Electrochemical metallization

L. E. Vaaler and co-workers, BatteUe Columbus Laboratories, Final Report on a Survey of Electrochemical Metal WinningProcesses, ANL/OEPM-79-3, Argonne National Laboratory, Mar. 1979. Available from National Technical Information Service, Washington, D.C. 

Table 2-1 Conversion factors and standard potentials for electrochemical metal-metal ion reactions...

Figure 4.3 Schematic diagram of the electrochemical metal—aqueous interface, with the electrode, inner layer, diffuse layer, outer Helmholtz plane (OHP), and inner-layer thickness Xji indicated.

Electrochemical reactions are driven by the potential difference at the solid liquid interface, which is established by the electrochemical double layer composed, in a simple case, of water and two types of counter ions. Thus, provided the electrochemical interface is preserved upon emersion and transfer, one always has to deal with a complex coadsorption experiment. In contrast to the solid/vacuum interface, where for instance metal adsorption can be studied by evaporating a metal onto the surface, electrochemical metal deposition is always a coadsorption of metal ions, counter ions, and probably water dipols, which together cause the potential difference at the surface. This complex situation has to be taken into account when interpreting XPS data of emersed electrode surfaces in terms of chemical shifts or binding energies. 

Given the tremendous development of SAMs over the past two decades it is dear that this chapter is able to cover only a fraction of the spectrum of topics related to the combination of SAMs and electrochemistry. For a comprehensive picture the reader is referred to a number of additional review articles, one of which is the excellent and extensive account of organized monolayers on electrodes by Finklea [23]. Besides this one, which comprehensively covers the literature up to the mid-1990s, other more focused reviews are available that address various developments over the past decade in areas of sensor development and electroanalytical applications [22, 24—28] and electrochemical metal deposition on SAM-modified electrodes [29, 30]. 

As far as SAM-controlled electrochemical metal deposition is concerned, substantial interest derives from microelectronics with its need to control the generation of interconnects and, thus, to understand the influence of organic layers on the metal nucleation and growth. However, the scope of this topic reaches well bqfond that, as illustrated by the substantial range of potential applications where small-scaled metal structures are of crucial importance, for example in electrochemical [31] and optical [32] sensing, molecular electronics [33], for plasmonics [34], or as metamaterials [35]. 

The possibilities afforded by SAM-controlled electrochemical metal deposition were already demonstrated some time ago by Sondag-Huethorst et al. [36] who used patterned SAMs as templates to deposit metal structures with line widths below 100 nm. While this initial work illustrated the potential of SAM-controlled deposition on the nanometer scale further activities towards technological exploitation have been surprisingly moderate and mostly concerned with basic studies on metal deposition on uniform, alkane thiol-based SAMs [37-40] that have been extended in more recent years to aromatic thiols [41-43]. A major reason for the slow development of this area is that electrochemical metal deposition with, in principle, the advantage of better control via the electrochemical potential compared to none-lectrochemical methods such as electroless metal deposition or evaporation, is quite critical in conjunction with SAMs. Relying on their ability to act as barriers for charge transfer and particle diffusion, the minimization of defects in and control of the structural quality of SAMs are key to their performance and set the limits for their nanotechnological applications. 

Among the factors crucial for a SAM structure, the thiol-substrate bond has been a subject of particular controversy over more than 15 years. It is worth discussing the SAM/substrate interface in some detail since it is important not only for understanding SAM structures in general but also for electrochemical metal deposition when metal is intercalated at the SAM/Au interface (Section 5.4.3). As indicated above we limit the discussion to SAMs on Au(l 11) as this is the interface where theoretical and experimental work is sufficiently detailed to allow for a discussion at the atomic level. 

Table 5.1 Compilation of thiol SAMs for which electrochemical metal deposition has been studied. 

Figure 5.13 (a) Potential drop across a SAM and the adjacent double layer (DL) in a region free of defects shown in top part. Solid and dashed lines are two examples of different positions of the Fermi level Ep. Dash-dot line indicates the course of an equipotential line representing the potential at the outer surface of the SAM including defects, (b) Illustration of processes involved in electrochemical metal deposition. For details, see text. 

Figure 5.23 Electrochemical Cu deposition onto a SAM of MBP12 on Au. (a) Patterning scheme involving patterning by electron irradiation through a mask and subsequent localized electrochemical metal deposition, (b) optical micrographs (right) and SEM image (left) of...

Electrodeposition of Cu for IC fabrication has been used successfully since 1997 for the production of interconnection lines down to 0.20 )Lim width. Electrochemical metal deposition methods represent a very attractive alternative to the conventional IC fabrication processes (33). Development of electrochemical deposition technology for IC fabrication also represents an excellent opportunity for the electrochemists community. This opportunity stems from the fact that new electrochemical deposition processes, producing deposits of different structure and properties, are needed to meet requirements of new, sub-micrometer-range computer technologies. 

B. E. Conway and J. O M. Bockris, Proc. Roy. Soc. London 248A 394 (1958). Calculations on competing steps in electrochemical metal deposition. 

Andco Environmental Processes, Inc., "Laboratory Test Results on Electrochemical Metal Removal."... 

Electrodeposition of Cu for IC fabrication has been successfully used since 1997 for the production of interconnection lines down to 0.20 /tun width. Electrochemical metal deposition methods represent a very attractive alternative to the conventional IC fabrication processes (33). 

Stripping-based electrochemical metal sensors for environmental monitoring... 

F. Darain, S.-U. Park and Y.-B. Shim, Disposable amperometric immunosensor system for rabbit IgG using a conducting polymer modified screen-printed electrode, Biosens. Bioelectron., 18 (2003) 773-780. M. Dequaire, C. Degrand and B. Limoges, An electrochemical metal-loimmunoassay based on a colloidal gold label, Anal. Chem., 72 (2000) 5521-5528. 

Formation of the tribofilm layer on friction surfaces occurs under the effect of the field in the electrochemical metal,-lubricant-metal2 system, owing to formation of electro-potential (emf), forming free copper tribofilm (Shpenkov, 1995a). Since the process of tribofilm formation takes place during the friction process, disintegration of the reverse micelles takes place in a tribochemical reaction, where a redox reaction occurs, and copper oxide reduces to free copper. 

The ability to switch the operation of electrochemical metal sensors between active and passive modes on demand offers substantial improvements in their stability in the presence of common surfactants, as demonstrated in stripping-voltammetric signals obtained from cadmium in the presence of gelatin and Tween 80. Bare electrodes display a substantial diminution of the cadmium peak in the presence of both surfactants. In contrast, the adaptive-nanowire electrode system exhibits a highly stable response with a negligible change of the peak current over multiple measurements. 

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