Additives

Additives have been and continue to be a critical component of industrial plating baths. Most additive work has been done empirically, although recent years have seen increased fundamental understanding of these complex systems. A detailed treatment of additive effects is left to more specialized texts. Instead, we provide just a brief introduction to this important topic.

Additives act in a variety of ways to influence the deposition process. For example, additives may complex with ions in solution and influence solution stability, deposition potentials and rates, and reaction mechanisms. Additives can also affect the stability of surface layers and prevent, for example, the formation of passive layers on anodes.

Organic additives act by adsorbing onto the deposit surface and can impact deposition by changing, for example, the concentration of available growth sites, the concentration of adions, and the rate of surface diffusion of adions. Such changes may lead to an increase in the local adion concentration, which may enhance nucleation and significantly change the structure and properties of the deposit. Preferential deposition of additives at specific sites may dramatically alter the growth mechanism (e.g., by blocking preferred growth sites) and the resulting deposit morphology. Industrially, organic additives can be classified as follows:

Brighteners: These are additives that lead to bright deposits by reducing the deposit roughness so that light is reflected rather than scattered. One way to reduce roughness is to enhance nucleation and randomize deposition in order to form small grains uniformly over the entire surface.
Levelers: Levelers control deposit roughness on a more “macroscale.” For example, some levelers act by preferentially adsorbing onto areas of high deposition, such as areas favored by diffusion, leading to a reduced rate of deposition at those areas.
Structure Modifiers: These additives can change the crystal orientation of the deposit or otherwise impact the structure and stresses in the deposit.
Wetting Agents: Wetting agents function by reducing the surface tension in order to prevent hydrogen bubbles from adhering to the surface and negatively impacting the deposit by the formation of pits under bubbles or by the entrapment of bubbles and possible hydrogen embrittlement. The wetting agent accelerates the release of hydrogen from the surface before bubbles of sufficient size to cause damage can be formed.
The above classification is not used universally. For example, in the semiconductor industry, the plating of copper to fill high-aspect ratio trenches is enabled by plating additives that are typically classified as accelerators, suppressors, and levelers. Independent of how they are classified, additives play a critical role in electrodeposition by interacting with the fundamental processes, either in the bath or on the deposit surface, that determine deposit growth and morphology. Table 13.1 provides an example of plating baths used industrially.

Table 13.1 Examples of Industrial Plating Baths and Conditions

Source: Adapted from Pletcher 1993.

Metal Electrolyte composition [kg·m3] T [°C] Current density [A·m−2] Additives Anode Current efficiency (%)
Cu CuSO4 (200–250)
H2SO4 (25–50) 20–40 200–500 Dextrin, gelatin, S-containing brighteners, sulfonic acids P-containing rolled Cu 95–99
Ni NiSO4 (250)
NiCl2 (45)
H3BO3 (30)
pH 4–5 40–70 200–500 Coumarin, saccharin, benzenesulfonamide, acetylene derivatives Ni pellets or pieces 95
Zn ZnO (20–40)
NaCN (60–120)
NaOH (60–100)