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Copper was implemented as a conductor for advanced back-end-of-line (BEOL) interconnects based on its low electrical resistivity and improved electromigration life relative to the aluminum baseline.1, 2 Over the past two decades, the smallest dimension of Cu interconnects has decreased from about 300 to the range of 10-20 nanometers.3 Until recently, damascene Cu electroplating on PVD Cu seed has been optimized to provide void-free Cu metallization using a process relying on inorganic and organic additives toenable bottom-up growth from the base of vias and trenches.1,2,4 With ever-decreasing feature size, the conventional damascene process is becoming increasingly limited by the inability to deposit a sufficiently continuous Cu seed on a barrier/Co liner stack without resulting in pinch-off of the PVD seed. In this talk we will report on the development of direct Cu plating on Co using a range of deposition processes to illustrate the challenges and possible direction for improvement of the nucleation and subsequent growth of Cu on the Co seed. One challenge is to create a Cu layer without significantly etching the Co in the plating bath. This can be achieved by reducing the electrode potential difference between Cu electrodeposition and Co corrosion using proper additives in the plating bath to increase suppression (Figure 1). Such a large overpotential translates into improved nucleation density, which effectively inhibits the complete dissolution of the ultrathin Co layers. This effect is also illustrated by measuring the galvanic displacement reaction between Cu ions and Co at open circuit potential (OCP). Figure 2 reveals quick deposition of a monolayer equivalent of Cu followed by a slight increase in Cu thickness over time. A second challenge involves generation of dense nucleation which results in a smooth initially plated Cu film. The number of protrusions is reduced noticeably when a potential-controlled entry is replaced by leaving the electrode at OCP for a short time before any current is applied (Figures 3a and 3b). A short OCP time allows Co oxides to first dissolve, thereby making more surface sites available for nucleation. The smoothness can be further enhanced by increasing the acidity of the bath (Figure 3c). Higher acid could have modulated the reactivities of additive(s) to promote lateral growth. Improved nucleation can also be attained through substrate engineering. This is shown in Figure 4(a-c), where the morphology/continuity of the electroless film exhibits a strong dependency on Co thickness. Furthermore, agglomeration caused by diffusion of Cu across the substrate during deposition can have a large effect on the final film morphology. This can be seen in figure 4 (d-f), in which differences in deposition morphology across a range of conditions are captured in a simple lateral diffusion simulation. We will discuss the mechanisms leading to these effects and their impact on the design of systems to produce improved film morphology. References (1) P.C. Andricacos, C. Uzoh, J. O. Dukovic, J. Horkans, H. Deligianni, IBM J. Res. Develop. 42, 567 (1998) (2) J. Reid, Jpn. J. Appl. Phys. 40, 2650 (2001) (3) I. Ciofi, P. J. Roussel, R. Baert, A. Contino, A. Gupta, K. Croes, C. J. Wilson, D. Mocuta, Z. Tokei, IEEE Trans. Electron. Devices, 66, 2339 (2019). (4) T.P. Moffat, D. Wheeler, S. K. Kim, D. Josell, J. Electrochem. Soc. 153, C127, (2006) Figure 1

