Electroless deposition steady-state

Process Control. Some hot nickel and flash electroless copper solutions are plated to the point of exhaustion and then discarded. Most baths are formulated to give bath fives of >6 turnovers of the bath constituents some reach steady-state buildup of the by-products and can be used indefinitely. AU. regenerable solutions should be filtered to remove particulates that can cause deposit roughness and bath instability. 

The O2 reduction reaction affects not only the steady-state deposition kinetics, but also the initiation of deposition, the so-called induction time [126, 127], At the beginning of the deposition process, the open circuit potential (Eoc) of either a uniformly catalytically active substrate, or a catalyst particle on an insulator, will be higher than that required for electroless deposition to occur. This is a consequence of the surface of the catalyst being covered with O or OH species which mask the catalytic activity of the surface the value of would be expected to be in the range of... 

Here F is the Faraday constant C = concentration of dissolved O2, in air-saturated water C = 2.7 x 10-7 mol cm 3 (C will be appreciably less in relatively concentrated heated solutions) the diffusion coefficient D = 2 x 10-5 cm2/s t is the time (s) r is the radius (cm). Figure 16 shows various plots of zm(02) vs. log t for various values of the microdisk electrode radius r. For large values of r, the transport of O2 to the surface follows a linear type of profile for finite times in the absence of stirring. In the case of small values of r, however, steady-state type diffusion conditions apply at shorter times due to the nonplanar nature of the diffusion process involved. Thus, the partial current density for O2 reduction in electroless deposition will tend to be more governed by kinetic factors at small features, while it will tend to be determined by the diffusion layer thickness in the case of large features. 

Steady-state electroless metal deposition at mixed potential is preceded by a non-steady-state period, called the induction period. 

Induction Period. The induction period is defined as the time necessary to reach the mixed potential at which steady-state metal deposition occurs. It is determined in a simple experiment in which a piece of metal is immersed in a solution for electroless deposition of a metal and the potential of the metal is recorded from the time of immersion (or the time of addition of the reducing agent, i.e., time zero) until the steady-state mixed potential is established. A typical recorded curve for the electroless deposition of copper on copper substrate is shown in Figure 8.11. 

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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