42
2 Electrochemistry and Electrodeposition
process is the growth of existing crystals, and the surface density of the adatoms at
the crystal planes differs from its equilibrium value to a small extent only. In parallel,
it is also true that the surface concentration of the precursor ions of the growing metal
is just slightly smaller that its bulk concentration, even though the deposition leads
to a little depletion near the cathode surface (see the bottom graph on the right-hand
side of Fig. 2.6).
A “thumb rule” for the interrelation of the crystal size and the deposition conditions was first outlined by Dini [2] and later completed by Winand [17]. According
to Dini’s concept, essentially all factors lead to a reduction in the crystal size of
a metal being deposited which decrease the surface concentration of the precursor
metal ions. In contrast, if a modification of the deposition conditions leads to a better
replenishment of the precursor ions, an enhancement of the crystal size is expected.
In accord with these considerations, the increase in either the stirring rate or the
temperature as well as the application of a more concentrated solution of the salt
of the precursor metal results in a grain coarsening. The increase in the deposition
rate, should it be achieved by the application of either a more negative deposition
potential or a larger cathodic current density, leads to a grain refinement (i.e., smaller
crystallites).
A factor that requires the consideration of additional effect is the application of
chemicals in the plating bath that modify the deposition. The compounds applied
for such purposes do not make a distinct compound family since inorganic salts
and organic materials with a large variety of functional groups belong to these bath
components. The common name of these compounds, additives, refers to that they are
often a minor component in the plating bath. Additives themselves may also undergo
electrochemical reactions and their decomposition products may incorporate into the
deposit, too; nevertheless, their decomposition is a less important process. Additives
are somewhat similar to catalysts and inhibitors in the sense that they modify a process
without being transformed. Additives are usually classified on the basis of the primary
impact of their application. Brighteners, levelling agents and stress relievers are the
most common types of additives. In spite of the different target parameters, levelling
agents and brighteners exhibit similar impact mechanisms.
The common mechanism by which additives modify metal deposition is that they
adsorb at the typical growth centres. Since the atoms at the near-equilibrium growth
centres of the metal crystal surface are the most exposed ones to the solution and least
surrounded by atoms of the crystal, the adsorption of a molecule with one or several
free non-bonding electron pair(s) is more likely on these crystal surface features. The
adsorption of the additives reduces the probability that the crystal growth proceeds
but increases the nucleation of new crystals from the adatoms that form during the
discharge of the metal ions.
It is important at this point to highlight the impact mechanism of the additives
for both potentiostatic and galvanostatic deposition. If potentiostatic conditions are
applied and the concentration of additives is increased, the current decreases since the
same driving force can lead to the discharge of less ions (the deposition is hindered).
The discharge of the metal ions will result in an increase in the surface concentration
of the adatoms whose dissolution rate also increases. As a result, the application of
2 Electrochemistry and Electrodeposition
process is the growth of existing crystals, and the surface density of the adatoms at
the crystal planes differs from its equilibrium value to a small extent only. In parallel,
it is also true that the surface concentration of the precursor ions of the growing metal
is just slightly smaller that its bulk concentration, even though the deposition leads
to a little depletion near the cathode surface (see the bottom graph on the right-hand
side of Fig. 2.6).
A “thumb rule” for the interrelation of the crystal size and the deposition conditions was first outlined by Dini [2] and later completed by Winand [17]. According
to Dini’s concept, essentially all factors lead to a reduction in the crystal size of
a metal being deposited which decrease the surface concentration of the precursor
metal ions. In contrast, if a modification of the deposition conditions leads to a better
replenishment of the precursor ions, an enhancement of the crystal size is expected.
In accord with these considerations, the increase in either the stirring rate or the
temperature as well as the application of a more concentrated solution of the salt
of the precursor metal results in a grain coarsening. The increase in the deposition
rate, should it be achieved by the application of either a more negative deposition
potential or a larger cathodic current density, leads to a grain refinement (i.e., smaller
crystallites).
A factor that requires the consideration of additional effect is the application of
chemicals in the plating bath that modify the deposition. The compounds applied
for such purposes do not make a distinct compound family since inorganic salts
and organic materials with a large variety of functional groups belong to these bath
components. The common name of these compounds, additives, refers to that they are
often a minor component in the plating bath. Additives themselves may also undergo
electrochemical reactions and their decomposition products may incorporate into the
deposit, too; nevertheless, their decomposition is a less important process. Additives
are somewhat similar to catalysts and inhibitors in the sense that they modify a process
without being transformed. Additives are usually classified on the basis of the primary
impact of their application. Brighteners, levelling agents and stress relievers are the
most common types of additives. In spite of the different target parameters, levelling
agents and brighteners exhibit similar impact mechanisms.
The common mechanism by which additives modify metal deposition is that they
adsorb at the typical growth centres. Since the atoms at the near-equilibrium growth
centres of the metal crystal surface are the most exposed ones to the solution and least
surrounded by atoms of the crystal, the adsorption of a molecule with one or several
free non-bonding electron pair(s) is more likely on these crystal surface features. The
adsorption of the additives reduces the probability that the crystal growth proceeds
but increases the nucleation of new crystals from the adatoms that form during the
discharge of the metal ions.
It is important at this point to highlight the impact mechanism of the additives
for both potentiostatic and galvanostatic deposition. If potentiostatic conditions are
applied and the concentration of additives is increased, the current decreases since the
same driving force can lead to the discharge of less ions (the deposition is hindered).
The discharge of the metal ions will result in an increase in the surface concentration
of the adatoms whose dissolution rate also increases. As a result, the application of