Metal Deposition Mechanisms

This paper describes a process for activating polyimide surfaces for electroless metal plating. A thin surface region of a polyimide film can be electro-chemically reduced when contacted with certain reducing agent solutions. The electroactivity of polyimides is used to mediate electron transfer for depositing catalytic metal (e.g., Pd, Pt, Ni, Cu) seeds onto the polymer surface. The proposed metal deposition mechanism presented is based on results obtained from cyclic voltammetric, UV-visible, and Rutherford backscattering analysis of reduced and metallized polyimide films. This process allows blanket and full-additive metallization of polymeric materials for electronic device fabrication. 

The metal surface properties also change with the bath constituents and thereby affect the particle-electrode interaction. Metal deposition constitutes a multi-step reaction mechanism that depends on the bath composition. In quite a number of reaction mechanism adsorbed intermediates, e.g. the presence of chromium and catalyst polyoxides on the metal surface during chromium plating, are involved. Not the metal surface, but the adsorbed intermediates will determine the particle-electrode interaction and might even compete for adsorption sites on the electrode surface with the particle. Although the reverse, i.e., the change in metal deposition mechanism due to the presence of particles has been investigated (see Section 3.U), no studies on the effect of the deposition mechanisms on particle codeposition have been reported. 

The first theoretical treatment of the metal deposition mechanism via surface diffusion was given by Lorenz [2.24], who adapted the theory of Burton, Cabrera and Frank [2.1] to the case of electrolytic metal deposition. Later the problem was treated by Vermilyea [2.29], Despic and Bockris [2.25], Fleischmann and Thirsk [2.28], and Damjanovic and Bockris [2.26], who brought the theory of surface diffusion to its present state. 

The metal deposition mechanism via direct transfer was suggested for the first time by Volmer [2.33], and was discussed later by Lorenz [2.24], Fleischmann and Thirsk [2.28], Mott and Watts-Tobin [2.34], and others. A comprehensive treatise on this problem is given by Vetter [2.27]. 

According to Spieker and Regalbuto [20], the following three metal deposition mechanisms can be identified ... 

Further investigations will be performed, for a better understanding of metal deposition mechanism associated with obtained morphology as an important factor to influence the final performance and to establish industrially compliant technological processes. 

As pointed out above, an STM tip can be used to nucleate and grow single clusters. In this type of experiment, cluster deposition on a STM tip is achieved when it is retracted about 10 to 20 run from the substrate surface. Under these conditions, where the feedback loop is disabled, absence of mechanical contact between the tip and the substrate in ensured. Then a positive potential pulse is applied to the tip, the metal deposited on it is dissolved, and it diffuses toward the substrate surface, where a nucleus develops and grows to yield a cluster, typically 20 nm wide. 

It was mentioned on page 306 (see Fig. 5.24) that, even at room temperature, a crystal plane contains steps and kinks (half-crystal positions). Kinks occur quite often—about one in ten atoms on a step is in the half-crystal position. Ad-atoms are also present in a certain concentration on the surface of the crystal as they are uncharged species, their equilibrium concentration is independent of the electrode potential. The half-crystal position is of basic importance for the kinetics of metal deposition on an identical metal substrate. Two mechanisms can be present in the incorporation of atoms in steps, and thus for step propagation ... 

The polymerisation appears to involve a nucleation process similar to that of the deposition of metals [182], and electrochemically-prepared polyaniline will form dense, non-fibrillar thin films [165, 173], but thicker films ( > 150nm) become less densely packed and more fibrous [176, 182]. This may be due a change in the deposition mechanism when the film becomes sufficiently thick to inhibit direct access to the platinum by unreacted monomer [176],... 

The principal evidence in favor of the electrochemical mechanism is the observation that metal deposition can occur, albeit at a much reduced rate, in a two-compartment cell. When one of the compartments contains only the metal ions and the other only the sodium hypophosphite, metal deposition occurs on the electrode in the first compartment, being driven by hypophosphite oxidation at the electrode in the other half-cell [36, 37],... 

