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Electrochemical alloying of Cu substrates through a reduction-diffusion method was investigated using an ionic liquid, trimethyln-hexylammonium bis͓͑trifluoromethyl͒sulfonyl͔amide, as a solvent for an electrolytic bath. The use of the ionic liquid made it possible to raise the processing temperature beyond 100°C and form the Cu-Sn layers faster than with an aqueous media. The layers obtained from a Cu thin layer under a potentiostatic condition were silver-gray speculum metal composed of Cu 6 Sn 5 , Cu 3 Sn, and Cu 10 Sn 3 intermetallic phases, while those prepared under a galvanic contact condition involved a ␤-Sn phase containing a trace amount of copper. The formation of each Cu-Sn phase is discussed in terms of alloy formation thermodynamics.The electrodeposition of alloys and compounds is a key technology for thin-layer processing in the fields of, for example, surface finishing and electronic materials. Cu-Sn alloy coating with composition of 40-60 wt % Sn, called "speculum metal," is silver-gray and has been investigated as promising alternative to an allergenic nickel coating, 1-3 which is used, for example, as an underlayer for chromium or gold electroplating. The layer also has potential as a novel anode material for lithium batteries. 4-6 In both cases, an electroplating technique will provide a useful route to form the Cu-Sn layer. Although electrodeposition technology is generally considered to be a "soft solution process," the waste bath treatment sometimes involves cumbersome and complicated procedures. Especially, the treatment of conventional alloy deposition bath, entropy of which is high, because the bath contains two or more metal components, is energy-consuming in some cases. In 2003, Fujiwara 7 demonstrated that a Cu-Sn speculum metal layer can be electrochemically formed on a Cu substrate by a reduction-diffusion ͑RD͒ method using an electrochemical contact immersion, or galvanic contact, process, where the substrate is immersed into an aqueous citrate bath containing only Sn͑II͒ ions and is short circuited to an auxiliary metallic Sn electrode dipped in the same bath. Here, the Cu substrate and Sn electrode constitute a kind of short-circuited galvanic cell, where the former acts as a cathode and the latter as an anode. By applying this technique, it is thought that a Cu-Sn alloy layer may be obtained on any conductive or nonconductive substrate through electrodeposition or electroless deposition of a Cu layer from appropriate baths containing only Cu͑II͒ ions followed by the RD process to give a Cu-Sn alloy. Figure 1, for example, compares the scheme of conventional decorative chromium electroplating process using nickel underlayer to that using Cu-Sn underlayer prepared through the RD alloying on both conductive and nonconductive substrates. Although this requires two steps to obtain the Cu-Sn alloy layer, the consecutive use of baths, each containing a single-metal component, makes the waste bath treatment simple and easy.For the continuous growth of Cu-Sn alloy by the RD process, rapid...

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On page C70, right column, line 7, the value of the mean size of the crystallites should be 0.5-1 m.Electrochemical and Solid-State Letters, 9 ͑5͒ L5-L5 ͑2006͒

Electrochemical alloying of Cu substrate through a reduction-diffusion method was investigated using ionic liquid, trimethyl-n-hexylammonium bis[(trifluoromethyl)sulfonyl]amide, as a solvent for an electrolytic bath. The use of the ionic liquid rendered it possible to raise the processing temperature beyond 100 åC so as to form the Cu-Sn layers faster than in the case of an aqueous media. The layers obtained from Cu thin layer under potentiostatic condition were silver-gray speculum metal composed of Cu6Sn5, Cu3Sn, and Cu10Sn3 intermetallic phases, while those prepared under galvanic contact condition involved a β-Sn phase containing a trace amount of copper. The formation of each Cu-Sn phase was discussed in terms of thermodynamics of alloy formation.