Seeking smoother plating deposits? It may be all in the waves. Our studies have shown that superimposition of a small sinusoidal potential wave (smooth, repetitive oscillation) onto a potentiostatic bias can lead to more level electrodeposits in finishing.
In principle—at least in the investigated case of an Au cyano complex bath—a relatively simple modification of the current supply mode can give rise to plating-quality results that are traditionally achieved with the use of additives that are generally environmentally unfriendly and complex to manage.
It is worth noting that the electrical control we are proposing is utterly different from pulse plating, from both the instrumental and theoretical viewpoints. As far as instrumentation is concerned, simply a controlled ripple has to be applied to an otherwise continuous cell voltage. Concerning theory, while pulse plating essentially relies on the generation of a pulsating concentration double layer, our approach rests on a theory we have been recently developing: explaining roughness development in metal plating on the basis of the dynamic coupling of surface morphology and composition, of which a review of this work can be found in .
Owing to the peculiarities of this dynamic—specifically the interaction between the frequency of a forcing added to the source term for morphology and the different intrinsic timescales of the model—morphological stabilization can be achieved because the controlled potential perturbation counteracts the tendency of roughness to diverge.
The rationale for our idea of adding an electrochemical forcing term is rooted in the evolution of the electrodeposit surface profile obtained as the solution of a balance equation. In the relevant experimental case, the ligand was released during the plating process when it was coupled to a balance equation for an adsorbate. In the morphological balance equation, the flow terms account for adatom surface diffusion, contributing to the build-up of morphology, while the source terms include deposition and corrosion, or desorption, of the relevant material.
Of course, the underlying physics is atomistic, but a continuous model such as ours is acceptable for the description of plating profile dynamics if specific conditions on length and timescales are defined (for details, see the Appendix of ). Under the realistic hypotheses discussed in , the model given in Equation (1) has been shown in a series of recent works of ours (among which [3, 4] stress mathematical aspects and [5, 6] experimental ones) to express a notably varied phenomenology, able to capture the key aspects of roughness formation in electroplating.
The focus in  has been to pinpoint the bearing of a small-signal applied potential on roughness and Turing pattern development.
The equation for the morphological dynamics in the presence of external sinusoidal forcing