Autocatalytic deposition process

This can be accomplished by means of two different processes (1) an electrodeposition process in which z electrons (e) are provided by an external power supply, and (2) an electroless (autocatalytic) deposition process in which a reducing agent in the solution is the electron source (no external power supply is involved). These two processes, electrodeposition and electroless deposition, constitute the electrochemical deposition. In this book we treat both of these processes. In either case our interest is in a metal electrode in contact with an aqueous ionic solution. Deposition reaction presented by Eq. (1.1) is a reaction of charged particles at the interface between a solid metal electrode and a liquid solution. The two types of charged particles, a metal ion and an electron, can cross the interface. 

In consideration of the so-called autocatalytic deposition processes, the following facts must be taken into consideration. 

H2 evolution. The phosphorous content in the NiP deposit was about 16% by mass, which is a result of the phosphite reduction. At the glass frit, NiP deposition also started, showing that there is no need of a metallic surface to initiate the autocatalytic deposition process. This observation can be attributed to the nickel hydroxide species adsorbed at the glass surface, which is obviously a suitable catalyst for the initiation of the deposition process. 

Better results can be obtained for the electroless deposition (ELD) in solutions. In this case the protective ligands can probably be dissolved in the liquid phase and thus diffuse away from the seeds. Of the several deposition baths considered, each consisting of a soluble gold salt and reducing reagent, only the one based on the sulfite complex of Au and hydroxylamine resulted in dense and relatively thick (up to 800 nm) gold layers. The reason for this behaviour remains unclear but there are hints that complex reactions are taking place in the autocatalytic deposition process. 

Electroless Electrolytic Plating. In electroless or autocatalytic plating, no external voltage/current source is required (21). The voltage/current is suppHed by the chemical reduction of an agent at the deposit surface. The reduction reaction must be catalyzed, and often boron or phosphoms is used as the catalyst. Materials that are commonly deposited by electroless plating (qv) are Ni, Cu, Au, Pd, Pt, Ag, Co, and Ni—Fe (permalloy). In order to initiate the electroless deposition process, a catalyst must be present on the surface. A common catalyst for electroless nickel is tin. Often an accelerator is needed to remove the protective coat on the catalysis and start the reaction. 

For articles that require only a very thin film of tin, seldom exceeding 0.8 p.m immersion tin coatings are appHed. The process is based on chemical displacement by immersion in a solution of tin salts. Recently, a new autocatalytic tin-deposition process was developed at the research level. It promises to be useful to coat any base material including plastics, in addition to providing coatings of any thickness desired (21). 

O Sullivan describes the fundamental theory, mechanistic aspects and practical issues associated with autocatalytic electroless metal deposition processes. Current approaches for gaining fundamental understanding of this complex process are described, along with results for copper, nickel and various alloys. Emphasis is placed on microelectronic applications that include formation of structures that are smaller than the diffusion layer thickness which influences structure formation. 

The decomposition mechanisms are difficult to understand because (i) the surface is not homogeneous with respect to its morphology and chemical composition and (ii) these features evolve continuously during the deposition process. Moreover, as has been clearly demonstrated for noble metals, autocatalytic phenomena can occur, dramatically increasing the growth rate while decreasing the nucleation rate. 

The activation of nonconducting materials by the deposition of metal-containing polymer films could improve the autocatalytic metallization process by eliminating the aqueous etching and sensitizing steps. In addition, substrates that are hard to etch and activate by the aqueous process could be plated by this technique. 

Y. Okinaka and T. Osaka describe the fundamental aspects and technological applications of autocatalytic metal deposition processes. In view of that electroless deposition has found important applications in the manufacture of microelectronic devices, a review of the pertaning electrochemical fundamentals has been long overdue. 

The autocatalytic deposition of Ni and Co was intensively studied, although the most realistic mechanism has not yet been proposed. In the explanation of the mechanisms of autocatalytic deposition, the authors use the principles of the electrodeposition, although there are significant differences among the two processes. 

The different morphology of the Ni-P deposits produced during the autocatalytic deposition (homogenous with pores) and electrodeposition (lamellar), when process is carried out at low temperature and containing phosphite or hypophosphite.11... 

During electroless plating, metal is deposited onto the desired surface from solution through an autocatalytic redox process [36]. Specifically, a reductant in the... 

Electroless deposition takes place by an autocatalytic redox process, in which the cation of the metal to be deposited is reduced by a soluble reductant at the surface... 

Autocatalytically controlled surface reactions Michaihk and coworkers [98] proposed an autocatalytic surface process representing the time dependence of formation of a 2D condensed film of triphenylethyl phospho-nium sulfate ((TPEP)2S04) deposited at a mercury-electrolyte interface [99] ... 

Autocatalytic deposition is the most widely used type of plating of metals from aqueous solutions, without an application of the external electrical current or potential. This process is frequently called electroless or chemical deposition, although, these terms do not precisely describe the autocatalytic deposition. 

The autocatalytic deposition proceeds by a controlled chemical reaction that is catalyzed by the metal or alloy being deposited [1]. This process can be presented with the following reaction ... 

By all means, further studies are required to clarify the mechanisms of the autocatalytic deposition. The observations suggest that every single reaction (autocatalytic deposition) must have its own mechanism. In this way, all the generalizations must be avoided, no matter how many of the autocatalytic processes are analogous or similar. 

Similarly, using the electroless deposition and an appropriate reducing agent in a way as described in Sect. 7.3.2, metallic nanoparticles can be successfully produced. Again, to produce metallic nanoparticles, attention should be paid to keep the process as short as possible to avoid the growth of particles larger than nanometer range via autocatalytic deposition. 

Electrochonical processes for metal deposition are broadly classified in electrolytic and electroless techniques. The electrolytic method implies the use of an external current source, whereas electroless methods—though relying on an electrochemical mechanism—only require the immersion of the substrate into the plating solution. Electroless methods comprise two essentially different processes, displacement or immersion plating and autocatalytic deposition. In Figure 11.2 the basic functioning of these processes is schematically represented. 

FIGURE 11.2 Schematic representation of the electrochemical processes used for deposition of metals. From left to right displacement deposition autocatalytic deposition electrodeposition. 

Autocatalytic deposition is far more important than immersion processes in the plating industry, and its prominence in the field also explains why it is commonly referred to as electroless deposition tout court. The discrimination between electroless or autocatalytic processes and immersion or displacement deposition should be particularly cared about in the case of gold deposition, as emphasized in an early review of the subject [14]. 

Surfaces. Essentially any electrically conductive surface can be electroplated, although special techniques may be required to make the surface electrically conductive. Many techniques ate used to metalline nonconductive surfaces. These are weU-covered ia the Hterature (3) and can range from coating with metallic-loaded paints or reduced-silver spray, to autocatalytic processes on tin—palladium activated surfaces or vapor-deposited metals. Preparation steps must be optimized and closely controlled for each substrate being electroplated. 

Recent work has shown that tin may be deposited by an autocatalytic process using transition-metal ion reducing agents. Very thick coatings may also be economically applied to a variety of substrates by the process of roll bonding . 

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