Figure 1 provides several electrochemical windows of important, relevant processes, including the reduction of alkyl carbonates, ethers, Li insertion into graphite, and Li metal deposition. Recent studies revealed two major failure mechanisms of graphite electrodes in repeated Li insertion/ deinsertion processes 21... 

To put the various contributions to the modeling of codeposition of particles into perspective, not only must the relevant electrochemical literature be reviewed, but also that in the field of filtration. Theories on the mechanisms of capture and encapsulation of particles during metal deposition have only begun to appear in the last two decades. The scope of this section will be to summarize the models published since 1970, and give the basis for their explanation. (A review of the older work can be found in the Ph.D. thesis of C. Buelens [58].) Let us see review the models in chronological order. 

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. 

Salvaga and Cavallotti [43], and Randin and Hintermann [44] advanced a metal hydroxide mechanism which involved hydrolyzed nickel ions as the actual nickel reactant in Ni-P deposition ... 

Although electroless deposition seems to offer greater prospects for deposit thickness and composition uniformity than electrodeposition, the achievement of such uniformity is a challenge. An understanding of catalysis and deposition mechanisms, as in Section 3, is inadequate to describe the operation of a practical electroless solution. Solution factors, such as the presence of stabilizers, dissolved O2 gas, and partially-diffusion-controlled, metal ion reduction reactions, often can strongly influence deposit uniformity. In the field of microelectronics, backend-of-line (BEOL) linewidths are approaching 0.1 pm, which is much less than the diffusion layer thickness for a... 

Photobleach mechanism, 19 203 Photobleach reversal grains, 19 201 Photocatalysis, 19 73-106. See also Photocatalysts Photoreactors aqueous pollutants eliminated and mineralized by, 19 89t catalyst modifications in, 19 94-95 catalysts in, 19 75-76 challenges in, 19 101-102 fate of photo-holes in titania, 19 82-85 in fine chemistry applications, 19 102 influence of oxygen pressure in, 19 82 ion doping in, 19 94-95 mass of catalyst in, 19 77-78 noble metal deposit in, 19 94 parameters governing kinetics in, 19 77-82... 

Kinetic Scheme. Generally, metal ions in a solution for electroless metal deposition have to be complexed with a ligand. Complexing is necessary to prevent formation of metal hydroxide, such as Cu(OH)2, in electroless copper deposition. One of the fundamental problems in electrochemical deposition of metals from complexed ions is the presence of electroactive (charged) species. The electroactive species may be complexed or noncomplexed metal ion. In the first case, the kinetic scheme for the process of metal deposition is one of simple charge transfer. In the second case the kinetic scheme is that of charge transfer preceded by dissociation of the complex. The mechanism of the second case involves a sequence of at least two basic elementary steps ... 

In this section we show that some electroless deposits have unique properties compared to electrodeposited, evaporated, or sputtered metal deposits. Our discussion is limited to mechanical and diffusion barrier properties. 

X-ray absorption spectroscopy can provide information concerning oxidation state and local structure of metals deposited on FGG catalysts and related supports, even at the several hundred ppm level. This information is valuable towards the understanding of catalyst deactivation and passivation mechanisms, and ultimately will lead to the development of new passivation routes. 

As seen in reaction (6.5.3) photogenerated holes are consumed, making electron-hole separation more effective as needed for efficient water splitting. The evolution of CO2 and O2 from reaction (6.5.6) can promote desorption of oxygen from the photocatalyst surface, inhibiting the formation of H2O through the backward reaction of H2 and O2. The desorbed CO2 dissolves in aqueous suspension, and is then converted to HCOs to complete a cycle. The mechanism is still not fully understood, with the addition of the same amount of different carbonates, see Table 6.2, showing very different results [99]. Moreover, the amount of metal deposited in the host semiconductor is also a critical factor that determines the catalytic efficiency, see Fig. 6.7. 

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