Selective metal deposition

Scheibel T, Parthasarathy R, Sawicki G, Lin X-M, Jaeger H, Lindquist SL. Conducting nanowires built by controlled self-assembly of amyloid fibers and selective metal deposition. Proc Natl Acad Sci USA 2003 100 4527-4532. 

Electrodeposition of metals on semiconductor surfaces has been used by Allongue et al. to form nearly ideal Schottky barriers on GaAs [6.176] and InP [6.177], and to stabilize photoelectrodes with ultrathin and transparent metal films [6.174, 6.178, 6.179]. Selective metal deposition has also been performed to reveal p-n junctions and transistors on silicon chips [6.180]. 

When an n-type semiconductor which is in contact with a metal ion-containing electrolyte is illuminated, then two equal partial currents occur under open-circuit conditions (Fig. 11.25a). The anodic photocurrent is due to O2 formation in H2O, whereas the cathodic partial current corresponds to the reduction of the metallic ions. Since the holes cannot diffuse very far, most of them collect at the illuminated interface. In the case of an n-type semiconductor, sufficient electrons are availabe everywhere, so that metal deposition should occur at illuminated as well as at dark surface sites (Fig. 11.25b), according to which conclusion, selective deposition would be impossible. Experimentally, however, selective metal deposition has been observed, e.g. at CdS at illuminated surfaces [130] and at Ti02 at the dark sites [131]. In the case of CdS, this phenomenon was interpreted as a downward shift of the energy bands at the illuminated surface which is more favorable to an electron transfer there [130]. The result obtained with Ti02 has been explained by strong internal and external recombination... 

The second step of the processing sequence is visuahzed in Figure 2.83. After pore formation by current oscillation, the pores are deepened into the substrate and, subsequently, site-selective metal deposition into the pores is carried out... 

Ishii, D., Yabu, H., Shimomura, M. Selective metal deposition in hydrophobic porous cavities of self-organized honeycomb-patterned polymer films by all-wet electroless plating. Colloids Surf. A Physicochem. Eng. Asp. 313-314, 590-594 (2008)... 

Selective metal deposition is of interest in several applications such as the formation of conduction patterns for integrated circuits and semiconductor devices. Instead of depositing a complete metal film and producing the pattern by selective etching, there is the interesting goal of forming the pattern directly by photodeposition. The basic concept of the procedure was already developed 25 years... 

Experimentally, however, selective metal deposition has been observed, for example at CdS at illuminated surfaces [133] and at TiOj at the dark sites [134]. In the case of CdS, this phenomenon was interpreted as a downward shift of the energy bands at the illuminated surface which is more favorable to an electron... 

The ligand-based chemistry also has several attractive features for use as an advanced metallization process where high throughput, adhesive, selective metallization on a variety of substrates is crucial (55). The versatility of the process with regard to surface attachment of UTFs to most key technological materials such as polymers, plastics, and diamond has already been demonstrated 20, 21, 34). The elimination of an acceleration step simplifies and reduces the overall cost of the EL metallization process also, the selective metal deposition is... 

Two examples of selective metal deposition follow. Parallel cylindrical PS-b-PMMA is treated with a palladium colloid, which selectively deposits on top of the polystyrene domains.Electroless deposition of nickel then forms on the palladium particles. Alternatively, PS-b-P4VP with cylindrical P4VP aligned parallel to the substrate is soaked in a weakly acidic aqueous solution of metal salts such as gold chloride, sodium tetrachloroplatinate, and sodium tetrachloropalla-date. ° ° A plasma etch removes the polymer film and reduces the metal patterned in the P4VP domains. 

Cladding may be less expensive than selective electro deposition when coatings greater than 1 p.m of a noble metal are required, but may be more expensive than electro deposition for thinner coatings. Selective techniques are most easily used for sheet metal substrates that are to be machine stamped and formed into contacts. Clad noble metals are considerably more ductile (and less hard) than comparable electro deposits and, therefore, are better suited to forming operations. Contacts that are made into separate parts from rod by screw machining are usually coated on all exposed surfaces by barrel electroplating. 

Reference 38 is a good guide to the selection of plate thickness test methods. Test methods may vary with the purity and electrochemical activity of the deposit. Metals deposited from commercial plating solutions are seldom pure. For example, zinc deposits from the three commonly used baths, ie, cyanide, chloride, and zincate, vary significantly in purity and activity (39). Standard ASTM test methods for determining plate thickness are... 

Generally speaking, homogeneous nucleation needs the supersaturation level higher than heterogeneous one. In the system consisting of support solid and metal salt solution, the nucleation occurs on the surfaces of the solid. The selective reductive deposition is performed by the adsorption of metal ion or complexes on the surfaces and hereby the reduction. Namely, the initial adsorption of metal ions or complexes is the key point of this technique. Hence, key points of this method are... 

Average content (in parts per million or percent) of selected metals and SIOt in 37 hydrothermal sulfide samples from the Sunrise deposit (lizasa et ah, 1999)... 

Electrogravimetry is one of the oldest electroanalytical methods and generally consists in the selective cathodic deposition of the analyte metal on an electrode (usually platinum), followed by weighing. Although preferably high, the current efficiency does not need to be 100%, provided that the electrodeposition is complete, i.e., exhaustive electrolysis of the metal of interest this contrasts with coulometry, which in addition to exhaustive electrolysis requires 100% current efficiency. 

This technique allows the selective electro-deposition of a metal from a solution in the presence of ions of a less noble metal, provided that there is a sufficient difference between their standard reduction potentials the latter condition suggests remaining on the safe side (less negative) with the cathodic potential, so that the analysis may lose much in velocity on the other hand, the simplicity of procedure and apparatus is an advantage. 

Focusing on selected aspects, a few reviews have covered SAM-controlled electrometallization [29], [30, 183]. Reviewing work published over the past 15 years, the following sections summarize the current state-of-the-art and discuss the different routes currently pursued. For this purpose we first start with a brief account of metal deposition on a clean metal substrate [29]... 

A variation of the selective Cu deposition process, limited to electroless Cu deposition, is the lift-ojfprocess, a planarized metallization process (16). Figure 19.5 shows a stepwise process sequence for this technology. 

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