increases the rate of electrochemical Cu deposition. The enhanced electroplating observed with MPS could stem from a greater permeability of Cu2+ ions through the additive monolayer. The sulfonate group of MPS might produce a layer that is less densely packed and/or more hydrophilic than that made with BuSH. To assess differences in monolayer permeability, cyclic voltammetry of the Ru3+/Ru2+ redox couple in Ru(NH3)63+/2+ was

recorded at the Cu electrode in the presence of each additive.

Figure 4.9 shows cyclic voltammograms obtained from a solution containing 100 mM H2SO4, 2 mM HCl, and 1 mM [Ru(NH3)6]3+ with no additive (black), with 2 mM MPS (red), and

with 2 mM BuSH (blue).

Figure 4.9. Cyclic voltammetry on Cu(poly) of organic additives in 100 mM H2SO4 and 2 mM

HCl with 1 mM [Ru(NH3)6]Cl3.

In the absence of MPS (black), the reversible oxidation and reduction waves of Ru are clearly visible. The presence of MPS (red) has no effect on the behavior of these peaks,

confirming that an adsorbed adlayer of MPS is permeable to the cationic complex.84 At the most negative potential, the MPS voltammogram displays capacitive current that is greater than the that of others, presumably because it has some involvement with Ru reactions that occur at lower potentials. The presence of BuSH also has no effect on the Ru redox couple, although it does somewhat inhibit Cu stripping at the most positive potentials.

The ease with which the Ru complex could be electrochemically activated in all cases suggests that neither of the additives form a densely packed, impenetrable monolayer under these conditions. Therefore, it is likely that the greater accelerating ability of MPS over BuSH lies in the interactions between Cu ions and the sulfonate group, rather than by surface blocking afforded by the putative presence of a dense, hydrophobic BuSH monolayer. High surface packing by BuSH is already likely to be constrained because of the competitive adsorption of Cl– observed in the Raman spectra (see Figure 4.4C).

Coordination between Cu ions and sulfonate has been previously observed under atmospheric conditions, forming a Cu2+-SPS polymer.48 Moreover, 3,3-thiobis(1- propanesulfonate) (TBPS), which contains a thioether rather than a thiol or disulfide, has been found to be an effective Cu deposition catalyst.85 The proposed mechanism for its acceleration relies on the trapping of Cu2+ ions by the sulfonate, which effectively dehydrates the ion and makes it susceptible to inner sphere electron transfer from adsorbed chloride when brought near the electrode surface.22 Our SHINERS data corroborate this mechanism, supporting the necessity of both functional groups. BuSO3– lacks sufficient surface adsorption to tether the

Cu2+ with surface chloride, and BuSH lacks an anionic terminus with which the Cu2+ ion could be stabilized.

4.4 Conclusions

In this work, we present a spectroscopic, theoretical, and voltammetric investigation into the accelerating properties of MPS for Cu electrodeposition and its interaction with Cl–. Through SHINERS, it has been demonstrated that the thiol functional group is responsible for the adsorption of MPS to the Cu electrode and that the sulfonate group has no significant surface adsorption. MPS and Cl– are observed to coadsorb to the Cu electrode surface, suggesting a mechanism of acceleration in which both species cooperate near the surface. DFT calculations demonstrate that charge transfer from the thiol to the electrode promotes depolarization of the Cu–Cl bond, in agreement with the observed redshift in Cu–Cl vibration frequency in the presence of MPS. Finally, cyclic voltammetry examined whether the poor Cu plating performance of BuSH could be attributed to a poorly permeable hydrophobic layer. On the contrary, adsorbed BuSH under these conditions demonstrated no inhibition toward Ru redox activity. This observation implies that acceleration of Cu plating requires direct involvement of the sulfonate group, which will be the subject of a future publication.

4.5 Acknowledgments

This work was made possible through generous support from Atotech Deutschland GmbH. K.G.S. is very grateful for the J & M Witt Fellowship and a University Fellowship award from the Graduate College of the University of Illinois at Urbana–Champaign. We thank Prof. Catherine Murphy and her students for graciously sharing their equipment and expertise for SHIN synthesis and characterization. We express gratitude to Jen Esbenshade for her assistance in obtaining SHIN TEM images.

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