Concerns about the extendibility of copper as an interconnect conductor for future technology nodes have resulted in increasing interest in alternative conductor metals.1 With ever-decreasing interconnect dimensions, alternative metals bring hope of improved resistivity and reliability through more favorable electron scattering and cohesive energy and the possibility of improved scaling through barrier layer removal.2,3 Cobalt, in particular, has been the subject of several recent studies and has already found its way into at least one major semiconductor manufacturer’s scheme as a replacement for tungsten in trench contacts.4 Considerable work has focused on understanding the cobalt electrofill process to enable its implementation in current interconnect technology.5–8 While conventional copper damascene plating uses a three-additive system, cobalt superfill can be achieved while using only one organic component, with the hydrogen evolution reaction having been shown to also play a role.5,7,8 Recently, we have demonstrated that boric acid is also an important component of cobalt plating baths.6 As shown in Figure 1, boric acid forms a series of polyborate species in solution as a function of both concentration and pH. Presence of the polyborates is correlated with pH buffering under typical cobalt electroplating conditions, preventing precipitation of cobalt hydroxide and improving nucleation within the features. Just as the cobalt electrofill process represents a departure from the established damascene copper plating techniques, development of processes for other alternative metals for future technology nodes will require new and creative formulations and methods to overcome electroplating challenges for these metals. K. Lin, M. Chandhok, and M. Reshotko, in 2018 IEEE International Interconnect Technology Conference (IITC), p. 2–3, IEEE, Santa Clara, CA, USA (2018) https://ieeexplore.ieee.org/document/8430404/. D. Josell, S. H. Brongersma, and Z. Tőkei, Annu. Rev. Mater. Res., 39, 231–254 (2009). K. Sankaran, S. Clima, M. Mees, and G. Pourtois, ECS J. Solid State Sci. Technol., 4, N3127–N3133 (2015). J. Yoshida, EETimes (2020) https://www.eetimes.com/apple-huawei-use-tsmc-but-their-7nm-socs-are-different/. M. A. Rigsby et al., J. Electrochem. Soc., 166, D3167–D3174 (2019). M. A. Rigsby, T. A. Spurlin, and J. D. Reid, J. Electrochem. Soc., 167, 112507 (2020). J. Wu, F. Wafula, S. Branagan, H. Suzuki, and J. van Eisden, J. Electrochem. Soc., 166, D3136–D3141 (2019). C. H. Lee, J. E. Bonevich, J. E. Davies, and T. P. Moffat, J. Electrochem. Soc., 156, D301 (2009). Figure 1: Raman spectra showing speciation of (a) 10 g/L and (b) 40 g/L boric acid in water as a function of pH.6 Figure 1

A variety of metal oxides can catalyze the oxidation of water to molecular oxygen when polarized by a sufficiently high electrochemical potential. Minimizing the overpotential and increasing the rate of the oxygen-evolving reaction (OER) are key goals in making such materials a component of viable energy storage devices. However, the structural factors that imbue the metal oxides with their catalytic power are difficult to assess as these solids contain many distinct metal-ion sites, have a varying amount of defect sites within the lattice, and can be composed of multiple phases. In the present study, we determined the magnetic properties for a series of dimeric cobalt complexes in which the two metal centers are bridged by a dioxygen moiety. Our spectroscopically validated electronic structure description indicates that each species is best described as two Co(III) ions that are bound to a μ–η1η1 superoxide ligand. Intriguingly, we found evidence that the two compounds that possess oxygen-evolving activi...

In traditional copper damascene interconnect electroplating baths, three organic additives (accelerator, suppressor, leveler) are used to produce void-free superconformal fill. Here, a different mechanism for electroplating void-free cobalt interconnects is described. Superconformal void-free cobalt fill is generated using a sacrificial oxidant (H+) and the addition of a single suppressor-type additive. A differential plating rate on the field and feature bottom can be obtained through process tuning involving waveform, mass transport conditions, pH, and suppressor concentration.

Palladium(II)-catalyzed aerobic oxidative cyclization of alkenes with tethered tert-butanesulfinamides furnishes enantiopure 2,5-disubstituted pyrrolidines, originating from readily available and easily diversified starting materials. These reactions are the first reported examples of metal-catalyzed addition of sulfinamide nucleophiles to alkenes.

Analysis of the surface is paramount to understanding the reactivity, selectivity, and catalytic ability of substances. In particular, this understanding is required to make an efficient use of the catalytic surfaces in fuel cells. X-ray photoelectron spectroscopy (XPS) allows determination of changes in the electronic structure for different surface preparation and composition based, mainly, on shifts of the binding energies of core-level electrons. It is also an ideal method that allows identification of the surface or near surface species in relation to fuel cell catalysis. However, the fundamental theoretical concepts, which are used to analyze and interpret XPS spectra are sometimes not correctly understood or correctly applied. In this review, we not only report on XPS operational parameters in use for fuel cell electrocatalysis, but, more significantly, we review and provide rigorous definitions of fundamental concepts used to understand XPS spectra, including the separation of initial and final state effects and the relaxation of valence electrons to screen core-holes. An additional direction of our review is to show the relationships between XPS binding energy shifts and XPS satellite structure with chemical bonding and chemical interactions. However, our primary concern is to provide reviews of representative cases of the application of XPS to solving fuel cell and electrocatalysis-related problems, highlighting progress in this laboratory. We begin with descriptions of essential issues in fuel cell science and with a review of key concepts of XPS. Then, we briefly report on the XPS instrumentation, after which, studies of fundamental importance to electrochemical processes are reviewed. This review includes an overview of complex organic and biological systems in relation to fuel cell electrocatalysis (probed via XPS). We conclude with a discussion of modern developments in XPS methodology.

