Electroless overall reaction

The overall reactions of electrodeposition and electroless deposition may be used to compare these two processes. The process of electrodeposition of metal M is represented by... 

In this process z electrons are supplied by an external power supply (Fig. 2.1). The overall reaction of electroless metal deposition is... 

An electrochemical model for the process of electroless metal deposition was suggested by Paunovic (10) and Saito (8) on the basis of the Wagner-Traud (1) mixed-potential theory of corrosion processes. According to the mixed-potential theory of electroless deposition, the overall reaction given by Eq. (8.2) can be decomposed into one reduction reaction, the cathodic partial reaction. 

Wagner-Traud Diagram, According to the mixed-potential theory, the overall reaction of the electroless deposition, [Eq. (8.2)] can be described electrochemically in terms of three current-potential i-E) curves, as shown schematically in Eigure 8.2. First, there are two current-potential curves for the partial reactions (solid curves) (1) ic =f(E), the current-potential curve for the reduction of ions, recorded from the rest potential E eq M the absence of the reducing agent Red (when the activity of is equal to 1, eq,M E m) and (2) = f(E), the current-potential... 

Figure 8.2. Wagner-Traud diagram for the total (/total) rid component current potential curves (/, / ) for the overall reaction of electroless deposition.

Wagner-Traud Diagram. According to the mixed-potential theory, the overall reaction of the electroless deposition, [Eq. (8.2)] can be described electrochemically in terms of three current-potential (i-E) curves, as shown schematically in Figure 8.2. 

It is well established that the overall electroless deposition reactions are basically electrochemical in nature, consisting of cathodic and anodic partial reactions occurring simultaneously on the same substrate surface ... 

According to the mixed potential theory, the overall reaction should be interpretable simply by superimposing the respective electrochemical behavior of the two partial reactions, determined independently. More recent studies, however, show that electroless deposition processes are much more complicated than represented by the simple mixed potential theory described above. Interdependence of partial reactions and participation of a third reaction are among the complications which limit the significance of simple combination of independently studied partial reactions. Examples of such complications Eire discussed in the subsequent sections. 

Shippey and Donahue [11] were the first to show how to derive an empirical expression for the overall rate law for electroless deposition reactions. They studied an electroless copper system with tartrate as a complexing agent. Later, Molenaar et al. [12] performed similar kinetic studies concerning an electroless copper deposition reaction with EDTA as a complexing agent. The kinetics of electroless nickel deposition was investigated by Mallory and Lloyd [13]. 

Overall rate laws such as those discussed above are useful for obtaining information on which variables must be controlled more closely in order to maintain a constant deposition rate in practical electroless plating. However, overall rate laws do not provide any mechanistic information. Donahue and Shippey [14] proposed a method of deriving rate laws for partial anodic and cathodic processes in order to gain insight into the mechanism of electroless deposition reactions. If it is assumed that the anodic and cathodic partial processes may interact with each other, then the general rate laws for the partial reactions can be written as follows ... 

According to the mixed-potential theory, the overall reaction of the electroless... 

The metal ion in electroless solutions may be significantly complexed as discussed earlier. Not all of the metal ion species in solution will be active for electroless deposition, possibly only the uncomplexed, or aquo-ions hexaquo in the case of Ni2+, and perhaps the ML or M2L2 type complexes. Hence, the concentration of active metal ions may be much less than the overall concentration of metal ions. This raises the possibility that diffusion of metal ions active for the reduction reaction could be a significant factor in the electroless reaction in cases where the patterned elements undergoing deposition are smaller than the linear, or planar, diffusion layer thickness of these ions. In such instances, due to nonlinear diffusion, there is more efficient mass transport of metal ion to the smaller features than to large area (relative to the diffusion layer thickness) features. Thus, neglecting for the moment the opposite effects of additives and dissolved 02, the deposit thickness will tend to be greater on the smaller features, and deposit composition may be nonuniform in the case of alloy deposition. 

As an example, the case of electroless copper deposition is described briefly below. For the overall electroless copper deposition reaction... 

We have described one example of this type of electrochemical deposition in Sect. 3.4.1, when we considered processes at a strip of Zn placed in a solution of Q1SO4 (Fig. 25). We have stated that there are two partial reactions in that system, like in an electroless system. In the displacement deposition of Cu on Zn, electrons are supplied in the oxidation reaction of Zn, reaction (64), where Zn from the substrate dissolves into the solution and, hence, supplies the electrons necessary for the reduction, deposition reaction (65). The overall displacement deposition reaction, Eq. (66), is obtained via the combination of the two partial electrode reactions, oxidation and reduction, Eqs (64) and (65), respectively. Thus, in the displacement deposition of Cu on a Zn substrate, a layer of metallic Cu is deposited on the zinc, while Zn dissolves into solution (Fig. 25). We stated that this reaction is possible because the Zn/Zn2+ system has a lower electrode potential than the Cu/Cu2+ one (Fig. 24). 

It should be noted that such a stoichiometry of the phosphorus formation reaction during electroless Ni-P deposition was confirmed by using analytical methods to determine the amounts of Ni and P deposited, the overall amount of hypophosphite used, and the isotopic composition of the evolved gas, which allows to quantify the contribution of reaction (19.12) under these conditions [78,79]. Therefore, anodic partial current is equal to the sum of cathodic currents during electroless Ni-P plating under open-circuit conditions ... 

EQCM makes it possible to measure the rate of reaction (19.3) in situ under open-circuit conditions and as a function of the electrode potential [25,26,43 -7,51,60]. QCM study of the effect of H/D substitution in formaldehyde on the rate of electroless copper deposition suggests that the rupture of the C-H bond is the rate-determining step of the overall process [43]. Less attention, however, was paid to the detailed mechanism of reaction (19.19), which involves, apparently, the two-step reduction of cupric ions to Cu with the formation of intermediate Cu(I) species [91]. 

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