Electroless deposition metals

Metal deposition processes, early,. 9 760-761. See also Electroless deposition Metal deposition systems, in compound semiconductor processing, 22 188-189 Metal-dielectric composite-metal tandem solar absorbing surface, 23 11 Metal dithionates, 23 677-678 uses for, 23 677-678... 

A comparison was made between Ni and Co diffusion barriers produced by electroless, electro-, and evaporation deposition (64). This comparison shows that only electrolessly deposited metals and alloys, at a thickness of 1000 im, have barrier properties for Cu diffusion. For Co(P) 1000-pm-thick barriers, annealed for 14h, the amount of Cu interdiffused into Co(P) is less than 1 at %. Thicker barriers of Ni(P), Ni(B), and Co(B) are required for the same degree of Cu interdiffusion. The same metals, if electrodeposited, both do and do not have inferior barrier properties. This... 

Empirical Modeling. The effect of process variables on the rate of deposition and properties of electrolessly deposited metals is usually studied by one-factor-at-a-time experiments (one-factor experiments are discussed further later in the text). In these experiments the effect of a single variable (factor), such as xv in the multi-variable process with the response y, y =zf(xv xv xy. . ., xn), is studied by varying the value (level) of this variable while holding the values of the other independent variable xed, y —f x3). Any prediction (extrapolation) of the effect of a... 

In this section, the corrosion aspects of different electroless-deposited metals and/or alloys are discussed. 

Table I. Adhesion Of Electrolessly-Deposited Metals To Chemically Pretreated Unfilled Polyetherilmide Film. ...

Currently, scaiming probe microscopy (e.g., atomic force microscopy) is often used for testing instead of optical and electron microscopes. A few advanced destructive techniques have been reported for the evaluation of nanostructured material. " However, they are still not good enough for the evaluation of a nanoscaled thin film system (such as electrochemically/electrolessly deposited metal films) owing to critical defects such as voids, delaminations, and debonding sites of the system which often exist in the interior. Therefore, microscopy designed for surface analysis may not be suitable for application to the system directly. 

In electroless deposition, the substrate, prepared in the same manner as in electroplating (qv), is immersed in a solution containing the desired film components (see Electroless plating). The solutions generally used contain soluble nickel salts, hypophosphite, and organic compounds, and plating occurs by a spontaneous reduction of the metal ions by the hypophosphite at the substrate surface, which is presumed to catalyze the oxidation—reduction reaction. 

The ideal electroless solution deposits metal only on an immersed article, never as a film on the sides of the tank or as a fine powder. Room temperature electroless nickel baths closely approach this ideal electroless copper plating is beginning to approach this stabiHty when carefully controUed. Any metal that can be electroplated can theoretically also be deposited by electroless plating. Only a few metals, ie, nickel, copper, gold, palladium, and silver, are used on any significant commercial scale. 

The important beneficial effects that substrate roughness can bring were firmly established in the late sixties and early seventies, principally as a result of work in two areas. The first was associated with the electroless deposition of metals onto plastics such as ABS and polypropylene. In the process the plastics must be etched in a way which produces pits on a micrometre scale. Such a topography had been shown to be a necessary, but not sufficient condition for adequate adhesion [40]. 

Electroless nickel baths are usually preferred to electroless copper, since they tend to be more stable and are less likely to deposit metal on unwanted areas, such as plating racks. Electrolytic copper is then plated before the final application of nickel and chromium, where this is the required finish, as it... 

Porter LA, Choi HC, Ribbe AE, Buriak JM (2002) Controlled electroless deposition of noble metal nanoparticle films on Germanium surfaces. Nano Lett 2 1067-1071... 

According to Ref. [12], template for synthesis of nanomaterials is defined as a central structure within which a network forms in such a way that removal of this template creates a filled cavity with morphological or stereochemical features related to those of the template. The template synthesis was applied for preparation of various nanostructures inside different three-dimensional nanoporous structures. Chemically, these materials are presented by polymers, metals, oxides, carbides and other substances. Synthetic methods include electrochemical deposition, electroless deposition, chemical polymerization, sol-gel deposition and chemical vapor deposition. These works were reviewed in Refs. [12,20]. An essential feature of this... 

The electrochemical mechanism was rejected by Salvago and Cavallotti [26] on the basis that it does not explain several features of electroless deposition of ferrous metals it does not account for the isotopic composition of the H2 gas evolved it does not explain the effect of the various solution components on reaction rate and it does not account for the homogeneous decomposition of very active solutions or the fact that they can give deposition on insulating surfaces. These authors put forward a chemical mechanism, involving various hydrolyzed nickel species, which they claim explains the observed behavior of the system ... 

Brenner and Riddell are credited with developing the first successful electroless deposition process, that for Ni-P, in the 1940 s [1]. However, it should be indicated that they were probably the first to develop an example of a controlled electroless deposition process, since a host of metallization processes existed prior to this that bear similarities to modern-day electroless methods. For example, there existed the well-... 

Nevertheless, at least some of the currently used electroless solution formulations owe their beginnings to work carried out in the pre-1940s period. The increasing use of electroless deposition to metallize smooth, nonmetallic surfaces in microelectronics means that the achievement of deposit adhesion to smooth surfaces is as much an issue today as it was a century or more ago when chemical deposition processes were being developed for metallization of glass surfaces. 