A brief review of the field of underpotential deposition (UPD) processes in the theory/modeling perspective is given, and new insights into the UPD of copper and silver on Rh(1 1 1) and Rh(1 0 0) in perchloric and sulfuric acid media are reported. Voltammetric results show a clear dependence of the UPD shift on Rh substrate geometry and on the type of the electrolyte used. For copper, we identified one peak on Rh(1 0 0) and two peaks on Rh(1 1 1), separated by 210 and 350 mV in perchloric and sulfuric electrolytes, respectively. Preliminary XPS experiments were performed to investigate some specific aspects of copper deposition processes. Overall, we report a strong effect of the anion adsorption on positions of Cu UPD peaks, with the effect being more noticeable on Rh(1 1 1) than on Rh(1 0 0). In the experiments with silver on Rh(1 1 1), multiple peaks were obtained, whereas no Ag UPD peak was found either on Rh(1 0 0) or on polycrystalline Rh. In addition, we found that the Ag UPD process on Rh(1 1 1) is dependent on silver concentration. As indicated in the paper Tables, except for Cu UPD on Rh(1 0 0), there is a major deviation from the linear relationship between the UPD shift and the difference between the work function of the substrate and that for the underpotentially deposited adatoms.

This study seeks to explore the effects of the electronic structure of Pt/Ru alloy nanoparticles on reactivity of small organic molecules of relevance to fuel cell applications through the combined use of synchrotron radiation photoelectron spectroscopy and electrochemistry. Platinum core-level binding energies were found to increase linearly with the addition of ruthenium. This effect is a product of lattice strain and charge transfer, and is explained in terms of the d-band center theory proposed by Norskov and co-workers (Hammer, B.; Norskov, J. K. Surf. Sci. 1995, 343, 211). In the course of the study of electrooxidation of methanol we have found that it is very difficult, if not impossible, to separate the effects of the bifunctional mechanism and the electronic structure effects that might play a role in the activity. However, data for electrooxidation of formic acid, when studied at a short reaction time (where the indirect reaction pathways and poisoning intermediates are assumed to play a negligi...

The surface structure, stability, and electrocatalytic activity of the Pt3Co alloy toward oxygen reduction reaction (ORR) (with reference to Pt black) in both alkaline and acidic media were studied by X-ray photoelectron spectroscopy (XPS), cyclic voltammetry, and by the rotating disk electrode method. The XPS data show that two species of cobalt, metallic cobalt and cobalt oxides, coexist in the as-received Pt3Co sample. In combination with the voltammetric results, the XPS data demonstrate that there is dissolution of cobalt upon exposure of the sample to acidic media, whereas the cobalt forms in the catalyst are stable in alkaline media. In addition, the Pt 4f binding energy was found by XPS to be upshifted by 0.2 eV vs that of the pure Pt black reference. In acids, Pt3Co alloy has a slightly but demonstratively higher activity than Pt black toward the oxygen reduction reaction as shown by the positive half-wave potential shift of ca. 10 ± 2 mV, while in alkaline media a decrease in the ORR activity wa...

The manganese(III)-bis[poly(pyrazolyl)borate] complexes, Mn(pzb)2SbF6, where pzb- = tetrakis(pyrazolyl)borate (pzTp) (1), hydrotris(pyrazolyl)borate (Tp) (2), or hydrotris(3,5-dimethylpyrazolyl)borate (Tp*) (3), have been synthesized by oxidation of the corresponding Mn(pzb)2 compounds with NOSbF6. The Mn(III) complexes are low-spin in solution and the solid state (μeff = 2.9−3.8 μB). X-ray crystallography confirms their uncommon low-spin character. The close conformity of mean Mn−N distances of 1.974(4), 1.984(5), and 1.996(4) A in 1, 2, and 3, respectively, indicates absence of the characteristic Jahn−Teller distortion of a high-spin d4 center. N−Mn−N bite angles of slightly less than 90o within the facially coordinated pzb- ligands produce a small trigonal distortion and effective D3d symmetry in 1 and 2. These angles increase to 90.0(4)o in 3, yielding an almost perfectly octahedral disposition of N donors in Mn(Tp*)2+. Examination of structural data from 23 metal−bis(pzb) complexes reveals systematic...

