Rate of metal deposition

The previous models were developed for Brownian particles, i.e. particles that are smaller than about 1 pm. Since most times particles that are industrially codeposited are larger than this, Fransaer developed a model for the codeposition of non-Brownian particles [38, 50], This model is based on a trajectory analysis of particles, including convective mass transport, geometrical interception, and migration under specific forces, coupled to a surface immobilization reaction. The codeposition process was separated in two sub-processes the reduction of metal ions and the concurrent deposition of particles. The rate of metal deposition was obtained from the diffusion... 

It should be understood that even for micro surface features the potential is uniform and the ohmic resistance through the bath to peaks and valleys is about the same. Also, electrode potential against SCE will be uniform. What is different is that over micro patterns the boundary of the diffusion layer does not quite follow the pattern contour (Fig. 12.3). Rather, it thus lies farther from depth or vias than from bump peaks. The effective thickness, 8N, of the diffusion layer shows greater variations. This variation of 8N over a micro profile therefore produces a variation in the amount of concentration polarization locally. Since the potential is virtually uniform, differences in the local rate of metal deposition result if it is controlled by the diffusion rate of either the depositing ions or the inhibiting addition (leveling) agents. 

The additional reaction intermediates in the Ni-tetra(3-methylphenyl)-porphyrin network coupled with slower rates of metal deposition result in deeper penetration of the internal maxima and higher concentration of metals in the pellets center compared to Ni-etioporphyrin. Likewise, the selective enhancement in the metal deposition rates of Ni-T3MPP on the sulfided catalyst is apparent by the steeper profiles in Fig. 28 relative to the results in Fig. 27 on the oxide form of CoMo/A1203. ... 

The most widely used vacuum deposition techniques are evaporation and sputtering, often employed for smaller substrates. In the evaporation process, heating the metal by an electron beam or by direct resistance produces the vapours. The system is operated at a very high vacuum (between 10-5 and 10 6 Torr) to allow a free path for the evaporant to reach the substrate. The rate of metal deposition by evaporation processes varies from 100 to 250,000 A min h These processes can be operated on a batch or a continuous scale. On the other hand, in the case of the sputtering technique, the reaction chamber is first evacuated to a pressure of about 10-5 Torr and then back-filled with an inert gas up to a pressure of 100 mTorr. A strong electric field in the chamber renders ionisation of the inert gas. These inert gas ions... 

Finally, US-enhanced mass transport has also been found to influence the rate of metal deposition (e.g. that of cobalt on glassy carbon electrodes by cyclic and stripping voltammetry, and chronoamperometry [157]). 

What this means, in simple terms, is that even though a low Wagner number may give rise to an uneven current distribution on the surface, the rate of metal deposition is equal everywhere, since a higher local... 

Adsorption of the cyanide ion on the surface is also very likely, in view of its high local concentration. This is an added factor which is expected to reduce the rate of metal deposition, leading to higher values of the Wagner number. 

Firstly, electrodeposition makes it possible to fill lithographically defined cavities with nanometer-scale fidelity. This pattern replication capability has been vividly demonstrated in numerous cases [6], even where the cavity depth significantly exceeds its width. Secondly, higher rates of metal deposition can typically be achieved... 

Unless the current efficiency varies strongly with the current density, the local rate of metal deposition by electroplating is proportional to the component of current density normal to the electrode surface. If all points on an electrode surface receive the same current density, all points will advance at the same rate. If a feature receives... 

The effect of ultrasonic field on the polarization curves of Cu-Pb, and some brasses has been studied in chloride and sulfate solutions in the presence and absence of the respective metal ions [108]. The main effect of the ultrasound at low current densities is the acceleration of diffusion. The passivation current density in solutions free of the respective metal ions is considerably increased when ultrasound is applied. Stable passivity cannot be attained because of the periodic destruction of the salt film. The hydrogen evolution reaction is accelerated because of the destruction of the solvation shell. The oxygen depolarization reaction is also enhanced due to the increased diffusion. The rate of metal deposition is likewise increased by ultrasound. The steady-state potentials of reactions with anodic control are shifted in the negative direction when ultrasound is applied. 

This raises some important possibilities, which have not escaped the attention of the electroplating community. For example, while metal deposition is conducted in fairly concentrated solutions of the metal being plated, and at current densities well below the mass-transport limit, additives acting as inhibitors for metal deposition are often introduced at concentrations that are several orders of magnitude lower, to ensure that their supply to the surface will be mass-transport limited. In this way, the tendency for increased rate of metal deposition on certain features on the surface, such as protrusions, will be moderated by the faster diffusion of the inhibitor to the very same areas. Furthermore, if deposition occurs in the region of mixed control, which is usually the case, it must be remembered that the relevant roughness factor is quite different for the charge-transfer and the mass-transport processes, and this may well be a function of current density, since the Faradaic resistance is inherently potential dependent. 

When no current flows in the outer circuit and the metal dissolution is fast in comparison with metal deposition, the metal is charged negatively with respect to the electrolyte. The potential of the metal becomes more negative with respect to the electrolyte. In this way the rate of metal dissolution is retarded, and the rate of metal deposition is accelerated. The potential will become more negative until an equilibrium potential % is reached. This is equivalent to chemical equilibrium with a chemical reaction. In this case the rates of metal dissolution and deposition are equal. 

The metal-oxide interaction also depends upon the rate of the metal deposition. Experimental results show that an increase in the deposition rate affects the structure and composition of the interfacial layer in much the same way as an increase in substrate temperature [6]. The effects of the rate of metal deposition, the substrate temperature, and the layer thickness on the electrical and physical properties of solar cells may be understood in terms of the physical and chemical interactions in Ti-SiO -Si cells [5, 6, 8]. 

As in other chemical processes, temperature significantly influences the rate of deposition during the galvanic displacement. The rate of metal deposition generally increases with an increase in temperature, which is a consequence of the Arrhenius equation ... 

During the treatment of dilute solutions, the cathodic deposition of metal is often under mass transport control, either initially or during longer batchprocessing times (section 2.5.2), In such cases, it was seen in Chapter 2 that the maximum duty of the reactor may be expressed in terms of the limiting current (which is proportional to the rate of metal deposition). From the definition of the mass transport coefficient ... 

Water electrolysis, as mentioned, is probably a large contributor to the current. It increases with voltage and, consequently, the migration component of the metal ions grows with it, as it is proportional to the total current via the transport number. Despite the initial very low concentration of metallic ions, the rate of metal deposition is increased when the electrolysis is Carried out with a large current caused by the reduction of another substance. This phenomenon is called exaltation of the migration current by simultaneous electrolysis of another compound or substance (3). 

Molecules of the additive adsorbed on the surface prevent or inhibit metal deposition. To a first approximation it can be said that the rate of metal deposition is simply proportional to the fraction of the surface that is not covered by the additive. A more detailed analysis shows that adsorption on part of the surface could also have an effect on the rate of metal deposition on the bare sites, but this refinement need not concern us here. As a rule, the molar concentration of the additive in solution is very low compared to that of the metal ion being plated. Consequently, the rate of adsorption of the additive is controlled by mass-transport limitation, while the rate of metal deposition is mostly activation controlled, with possibly some mass transport limitation involved, depending on the ratio of j/ji, where j is the partial current density for deposition of the metal. This helps to produce a smooth surface for the same reason that a rough surface is formed in the absence of a suitable additive. On protruding parts on the surface the rate of mass transport is higher than on flat or... 

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