134
5 Compositionally Modulated and Multilayered Deposits
appropriate additives for each type of layers, which are highly specific for the metal
to be deposited.
Since the contamination of either of the baths used for depositing a layer with the
components of the other bath(s) is highly undesired, the electrode has to be rinsed
between the deposition steps. Rinsing is accompanied with a loss of the plating
bath (called drag-out) as well as with the production of a large volume of waste
solution. The large amount of waste solution produced is the common feature of all
multiple-bath methods, should they be based on an alternating electrode immersion
or a solution flow.
5.2.2 The Sequential Immersion Method
Historically, the dual bath technique was the first electrochemical method to produce
metallic multilayers [7], although the thickness of the constituent layer was in the
micrometre range in the first trials. The basis of this technique is that the cathode is
moved between separated solutions and, hence, it is exposed to the baths containing
the ions of various metals alternatingly. This chapter summarizes the methods
applying stagnant solutions between which the cathode is transferred.
The mechanical transfer of the sample makes the system required quite complex.
A manual sample transfer imposes a very tight limit for the number of layers to be
produced, even though the composition of the layers (and also that of the bath) may
vary arbitrarily, allowing to plate a large variety of desired layer permutations. The
available thickness of the layers was assessed to be at least 25 nm when the dual
bath technique was elaborated and automated [8], but nowadays much lower layer
thickness values also appear in dual immersion-plated samples [9–11].
The electrical control mode for the sequential immersion method is exclusively
galvanostatic. This can be rationalized with the simplicity of the cell construction as
well as with the fact that baths are usually optimized for a specific current density
rather than for a particular electrode potential.
It has long been believed to be a disadvantage of the alternating immersion
methods that the sample being transferred cannot be held under potential control,
which may lead to the oxidation of the sample in the ambient atmosphere. Although
it is theoretically possible to provide an inert gas atmosphere for the entire sample
transfer process (see the subsequent immersion method in Chap. 4.2.1), it is seldom
applied if the layers are thicker than a single atomic layer. This means that layers
with as small a thickness as a few nanometres can hardly be protected sufficiently
upon the transfer step, and the impact of the environment can be the more significant,
the thinner the layer.
Even if the oxidation in the ambient atmosphere is eliminated, another type of
corrosion process may occur when the sample is transferred into the solution of the
salt of the more noble metal. Taking the example of Co and Cu, when the Co-covered
specimen is immersed into the Cu
2+ -containing solution, the exchange reaction (also
termed as displacement or cementation) takes place. The process can be formulated as
5 Compositionally Modulated and Multilayered Deposits
appropriate additives for each type of layers, which are highly specific for the metal
to be deposited.
Since the contamination of either of the baths used for depositing a layer with the
components of the other bath(s) is highly undesired, the electrode has to be rinsed
between the deposition steps. Rinsing is accompanied with a loss of the plating
bath (called drag-out) as well as with the production of a large volume of waste
solution. The large amount of waste solution produced is the common feature of all
multiple-bath methods, should they be based on an alternating electrode immersion
or a solution flow.
5.2.2 The Sequential Immersion Method
Historically, the dual bath technique was the first electrochemical method to produce
metallic multilayers [7], although the thickness of the constituent layer was in the
micrometre range in the first trials. The basis of this technique is that the cathode is
moved between separated solutions and, hence, it is exposed to the baths containing
the ions of various metals alternatingly. This chapter summarizes the methods
applying stagnant solutions between which the cathode is transferred.
The mechanical transfer of the sample makes the system required quite complex.
A manual sample transfer imposes a very tight limit for the number of layers to be
produced, even though the composition of the layers (and also that of the bath) may
vary arbitrarily, allowing to plate a large variety of desired layer permutations. The
available thickness of the layers was assessed to be at least 25 nm when the dual
bath technique was elaborated and automated [8], but nowadays much lower layer
thickness values also appear in dual immersion-plated samples [9–11].
The electrical control mode for the sequential immersion method is exclusively
galvanostatic. This can be rationalized with the simplicity of the cell construction as
well as with the fact that baths are usually optimized for a specific current density
rather than for a particular electrode potential.
It has long been believed to be a disadvantage of the alternating immersion
methods that the sample being transferred cannot be held under potential control,
which may lead to the oxidation of the sample in the ambient atmosphere. Although
it is theoretically possible to provide an inert gas atmosphere for the entire sample
transfer process (see the subsequent immersion method in Chap. 4.2.1), it is seldom
applied if the layers are thicker than a single atomic layer. This means that layers
with as small a thickness as a few nanometres can hardly be protected sufficiently
upon the transfer step, and the impact of the environment can be the more significant,
the thinner the layer.
Even if the oxidation in the ambient atmosphere is eliminated, another type of
corrosion process may occur when the sample is transferred into the solution of the
salt of the more noble metal. Taking the example of Co and Cu, when the Co-covered
specimen is immersed into the Cu
2+ -containing solution, the exchange reaction (also
termed as displacement or cementation) takes place. The process can be formulated as