Several substituted iron-porphyrin complexes were evaluated for oxygen reduction reaction (ORR) electrocatalysis in different homogeneous and heterogeneous media. The selectivity for four-electron reduction to H2O versus two-electron reduction to H2O2 varies substantially from one medium to another for a given catalyst. In many cases, the influence of the medium in which the catalyst is evaluated has a larger effect on the observed selectivity than the factors attributable to chemical modification of the catalyst. For instance, introduction of potential proton relays has variable effects depending on the catalyst medium. Thus, comparisons of selectivity results from supported and soluble molecular ORR electrocatalysts must be interpreted with caution, as selectivity is a property not only of the catalyst, but also of the larger mesoscale environment beyond the catalyst. Still, in all the direct pairwise comparisons in the same medium, the catalysts with potential proton relays have similar or better selectivity for the preferred 4e(-) path.

A modular synthetic method has been developed for the preparation of Ru polypyridyl complexes bearing a terminal alkyne. This method proceeds through a readily accessible intermediate with a silyl-protected alkyne and allows access to a variety of five- and six-coordinate Ru complexes. These complexes can be easily attached to azide-functionalized electrode surfaces with only slight perturbation of the redox properties of the parent complex.

The products are obtained in an enantiopure form with mostly excellent diastereoselectivities.

Highly doped diamond films are new candidates for electrodes in reactive environments, such as electrocatalytic interfaces. Here the electronic structure of such films is investigated by X-ray absorption spectroscopy at the C 1s and B 1s edges, combined with X-ray and ultraviolet photoelectron spectroscopy, as well as optical measurements. A diamond surface functionalized covalently with Ru(tpy) 2, a model complex similar to ruthenium-based molecules used in photocatalysis and photovoltaics, is compared to a hydrogen-terminated diamond surface as a reference. Bulk-sensitive absorption spectra with photon detection reveal diamond gap states, while surface-sensitive spectra with electron detection reveal the adsorbate states and π-bonding at the diamond surface. The positions of the frontier orbitals of the dye relative to the band edges of diamond are inferred from the spectroscopic data. The implications of using diamond films as inert electron donors in photocatalysis and dye-sensitized solar cells are discussed. © 2012 American Chemical Society., This work was supported by the NSF with the awards CHE-1026245 and DMR-0537588 (SRC), by the DOE under the contracts DE-FG02-01ER45917 (end station) and DE-AC02-05CH11231 (ALS), by the Spanish Ministerio de Economia y Competitividad (MAT2010-21156-C03-01, C03-03, PIB2010US-00652), and by the Basque Government (IT-257-07). R.E.R. and R.J.H. acknowledge support from the NSF with grants CHE-0613010 and CHE-0911543.

Building upon recent study of cobalt-oxide electrocatalysts in fluoride-buffered electrolyte at pH 3.4, we have undertaken a mechanistic investigation of cobalt-catalyzed water oxidation in aqueous buffering electrolytes from pH 0-14. This work includes electrokinetic studies, cyclic voltammetric analysis, and electron paramagnetic resonance (EPR) spectroscopic studies. The results illuminate a set of interrelated mechanisms for electrochemical water oxidation in alkaline, neutral, and acidic media with electrodeposited Co-oxide catalyst films (CoO(x)(cf)s) as well as for a homogeneous Co-catalyzed electrochemical water oxidation reaction. Analysis of the pH dependence of quasi-reversible features in cyclic voltammograms of the CoO(x)(cf)s provides the basis for a Pourbaix diagram that closely resembles a Pourbaix diagram derived from thermodynamic free energies of formation for a family of Co-based layered materials. Below pH 3, a shift from heterogeneous catalysis producing O(2) to homogeneous catalysis yielding H(2)O(2) is observed. Collectively, the results reported here provide a foundation for understanding the structure, stability, and catalytic activity of aqueous cobalt electrocatalysts for water oxidation.

