Deposition of Alloys

In the sputtering process, each surface atomic layer is removed consecutively. If there is no diffusion in the target, the composition of the vapor flux leaving the surface is the same as the composition of the bulk of the material being sputtered, even though the composition of the surface may be different from the bulk. This allows the sputter deposition of alloy compositions, which can not be thermally vaporized as the alloy because of the greatly differing vapor pressures of the alloy constituents. 

Electrodeposition and Vacuum Deposition of Alloys, U. S. Bureau of Mines, Rept. Invst. No. 7308,... 

For practical as well as fundamental reasons, there has been considerable interest in the deposition of alloys containing metals such as W, Mo, and Sn. In their pure forms, these metals do not catalyze the oxidation of the usual electroless reducing agents. Therefore, their mechanism of codeposition is intriguing, and developing an understanding of it should help to better understand the mechanism of electroless deposition as whole. Obvious questions in ternary and quaternary alloy deposition include the effect of the third or fourth element containing ions in solution on the rate of electroless deposition, as well as on the P and B contents in the case of alloys such as Ni-P and Ni-B. 

Figure 11.1. Polarization curves for the deposition of alloys. (From Science and Technology of Surface Coating, a NATO Advanced Study Institute publication, 1974, with permission from Academic Press.)...

One recognizes three main steps in the cathodic deposition of alloys or single metals ... 

The approaches described previously assume that reactions will be suppressed, without giving any specific mechanism, and then rationalise the behaviour of the process using calculated metastable equilibria. A more innovative approach was taken by Saunders (1984) and Saunders and Miodownik (1985, 1987) for the prediction of phases formed by vapour co-deposition of alloys. It was postulated that the formation of phases on the substrate is controlled by the diffiisional breakdown of iiiUy intermixed depositing atoms so that three kinetic regimes are observed ... 

In the last 10 years, the electrodeposition of the new Zn-Me alloys and ternary alloys was studied. The influence of plating conditions and different additives for deposition of alloys of Zn with iron group metals was studied in detail. 

In the normal deposition of alloys, the more positive (noble) metal deposits preferentially. The result of this is that... 

In anomalous deposition of alloys the opposite equation is valid... 

In this section, the process of electrodeposition is reviewed briefly, and its place in the general context of electrode reactions and charge transfer across the metal/solution interface is set (Section 1.1). In Section 1.2, special emphasis is given to deposition of alloys, and particularly to anomalous deposition of alloys (Sections 1.2.3 and 1.2.4). Next, the phenomenon of induced codeposition is defined, and possible mechanisms are discussed briefly (Section 1.2.5). Several electroless (Section 1.2.6) and electrodeposition processes, in which induced codeposition plays a role, are mentioned. A more extensive discussion of electrodeposition of W-, Mo- and Re-based alloys is included in Section 2. Typical... 

Employ reactive sputter-deposition of alloys and oxides. 

Finally, an example for the low-temperature deposition of alloy films is the production of Ni/Fe films of the permalloy type by pyrolysis of Ni and Fe carbonyl mixtures on glass at a substrate temperature of 300°C [192,193]. 

III. ELECTROLYTIC DEPOSITION OF ALLOYS FROM AQUEOUS SOLUTIONS. 

Some of the methods using more sophisticated equipment for the deposition of Pd or Pd/alloy on porous substrates include, among others [7], the metal-organic chemical vapor deposition (MOCVD) [8, 9], and electron beam evaporation and ion-beam sputtering [10]. On the other hand, electroless plating uses relatively simple equipment. The techniques using sophisticated equipment are quite powerful and especially useful for the deposition of alloy films. However, their major drawbacks are low area of the prepared membranes, difficult for large scale production and/or... 

An alternative approach to deposition of alloys could be to put down layers of pure metals and use a diffusion anneal. Unfortunately, this complicates the process and reduces the overall deposition rate. 

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