5.4 Properties of Electrodeposited CMAs
153
in an alloy plating solution exhibiting regular codeposition is small and the current
density is quite high, the thickness of such an initial layer is little and was not the
subject of a direct observation. However, if pulse plating is applied, such a MN metal
layer occurs for the initial period of each single pulse, which has an impact on the
mean composition of the deposit [48]. The partial current density of the MN metal
at the beginning of the pulse can be even 100%, which is followed by a decay whose
rate is a function of many parameters (including the ratio of the concentration of the
metal ions, that of their transport rate and the current density).
Concerning the general aspects of the unintentional composition variation during
alloy plating, it was shown already in the 1960s that the mean composition of electrodeposited Fe–Ni alloys depends on the deposition time [78–80]. The reason why
the composition change was discovered on Fe–Ni alloys was that the deposition of
zero-magnetostriction Fe–Ni films, often called Permalloy, has been an important
goal of electroplating for decades. The thickness of the near-substrate zone with
a significant composition variation was assessed to be around 150–200 nm [79].
Although the observation of the mean deposit composition as a function of the deposition time is an integral method, it remained an important approach until nowadays
[81, 82]. However, it is straightforward that the change of the overall composition
with thickness must stem from the gradual change of the local composition. Therefore, a nanoscale composition-modulated alloy is formed unintentionally even during
d.c. deposition.
Sputtering-based in-depth composition analysis methods (that are also referred
to as depth profiling) are seldom suitable to detect subtle near-substrate composition
changes. Sputtering-based methods always induce some roughening, and the roughness of the actual surface during the sputtering process is not predictable since it
depends on the initial surface roughness and the sputtering-induced effects. Therefore, the surface during the sputtering-based analysis process practically always
differs from that occurring during the deposition, which makes the composition
changes far from the substrate much smeared out. Although scanning EDS for
thinned samples during a TEM study can yield a composition profile with quite
high resolution, this technique is seldom used, at least by far not routinely.
Near-substrate composition profile of electrodeposited samples can be obtained
with a high resolution if two conditions are fulfilled. First, the substrate has to be as
smooth as possible; second, the deposit has to be removed from the substrate with
no damage. Once the deposit is separated from the smooth substrate, the sputteringbased depth profile analysis can be started from the “back side” of the sample, hence
studying the near-substrate zone without any roughening artefact.
The technique of the sample preparation for sputtering from the reverse direction
was developed by the author of this work. In a nutshell, the process involves the
following steps. The substrate is a polished silicon wafer with sputtered adhesion
and conducting layers (which can be Cr and Cu with as small thicknesses as 5
and 20 nm, respectively). Then, the deposit of interest is plated onto the substrate.
If needed, an additional nickel supporting layer can be plated onto the deposit of
interest. Then, the silicon wafer is scratched at the back side and gently broken,
which makes it possible to peel off the deposit. Further details of the procedure are
153
in an alloy plating solution exhibiting regular codeposition is small and the current
density is quite high, the thickness of such an initial layer is little and was not the
subject of a direct observation. However, if pulse plating is applied, such a MN metal
layer occurs for the initial period of each single pulse, which has an impact on the
mean composition of the deposit [48]. The partial current density of the MN metal
at the beginning of the pulse can be even 100%, which is followed by a decay whose
rate is a function of many parameters (including the ratio of the concentration of the
metal ions, that of their transport rate and the current density).
Concerning the general aspects of the unintentional composition variation during
alloy plating, it was shown already in the 1960s that the mean composition of electrodeposited Fe–Ni alloys depends on the deposition time [78–80]. The reason why
the composition change was discovered on Fe–Ni alloys was that the deposition of
zero-magnetostriction Fe–Ni films, often called Permalloy, has been an important
goal of electroplating for decades. The thickness of the near-substrate zone with
a significant composition variation was assessed to be around 150–200 nm [79].
Although the observation of the mean deposit composition as a function of the deposition time is an integral method, it remained an important approach until nowadays
[81, 82]. However, it is straightforward that the change of the overall composition
with thickness must stem from the gradual change of the local composition. Therefore, a nanoscale composition-modulated alloy is formed unintentionally even during
d.c. deposition.
Sputtering-based in-depth composition analysis methods (that are also referred
to as depth profiling) are seldom suitable to detect subtle near-substrate composition
changes. Sputtering-based methods always induce some roughening, and the roughness of the actual surface during the sputtering process is not predictable since it
depends on the initial surface roughness and the sputtering-induced effects. Therefore, the surface during the sputtering-based analysis process practically always
differs from that occurring during the deposition, which makes the composition
changes far from the substrate much smeared out. Although scanning EDS for
thinned samples during a TEM study can yield a composition profile with quite
high resolution, this technique is seldom used, at least by far not routinely.
Near-substrate composition profile of electrodeposited samples can be obtained
with a high resolution if two conditions are fulfilled. First, the substrate has to be as
smooth as possible; second, the deposit has to be removed from the substrate with
no damage. Once the deposit is separated from the smooth substrate, the sputteringbased depth profile analysis can be started from the “back side” of the sample, hence
studying the near-substrate zone without any roughening artefact.
The technique of the sample preparation for sputtering from the reverse direction
was developed by the author of this work. In a nutshell, the process involves the
following steps. The substrate is a polished silicon wafer with sputtered adhesion
and conducting layers (which can be Cr and Cu with as small thicknesses as 5
and 20 nm, respectively). Then, the deposit of interest is plated onto the substrate.
If needed, an additional nickel supporting layer can be plated onto the deposit of
interest. Then, the silicon wafer is scratched at the back side and gently broken,
which makes it possible to peel off the deposit. Further details of the procedure are