We demonstrate a modular "click"-based functionalization scheme that allows inexpensive conductive diamond samples to serve as an ultrastable platform for surface-tethered electrochemically active molecules stable out to ∼1.3 V vs Ag/AgCl. We have cycled surface-tethered Ru(tpy)(2) to this potential more than 1 million times with little or no degradation in propylene carbonate and only slightly reduced stability in water and acetonitrile.

The manganese(III)-bis[poly(pyrazolyl)borate] complexes, Mn(pzb)2SbF6, where pzb- = tetrakis(pyrazolyl)borate (pzTp) (1), hydrotris(pyrazolyl)borate (Tp) (2), or hydrotris(3,5-dimethylpyrazolyl)borate (Tp*) (3), have been synthesized by oxidation of the corresponding Mn(pzb)2 compounds with NOSbF6. The Mn(III) complexes are low-spin in solution and the solid state (microeff = 2.9-3.8 microB). X-ray crystallography confirms their uncommon low-spin character. The close conformity of mean Mn-N distances of 1.974(4), 1.984(5), and 1.996(4) A in 1, 2, and 3, respectively, indicates absence of the characteristic Jahn-Teller distortion of a high-spin d4 center. N-Mn-N bite angles of slightly less than 90 degrees within the facially coordinated pzb- ligands produce a small trigonal distortion and effective D3d symmetry in 1 and 2. These angles increase to 90.0(4)degrees in 3, yielding an almost perfectly octahedral disposition of N donors in Mn(Tp*)2+. Examination of structural data from 23 metal-bis(pzb) complexes reveals systematic changes within the metal-(pyrazolyl)borate framework as the ligand is changed from pzTp to Tp to Tp*. These deformations consist of significant increases in M-N-N, N-B-N, and N-N-B angles and a minimal increase in Mn-N distance as a consequence of the steric demands of the 3-methyl groups. Less effective overlap of pyrazole lone pairs with metal atom orbitals resulting from the M-N-N angular displacement is suggested to contribute to the lower ligand field strength of Tp* complexes. Mn(pzb)2+ complexes undergo electrochemical reduction and oxidation in CH3CN. The electrochemical rate constant (ks,h) for reduction of t2g4 Mn(pzb)2+ to t2g3eg2 Mn(pzb)2 (a coupled electron-transfer and spin-crossover reaction) is 1-2 orders of magnitude smaller than that for oxidation of t2g4 Mn(pzb)2+ to t2g3 Mn(pzb)22+. ks,h values decrease as Tp*pzTpTp for the Mn(pzb)2+/0 electrode reactions, which contrasts with the behavior of the comparable Fe(pzb)2+/0 and Co(pzb)2+/0 couples.

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Several binuclear cobalt(III) complexes that mimic Ru-based water oxidation catalysts have been prepared. The initial complexes exhibited thermodynamic instability and kinetic lability that complicated efforts to use these cobalt complexes as electrocatalysts for water oxidation. Binuclear cobalt(III) complexes supported by a bridging bispyridylpyrazolate (bpp) ligand overcome these limitations. Two bpp-ligated dicobalt(III)-peroxo complexes were prepared and structurally characterized, and electrochemical investigation of these complexes supports their ability to serve as molecular electrocatalysts for water oxidation under acidic conditions (pH 2.1).

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The nature of Cu and Ag underpotentially-deposited on Rh(111) was investigated by X-ray photoelectron spectroscopy (XPS). These underpotential deposition (UPD) systems present multipeak features, which were evaluated by cyclic voltammetry and in terms of binding energy shifts. The use of different electrolytes (sulfuric and perchloric acid) strongly affected the metallic coverage and slightly affected the UPD shifts. Through XPS, the examined surfaces were found to contain C, O, and S or Cl, which confirms the presence of anions and/or products of their reduction at the Rh(111) surface, in addition to the deposited metals. The binding energies for underpotentially-deposited Cu and Ag displayed negative shifts (to lower binding energy) with respect to both the bulk deposits and clean metal references. These negative shifts were discussed in terms of contributions from charge transfer, orbital rehybridization, and lattice strain.

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