Current-potential curves electroless deposition

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. 

Fig. 2. Current-potential curves in Evans diagram [29] format for reduction of Cu2+ ions and oxidation of H2CO. and are the equilibrium, or open circuit, potentials for the Cu2+ reduction and H2CO oxidation reactions, respectively. Assuming negligible interfering reactions, the vertical dashed lines indicate the exchange current densities for the two half reactions, and the deposition current for the complete electroless solution. Adapted from ref. 23.

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.

Electroless Deposition of Copper. The basic ideas of the mixed-potential theory were tested by Paunovic (10) for the case of electroless copper deposition from a cupric sulfate solution containing ethylenediaminetetraacetic acid (EDTA) as a complexing agent and formaldehyde (HCHO) as the reducing agent (Red). The test involved a comparison between direct experimental values for and the rate of deposition with those derived theoretically from the current-potential curves for partial reactions on the basis of the mixed-potential theory. 

Electroless Deposition in the Presence of Interfering Reactions. According to the mixed-potential theory, the total current density, is a result of simple addition of current densities of the two partial reactions, 4 and However, in the presence of interfering (or side) reactions, 4 and/or may be composed of two or more components themselves, and verification of the mixed-potential theory in this case would involve superposition of current-potential curves for the electroless process investigated with those of the interfering reactions in order to correctly interpret the total i-E curve. Two important examples are discussed here. 

Interaction Between Partial Reactions. The original mixed-p)otential theory assumes that the two partial reactions are independent of each other (1). In some cases this is a valid assumption, as was shown earlier in this chapter. However, it was shown later that the partial reactions are not always independent of each other. For example, Schoenberg (13) has shown that the methylene glycol anion (the formaldehyde in an alkaline solution), the reducing agent in electroless copper deposition, enters the first coordination sphere of the copper tartrate complex and thus influences the rate of the cathodic partial reaction. Ohno and Haruyama (37) showed the presence of interference in partial reactions for electroless deposition of Cu, Co, and Ni in terms of current-potential curves. 

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) ... 

An alternative method of presenting the current-potential curves for electroless metal deposition is the Evans diagram. In this method, the sign of the current density is suppressed. Figure 22 shows a general Evans diagram with current-potential functions i = f(E) for the individual electrode processes, Eqs (43 and 44). According to this presentation of the mixed-potential theory, the current-potential curves for individual processes, ic = iu = f(E) and ia = = f(E), intersect. The... 

Fig. 23 Evans diagram for electroless deposition of copper. Current-potential curves for the reduction of Cu2+ ions and for the oxidation of reducing agent Red, formaldehyde, combined into one graph. Solution for the Tafel line for the reduction of Cu2+ ions 0.1 M CuS04, 0.175 M EDTA, pH 12.50, Eeq (Cu/Cu2+) = -0.47 V versus SCE for the oxidation of formaldehyde 0.05 M HCHO and 0.075 M EDTA, pH 12.50, Eeq(HCHO) = —1.0 V versus SCE temperature 25°C ( 0.5°C) (from Ref. 43 with permission from American Electroplaters and Surface Finishers Society).

The studies of ruthenium deposition are mainly carried out by electrochemical and spontaneous (electroless) deposition procedures. Friedrich and colleagues found that the well-reproduced current-potential curves of ruthenium-modified Pt(lll) electrodes were obtained when... 

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... 

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. 

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