2.12 Composition Aspects of Alloy Electrodeposition
49
characterized with irregular codeposition mostly include complex ion systems such
as the Cu–Zn from a cyanide bath, while the anomalous codeposition is valid for
any pair among the Fe–Co–Ni–Zn group. The common feature for these categories
is that the metals indeed form an alloy.
For a better understanding of the impact of the deposition conditions on the deposit
composition, it is worthwhile of studying Fig. 2.21b. It shows that the increase in the
supply of the preferentially deposited metal, should it be either the more noble one (for
regular and irregular codeposition) or the less noble one (anomalous codeposition),
results in the increase of the mole fraction of this metal in the deposit. In contrast,
when the abundance of the ions of the preferentially deposited metal decreases in the
vicinity of the cathode because of, e.g., the increase in current density, an adverse
effect can be seen. Figure 2.21b yields a guideline for composition modulation during
the electrodeposition from a single solution by means of agitation and current density
modulation.
Composition diagrams usually refer to a steady-state deposition process which is
not influenced by the transition that takes place at the beginning of the deposition.
In the initial transition period of the alloy deposition, the concentrations (and hence,
also the concentration gradients) of the precursor ions change until the deposition
process is stabilized. In the near-substrate zone of the deposit, the mole fraction of
the preferentially deposited metals is always larger than in a steady-state deposit.
At room temperature and in a stagnant solution, the thickness of the transition zone
ranges up to 200 nm. Figure 2.21b can also be used for assessing the impact of the
initial transient on the composition: the composition line for a very thin deposit moves
in the same direction as if the mass transport war stimulated. The explanation is that
at the beginning of the deposition process, the near-substrate solution composition
is closer to the bulk one than in the steady-state.
The impact of pulse plating on the composition diagram can be elucidated on
the basis of the same train of thought that was applied for the initial transient of a
d.c. deposit. As a result of the frequently intermitted current, the solution depletion
near the cathode is counteracted by the mass transport during the off-time. Hence,
it is straightforward that the application of current pulses modifies the composition
the same way as discussed for the initial zone or for an enhanced bath agitation
(provided that the d.c. current density is the same as the on-time current density
during the pulse plating). Pulse plating can also be regarded from the view-point of
the deposit composition as if only the beginning of a d.c. deposit was plated a large
number of times.
For induced codeposition, no curves can be displayed in the conventional composition diagram like Fig. 2.21 in the entire solution composition range. The reason
is that the metal whose deposition is induced by the other one cannot be deposited
alone (at least, not with an appreciable current efficiency). This takes place when
iron-group metals (that play the role of the inducing component) are codeposited with
oxoanion-forming elements (metals like Mo and W and also non-metallic elements
like P) or germanium. As the ratio of the ions of the induced metal increases, an
49
characterized with irregular codeposition mostly include complex ion systems such
as the Cu–Zn from a cyanide bath, while the anomalous codeposition is valid for
any pair among the Fe–Co–Ni–Zn group. The common feature for these categories
is that the metals indeed form an alloy.
For a better understanding of the impact of the deposition conditions on the deposit
composition, it is worthwhile of studying Fig. 2.21b. It shows that the increase in the
supply of the preferentially deposited metal, should it be either the more noble one (for
regular and irregular codeposition) or the less noble one (anomalous codeposition),
results in the increase of the mole fraction of this metal in the deposit. In contrast,
when the abundance of the ions of the preferentially deposited metal decreases in the
vicinity of the cathode because of, e.g., the increase in current density, an adverse
effect can be seen. Figure 2.21b yields a guideline for composition modulation during
the electrodeposition from a single solution by means of agitation and current density
modulation.
Composition diagrams usually refer to a steady-state deposition process which is
not influenced by the transition that takes place at the beginning of the deposition.
In the initial transition period of the alloy deposition, the concentrations (and hence,
also the concentration gradients) of the precursor ions change until the deposition
process is stabilized. In the near-substrate zone of the deposit, the mole fraction of
the preferentially deposited metals is always larger than in a steady-state deposit.
At room temperature and in a stagnant solution, the thickness of the transition zone
ranges up to 200 nm. Figure 2.21b can also be used for assessing the impact of the
initial transient on the composition: the composition line for a very thin deposit moves
in the same direction as if the mass transport war stimulated. The explanation is that
at the beginning of the deposition process, the near-substrate solution composition
is closer to the bulk one than in the steady-state.
The impact of pulse plating on the composition diagram can be elucidated on
the basis of the same train of thought that was applied for the initial transient of a
d.c. deposit. As a result of the frequently intermitted current, the solution depletion
near the cathode is counteracted by the mass transport during the off-time. Hence,
it is straightforward that the application of current pulses modifies the composition
the same way as discussed for the initial zone or for an enhanced bath agitation
(provided that the d.c. current density is the same as the on-time current density
during the pulse plating). Pulse plating can also be regarded from the view-point of
the deposit composition as if only the beginning of a d.c. deposit was plated a large
number of times.
For induced codeposition, no curves can be displayed in the conventional composition diagram like Fig. 2.21 in the entire solution composition range. The reason
is that the metal whose deposition is induced by the other one cannot be deposited
alone (at least, not with an appreciable current efficiency). This takes place when
iron-group metals (that play the role of the inducing component) are codeposited with
oxoanion-forming elements (metals like Mo and W and also non-metallic elements
like P) or germanium. As the ratio of the ions of the induced metal increases, an