4.2 Combination of UPD with Other Surface-Area-Limited Processes
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an inert atmosphere for avoiding the unwanted oxidation of the sacrificial metal.
Although the absence of oxygen is a prerequisite for all possible configurations,
the requirements for providing a non-reactive environment is the most difficult if
sequential immersion is applied.
The build-up of an SLRR layer can also be organized into cycles provided that a
suitable UPD layer can form again onto the metal deposited in the previous SLRR
process. This newly-formed UPD layer can be stripped off again by a more noble
metal. Due to the high demand for the exclusion of the mutual solution contamination,
the method with subsequent immersions is impractical for several cycles. This results
in the situation that in the technical sense, the apparatus to deposit several sequential
SLRR monolayers is analogous to that used in EC-ALD. The first step of the cycle
is the deposition of a UPD layer, but the next one is a replacement process where
the thickness does not increase. Instead, the composition of the surface changes, and
no electrochemical control is applied in this step (the process goes on under open
circuit conditions) except for monitoring the electrode potential. The big majority
of the SLRR studies apply this type of instrumentation. The apparatus involving a
solution replacement is easy to use also for the deposition of an alloyed layer if the
solutions used in the subsequent displacement half-cycles contain the ions of various
growing metals [107]. In such a case, the UPD of the sacrificial metal has to undergo
on the surface of both depositing metals.
Another approach for SLRR is based on a single solution in which the ions of the
sacrificial and the depositing metals are present in different concentrations [108–114].
Although the control of the deposit composition is less accurate than for separated
solutions, should this mean an alternating immersion or a flow cell approach, the
simplification offers the opportunity of the repeated application of the same bath
without a high consumption of rinsing solution. The deposition of the UPD layer of
the sacrificial metal takes place during a relatively short potential pulse with kinetic
control, and then the replacement process is left to take place in a much longer time
with the diffusion control of the reaction. The concentration ratio of the sacrificial
and the deposited metal is typically 2–100 for the one-pot method. The larger the
ratio of the apparent deposition rate constants of the sacrificial metal to the deposited
one, the lower concentration ratio of their ions can be allowed. The incorporation of
the sacrificial metal into the desired layer was occasionally found [109], which may
necessitate an anodic dealloying step after the completion of the deposition cycles.
By omitting the dealloying step, the incorporation of the sacrificial metal into the
deposited one can also be used intentionally for producing alloyed layers.
The production of the sacrificial UPD layer to be displaced is also possible in an
electroless manner [115, 116], although the mechanism of the deposition process is
purely electrochemical. In this case, a simple galvanic couple is created by connecting
the substrate to be covered with a so-called “executive” metal on which a dissolution
process takes place to the same extent as the UPD layer formation on the substrate.
The driving force of the deposition of the sacrificial metal is hence provided by
creating an electrochemical cell instead of applying an electronic power source.
Electroless SLRR is also possible if the substrate can absorb hydrogen which is
applied as a sacrificial material in the deposition half-cycle [117]. Of course, the
99
an inert atmosphere for avoiding the unwanted oxidation of the sacrificial metal.
Although the absence of oxygen is a prerequisite for all possible configurations,
the requirements for providing a non-reactive environment is the most difficult if
sequential immersion is applied.
The build-up of an SLRR layer can also be organized into cycles provided that a
suitable UPD layer can form again onto the metal deposited in the previous SLRR
process. This newly-formed UPD layer can be stripped off again by a more noble
metal. Due to the high demand for the exclusion of the mutual solution contamination,
the method with subsequent immersions is impractical for several cycles. This results
in the situation that in the technical sense, the apparatus to deposit several sequential
SLRR monolayers is analogous to that used in EC-ALD. The first step of the cycle
is the deposition of a UPD layer, but the next one is a replacement process where
the thickness does not increase. Instead, the composition of the surface changes, and
no electrochemical control is applied in this step (the process goes on under open
circuit conditions) except for monitoring the electrode potential. The big majority
of the SLRR studies apply this type of instrumentation. The apparatus involving a
solution replacement is easy to use also for the deposition of an alloyed layer if the
solutions used in the subsequent displacement half-cycles contain the ions of various
growing metals [107]. In such a case, the UPD of the sacrificial metal has to undergo
on the surface of both depositing metals.
Another approach for SLRR is based on a single solution in which the ions of the
sacrificial and the depositing metals are present in different concentrations [108–114].
Although the control of the deposit composition is less accurate than for separated
solutions, should this mean an alternating immersion or a flow cell approach, the
simplification offers the opportunity of the repeated application of the same bath
without a high consumption of rinsing solution. The deposition of the UPD layer of
the sacrificial metal takes place during a relatively short potential pulse with kinetic
control, and then the replacement process is left to take place in a much longer time
with the diffusion control of the reaction. The concentration ratio of the sacrificial
and the deposited metal is typically 2–100 for the one-pot method. The larger the
ratio of the apparent deposition rate constants of the sacrificial metal to the deposited
one, the lower concentration ratio of their ions can be allowed. The incorporation of
the sacrificial metal into the desired layer was occasionally found [109], which may
necessitate an anodic dealloying step after the completion of the deposition cycles.
By omitting the dealloying step, the incorporation of the sacrificial metal into the
deposited one can also be used intentionally for producing alloyed layers.
The production of the sacrificial UPD layer to be displaced is also possible in an
electroless manner [115, 116], although the mechanism of the deposition process is
purely electrochemical. In this case, a simple galvanic couple is created by connecting
the substrate to be covered with a so-called “executive” metal on which a dissolution
process takes place to the same extent as the UPD layer formation on the substrate.
The driving force of the deposition of the sacrificial metal is hence provided by
creating an electrochemical cell instead of applying an electronic power source.
Electroless SLRR is also possible if the substrate can absorb hydrogen which is
applied as a sacrificial material in the deposition half-cycle [117]. Of course, the