154
5 Compositionally Modulated and Multilayered Deposits
available in the original works [83–87]. The films thus removed from the substrate
with a thickness of a few nanometre can be studied with a sputtering-based analysis
method, preferably with secondary neutral mass spectrometry (SNMS) in which the
sample is entirely in the vacuum chamber of the device (and hence, it is not required
to withstand against a pressure difference, unlike in glow discharge devices).
The result of the studies performed for alloy deposits obtained from stagnant
solutions was that the preferentially deposited component(s) of the alloy accumulated
in the near-substrate zone of the deposit. This can happen because at the moment when
the deposition starts, the solution composition near the cathode surface is identical
to the bulk solution composition. However, as the deposition proceeds, the depletion
influences the near-substrate concentration of the ions of the preferentially deposited
metal to the largest extent. As the steady-state composition profile is achieved for
the precursor ions, the deposition rate of the preferentially deposited metal drops.
This trend proved to be valid for both regular and anomalous deposition modes
of binary and ternary alloys, and the near-substrate zone in which the composition
stabilization of the deposit takes place was at most 200 nm. A few typical composition
depth profile curves of various alloys is the near-substrate zone is shown in Fig. 5.6.
For binary alloys, there is a maximum in the mole fraction of the preferentially
deposited component near the substrate. In contrast, for ternary alloys, there are
subsequent maxima in the mole fractions in the order of the deposition preference
of the components, and the decay of the spontaneous composition change often has
a complicated character with various oscillation patterns.
The appropriate countermeasures for avoiding the spontaneous composition
modulation near the substrate include either the thorough control of the hydrodynamics during the deposition [87] or the application of pulse plating instead of
constant current [86]. In both cases, the initial composition change in the deposit is
minimized, although the reasons for the result are different. If the cathode is rotated,
the depletion of the solution with respect for the precursor ions is much depressed,
0
100
200
300
400
0.00
0.04
0.08
0.12
0.16
Ni 80 Fe 20
Ni 98.8 Cd 1.2
Ni 96 Co 4
mole fraction (Co, Cd)
thickness / nm
0.0
0.1
0.2
0.3
0.4
mole fraction (Fe)
0
200
400
600
0.0
0.2
0.4
0.6
0.8
Ni
Co
Fe
mole fraction
thickness / nm
a
b
Fig. 5.6 Reverse composition depth profile of various d.c.-plated alloys near the substrate as
measured with SNMS. a Ni–Cd, Ni–Co and Ni–Fe binary alloys (mole fraction of Ni is not shown);
b Fe–Co–Ni ternary alloy. For all curves, zero thickness indicates the substrate/deposit interface
[88]
5 Compositionally Modulated and Multilayered Deposits
available in the original works [83–87]. The films thus removed from the substrate
with a thickness of a few nanometre can be studied with a sputtering-based analysis
method, preferably with secondary neutral mass spectrometry (SNMS) in which the
sample is entirely in the vacuum chamber of the device (and hence, it is not required
to withstand against a pressure difference, unlike in glow discharge devices).
The result of the studies performed for alloy deposits obtained from stagnant
solutions was that the preferentially deposited component(s) of the alloy accumulated
in the near-substrate zone of the deposit. This can happen because at the moment when
the deposition starts, the solution composition near the cathode surface is identical
to the bulk solution composition. However, as the deposition proceeds, the depletion
influences the near-substrate concentration of the ions of the preferentially deposited
metal to the largest extent. As the steady-state composition profile is achieved for
the precursor ions, the deposition rate of the preferentially deposited metal drops.
This trend proved to be valid for both regular and anomalous deposition modes
of binary and ternary alloys, and the near-substrate zone in which the composition
stabilization of the deposit takes place was at most 200 nm. A few typical composition
depth profile curves of various alloys is the near-substrate zone is shown in Fig. 5.6.
For binary alloys, there is a maximum in the mole fraction of the preferentially
deposited component near the substrate. In contrast, for ternary alloys, there are
subsequent maxima in the mole fractions in the order of the deposition preference
of the components, and the decay of the spontaneous composition change often has
a complicated character with various oscillation patterns.
The appropriate countermeasures for avoiding the spontaneous composition
modulation near the substrate include either the thorough control of the hydrodynamics during the deposition [87] or the application of pulse plating instead of
constant current [86]. In both cases, the initial composition change in the deposit is
minimized, although the reasons for the result are different. If the cathode is rotated,
the depletion of the solution with respect for the precursor ions is much depressed,
0
100
200
300
400
0.00
0.04
0.08
0.12
0.16
Ni 80 Fe 20
Ni 98.8 Cd 1.2
Ni 96 Co 4
mole fraction (Co, Cd)
thickness / nm
0.0
0.1
0.2
0.3
0.4
mole fraction (Fe)
0
200
400
600
0.0
0.2
0.4
0.6
0.8
Ni
Co
Fe
mole fraction
thickness / nm
a
b
Fig. 5.6 Reverse composition depth profile of various d.c.-plated alloys near the substrate as
measured with SNMS. a Ni–Cd, Ni–Co and Ni–Fe binary alloys (mole fraction of Ni is not shown);
b Fe–Co–Ni ternary alloy. For all curves, zero thickness indicates the substrate/deposit interface
[88]