140
5 Compositionally Modulated and Multilayered Deposits
stopped for the deposition periods (which were reported to make the optimization
of the deposition of the layers easier [33]). Therefore, the electrochemical cells used
are thin-layer cells, similarly to the EC-ALD studies.
Secondly, the choice of the electrode potential during the deposition of the subsequent layers is different from that applied in the EC-ALD system. When a UPD
layer is formed, the strong interaction between the alternating atomic layers ensures
that the stripping of the layers takes place at much more positive potentials than the
deposition potential of either of these layers. However, in multilayer systems, one of
the constituent layers usually has a much more negative onset potential of dissolution than the onset potential of the deposition of the other metal. This is the case, for
instance, for the deposition of a Co/Cu multilayer in which the Co dissolution can
take place about 500 mV more negative potential than the onset of Cu deposition (in
solutions containing no complexing agents). This means that the cathode potential
always has to be negative enough to avoid the dissolution of the least noble metal,
whichever deposition potential would be acceptable for the other metal on an inert
substrate. The synchronization of the electrode potential change with the solution
exchange is a delicate task for variable-bath deposition systems. Nevertheless, such
optimization for nanometre-scale layer thicknesses has not been reported so far in
detail in any available publication.
It is clear from the nature of the multiple-bath methods, should they be implemented with either an alternating cathode immersion or a solution exchange, that
one does not have to take into account any interference of the reagents that are not
present near the electrode at the same time. Therefore, if metallic elements form the
subsequent layers, the codeposition mode of these elements is of no importance, and
either of the layers can be produced as pure metal.
Examples for the flow cell-based dual bath electrodeposition include the plating
of Ni/Cu and Ni/Fe [33] and Co/Cu [34] multilayers. The lowest layer thickness
tested for the Ni/Cu and Ni/Fe systems was 50 nm, and the only observation made
was the occurrence of the natural crystal structure of each layer type in the X-ray
diffractograms [33]. During the preparation of the Co/Cu multilayers, the capability
of the alternating solution method was fully exploited and the lowest nominal layer
thickness was 1 nm [34]. Although the layer structure was assessed to be imperfect
by the authors in the latter case, the composition modulation was clearly shown by
the magnetoresistance effect measured for the layered samples.
5 Compositionally Modulated and Multilayered Deposits
stopped for the deposition periods (which were reported to make the optimization
of the deposition of the layers easier [33]). Therefore, the electrochemical cells used
are thin-layer cells, similarly to the EC-ALD studies.
Secondly, the choice of the electrode potential during the deposition of the subsequent layers is different from that applied in the EC-ALD system. When a UPD
layer is formed, the strong interaction between the alternating atomic layers ensures
that the stripping of the layers takes place at much more positive potentials than the
deposition potential of either of these layers. However, in multilayer systems, one of
the constituent layers usually has a much more negative onset potential of dissolution than the onset potential of the deposition of the other metal. This is the case, for
instance, for the deposition of a Co/Cu multilayer in which the Co dissolution can
take place about 500 mV more negative potential than the onset of Cu deposition (in
solutions containing no complexing agents). This means that the cathode potential
always has to be negative enough to avoid the dissolution of the least noble metal,
whichever deposition potential would be acceptable for the other metal on an inert
substrate. The synchronization of the electrode potential change with the solution
exchange is a delicate task for variable-bath deposition systems. Nevertheless, such
optimization for nanometre-scale layer thicknesses has not been reported so far in
detail in any available publication.
It is clear from the nature of the multiple-bath methods, should they be implemented with either an alternating cathode immersion or a solution exchange, that
one does not have to take into account any interference of the reagents that are not
present near the electrode at the same time. Therefore, if metallic elements form the
subsequent layers, the codeposition mode of these elements is of no importance, and
either of the layers can be produced as pure metal.
Examples for the flow cell-based dual bath electrodeposition include the plating
of Ni/Cu and Ni/Fe [33] and Co/Cu [34] multilayers. The lowest layer thickness
tested for the Ni/Cu and Ni/Fe systems was 50 nm, and the only observation made
was the occurrence of the natural crystal structure of each layer type in the X-ray
diffractograms [33]. During the preparation of the Co/Cu multilayers, the capability
of the alternating solution method was fully exploited and the lowest nominal layer
thickness was 1 nm [34]. Although the layer structure was assessed to be imperfect
by the authors in the latter case, the composition modulation was clearly shown by
the magnetoresistance effect measured for the layered samples.