Electroless deposition as we know it today has had many applications, e.g., in corrosion prevention [5-8], and electronics [9]. Although it yields a limited number of metals and alloys compared to electrodeposition, materials with unique properties, such as Ni-P (corrosion resistance) and Co-P (magnetic properties), are readily obtained by electroless deposition. It is in principle easier to obtain coatings of uniform thickness and composition using the electroless process, since one does not have the current density uniformity problem of electrodeposition. However, as we shall see, the practitioner of electroless deposition needs to be aware of the actions of solution additives and dissolved O2 gas on deposition kinetics, which affect deposit thickness and composition uniformity. Nevertheless, electroless deposition is experiencing increased interest in microelectronics, in part due to the need to replace expensive vacuum metallization methods with less expensive and selective deposition methods. The need to find creative deposition methods in the emerging field of nanofabrication is generating much interest in electroless deposition, at the present time more so as a useful process however, than as a subject of serious research. 

Electroless deposition should not be confused with metal displacement reactions, which are often known as cementation or immersion plating processes. In the latter, the less noble metal dissolves and eventually becomes coated with a more noble metal, and the deposition process ceases. Coating thicknesses are usually < 1 pm, and tend to be less continuous than coatings obtained by other methods. A well-known example of an immersion plating process that has technological applications is the deposition of Sn on Cu [17] here a strong complexant for Cu(I), such as thiourea, forces the Cu(I)/Cu couple cathodic with respect to the Sn(II)/Sn couple, thereby increasing the thermodynamic stability in solution of thiourea-complexed Cu(I) relative to Sn(II). 

In contrast to a mixture of redox couples that rapidly reach thermodynamic equilibrium because of fast reaction kinetics, e.g., a mixture of Fe2+/Fe3+ and Ce3+/ Ce4+, due to the slow kinetics of the electroless reaction, the two (sometimes more) couples in a standard electroless solution are not in equilibrium. Nonequilibrium systems of the latter kind were known in the past as polyelectrode systems [18, 19]. Electroless solutions are by their nature thermodyamically prone to reaction between the metal ions and reductant, which is facilitated by a heterogeneous catalyst. In properly formulated electroless solutions, metal ions are complexed, a buffer maintains solution pH, and solution stabilizers, which are normally catalytic poisons, are often employed. The latter adsorb on extraneous catalytically active sites, whether particles in solution, or sites on mechanical components of the deposition system/ container, to inhibit deposition reactions. With proper maintenance, electroless solutions may operate for periods of months at elevated temperatures, and exhibit minimal extraneous metal deposition. 

With regard to eqn. (2), which represents the metal deposition half reaction in electroless deposition, in a simplistic sense we see that it is analogous to an electrodeposition process. With respect to the reducing agent reaction, organic [20, 21] and relatively complex inorganic oxidation reactions [22] have similarly been widely studied electrochemically. It is therefore reasonable to think that electroless deposition could be described, or modeled, using an electrochemical approach. 

In the mixed potential theory (MPT) model, both partial reactions occur randomly on the surface, both with respect to time and space. However, given the catalytic nature of the reductant oxidation reaction, it may be contended that such a reaction would tend to favor active sites on the surface, especially at the onset of deposition, and especially on an insulator surface catalyzed with Pd nuclei. Since each reaction strives to reach its own equilibrium potential and impose this on the surface, a situation is achieved in which a compromise potential, known as the mixed potential (.Emp), is assumed by the surface. Spiro [27] has argued the mixed potential should more correctly be termed the mixture potential , since it is the potential adopted by the complete electroless solution which comprises a mixture of reducing agent and metal ions, along with other constituents. However, the term mixed potential is deeply entrenched in the literature relating to several systems, not just electroless deposition. 

The critical reaction in electroless deposition from the viewpoint of catalysis is oxidation of the reducing agent. The ability to catalyze oxidation of the reductant determines whether a metal or alloy can sustain electroless deposition in an otherwise properly formulated electroless solution. 

Figure 3 shows polarization curves for the anodic oxidation of H2CO at various metal electrodes recorded by Ohno et al. [38] in a solution maintained at 25 °C and containing EDTA (a commonly used complexant in electroless Cu solutions) and maintained at a pH = 12.5. After exhibiting exceptional activity at potentials less than -0.8 Y (SCE)2, the activity of Cu decreases at ca. 0.3 Y (SCE) this region of activity is more than adequate for electroless deposition of Cu. Although they... 

Many factors influence the deposition kinetics of P and B, including metal ion and complexant concentrations, solution pH, and temperature. Though unavoidable side products of the electroless deposits, P and B impart unique properties to electroless deposits, e.g., good corrosion resistance in the case of Ni-P deposits, where the P content can exceed 30 at% in certain solutions [10, 11],... 

The coevolution of H2 gas in electroless deposition processes is a phenomenon that needs to be understood not only to elucidate the mechanism of deposition, but also since it impacts the properties of deposits by H inclusion. Van den Meerakker [51] first proposed a correlation between simultaneous hydrogen evolution in electroless deposition and the heat of adsorption of hydrogen. In this useful endeavor, however, he has been criticized for erroneously calculating the heats of adsorption of H at Cu by Gottesfeld et al. [52], and Group I (or SP type) metals in general by Bindra and Tweedie [53]. 

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