100
4 Ultrathin Layers
electrical connection has to be interrupted when the solution carrying ions of the
deposited metal is introduced into the cell.
Concerning the composition of the deposits obtained with an SLRR process, these
are mostly pure metals, unlike for EC-ALD. Hence, the significance of SLRR does
not come from the fact that a material with special chemical composition is deposited,
but from the very small mean thickness of the layer that is at the same time surfaceconformal. Unlike the conventional electrodeposition of metals that seldom leads to
monoatomic layers (see Chap. 2.10), the deposition by SLRR is much more similar to
an ALD process in its surface-conformal nature. At the same time, the production of
special non-equilibrium atomic structures of multilayers is also possible (see later).
The control of SLRR processes, driving the displacement half-cycle reaction
strictly under open circuit condition, can be based on the observation of the electrode
potential. Kinetic models include the substrate–deposit and deposit–deposit atomic
interactions, adsorption isoterms and Gibbs free energy changes of the reactions
[118]. Kinetic studies showed [119] that the rate of the replacement reaction can be
elucidated in terms of surface limitation at a concentration of the ions of the growing
metal in the millimolar range, while the transport limitation is in effect when this
concentration is lowered to the level of 0.01 mM. The typical replacement time is
a few tens of seconds for the mM concentration range, which increases inversely
proportional to the concentration in stagnant solutions. The E(t) functions obtained
could be fitted with the appropriate isoterm, taking also into account the order of
the replacement reaction. Figure 4.9 shows typical surface coverage vs. electrode
potential and electrode potential vs. displacement time functions.
In a later work of the Brankovic group [120], in situ surface reflectivity measurements were also applied for following the kinetics of the displacement process, and
an excellent agreement was found between the theoretical and experimental values.
In the latter work, surface reflectivity data were taken advantage in the calculation
of kinetic coefficients under various circumstances.
Although the general goal is to achieve a complete replacement of the sacrificial
metal, the partial replacement may also be a goal [114]. To this end, the cut-off
potential should be set so that the potential decay is yet incomplete, and a significant
part of the sacrificial metal remains on the surface.
4.2.2 Substrates and Displacement Pairs in SLRR Processes
The majority of the studies refer to noble metals as substrates (Au, Pd, Ru), many
of them having a single-crystalline character or at least a preferred orientation
like Au(111) for sputtered gold. Beside noble metal substrates, it is worthwhile
of mentioning that the SLRR process has also been tested for materials where the
“classical” UPD process could not be observed. In the study of Fang et al. [121],
the substrate was a sputtered Co film, and the sacrificial metal was Pb that mediated
the deposition of Cu. Although the Pb UPD peak was not very clear, the Pb-to-Cu
replacement cycles were successful, and the technique suggested was suitable to
4 Ultrathin Layers
electrical connection has to be interrupted when the solution carrying ions of the
deposited metal is introduced into the cell.
Concerning the composition of the deposits obtained with an SLRR process, these
are mostly pure metals, unlike for EC-ALD. Hence, the significance of SLRR does
not come from the fact that a material with special chemical composition is deposited,
but from the very small mean thickness of the layer that is at the same time surfaceconformal. Unlike the conventional electrodeposition of metals that seldom leads to
monoatomic layers (see Chap. 2.10), the deposition by SLRR is much more similar to
an ALD process in its surface-conformal nature. At the same time, the production of
special non-equilibrium atomic structures of multilayers is also possible (see later).
The control of SLRR processes, driving the displacement half-cycle reaction
strictly under open circuit condition, can be based on the observation of the electrode
potential. Kinetic models include the substrate–deposit and deposit–deposit atomic
interactions, adsorption isoterms and Gibbs free energy changes of the reactions
[118]. Kinetic studies showed [119] that the rate of the replacement reaction can be
elucidated in terms of surface limitation at a concentration of the ions of the growing
metal in the millimolar range, while the transport limitation is in effect when this
concentration is lowered to the level of 0.01 mM. The typical replacement time is
a few tens of seconds for the mM concentration range, which increases inversely
proportional to the concentration in stagnant solutions. The E(t) functions obtained
could be fitted with the appropriate isoterm, taking also into account the order of
the replacement reaction. Figure 4.9 shows typical surface coverage vs. electrode
potential and electrode potential vs. displacement time functions.
In a later work of the Brankovic group [120], in situ surface reflectivity measurements were also applied for following the kinetics of the displacement process, and
an excellent agreement was found between the theoretical and experimental values.
In the latter work, surface reflectivity data were taken advantage in the calculation
of kinetic coefficients under various circumstances.
Although the general goal is to achieve a complete replacement of the sacrificial
metal, the partial replacement may also be a goal [114]. To this end, the cut-off
potential should be set so that the potential decay is yet incomplete, and a significant
part of the sacrificial metal remains on the surface.
4.2.2 Substrates and Displacement Pairs in SLRR Processes
The majority of the studies refer to noble metals as substrates (Au, Pd, Ru), many
of them having a single-crystalline character or at least a preferred orientation
like Au(111) for sputtered gold. Beside noble metal substrates, it is worthwhile
of mentioning that the SLRR process has also been tested for materials where the
“classical” UPD process could not be observed. In the study of Fang et al. [121],
the substrate was a sputtered Co film, and the sacrificial metal was Pb that mediated
the deposition of Cu. Although the Pb UPD peak was not very clear, the Pb-to-Cu
replacement cycles were successful, and the technique suggested was suitable to