Electroless deposition anodic partial reaction

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 ... 

The experimental evidence described above clearly shows that the cathodic and anodic partial reactions interact with each other, and that such interactions are an essential part of the mechanism of electroless copper deposition. It should be noted that the interdependence of partial reactions has also been demonstrated by Bindra et al. [127], based on their results of kinetic and mechanistic analysis. 

Paunovic [23] and Saito [24] first advanced the notion that an electroless deposition process could be modeled using a simple electrochemical approach. They reasoned that the potential of a surface undergoing electroless deposition could be regarded as a mixed potential intermediate in value between the potentials of its constituent anodic and cathodic partial reactions. These authors employed the mixed potential concept of corrosion reactions first outlined in a systematic manner by Wagner and... 

Fig. 1. Current-potential curves for a generalized electroless deposition reaction. The dashed line indicates the curve for the complete electroless solution. The partial anodic and cathodic currents are represented by ia and ic, respectively. Adapted from ref. 28. 

Figure 1 shows a generalized representation of an electroless deposition process obeying MPT [28]. Polarization curves are shown for the two partial reactions (full lines), and the curve expected for the full electroless solution (dashed curve). The polarization curve for anodic and cathodic partial reactions intersect the potential axis at their respective equilibrium potential values, denoted by / j]cd and respectively. At Emp, the anodic and cathodic partial current densities are equal, a... 

A discussion of the applicability of the MPT model to a particular electroless system ideally presumes knowledge of the kinetics and mechanisms of the anodic and cathodic partial reactions, and experimental verification of the interdependence or otherwise of these reactions. However, the study of the kinetics, catalysis, and mechanistic aspects of electroless deposition is an involved subject and is discussed separately. 

A comparison of the results using this method and the rate of electroless copper deposition determined gravimetrically shows that the best results are obtained with the Le Roy equation applied to the polarization data in the anodic range. It is interesting to note that here, in the metal deposition as in the corrosion (9), the partial reaction, which does not involve destruction or building of a crystal lattice of metal substrate, gives better results (this is hardly surprising, of course). 

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 ... 

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 ... 

The highest rate of electroless copper deposition under open-circuit conditions is achieved at the most positive Em values both Em and process rate values are determined by electrochemical characteristics of coupled partial reactions. Notably, Cu(II) reduction partial reaction is more sensitive to the nature of the complexing agent compared to anodic formaldehyde oxidation. The decrease in the rate of electroless copper deposition in solutions with Em value becoming more negative corresponds to a negative shift of the Cu(II)/Cu potential, due to the increase in the pK value of the Cu(II) complexes, as well as due to kinetic and structural factors [37]. 

Steady-State Kinetics, There are two electrochemical methods for determination of the steady-state rate of an electrochemical reaction at the mixed potential. In the first method (the intercept method) the rate is determined as the current coordinate of the intersection of the high overpotential polarization curves for the partial cathodic and anodic processes, measured from the rest potential. In the second method (the low-overpotential method) the rate is determined from the low-overpotential polarization data for partial cathodic and anodic processes, measured from the mixed potential. The first method was illustrated in Figures 8.3 and 8.4. The second method is discussed briefly here. Typical current—potential curves in the vicinity of the mixed potential for the electroless copper deposition (average of six trials) are shown in Figure 8.13. The rate of deposition may be calculated from these curves using the Le Roy equation (29,30) ... 

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