7.1 Composite Preparation by Codeposition of Metals
225
Pure Cu deposits are notorious of self-annealing also at room temperature that
leads to an increase in the grain size, and, consequently, the decrease in the specific
resistivity due to the diminished accumulated volume fraction of the grain boundary
zones. Silver doping of copper deposits can be used for the elimination of this selfannealing process by reducing the grain boundary diffusivity [33]. The inhibition of
the self-annealing of Cu can be observed at as low as 1.2 at.% Ag content [32] and
above.
Annealing at higher temperature is a core question in the relevant studies. An
increase in grain size was reported in all annealing-related studies. As the temperature dependence of the resistivity of the as-deposited samples shows, annealing up to
200 °C results in a small relative resistivity decrease due to an impurity redistribution
process. A further temperature increase up to 500 °C is required for silver precipitation and grain growth of the Cu matrix [35]. Nevertheless, one short annealing cycle
even up to 500 °C does not lead to the complete segregation of silver. The presence
of a residual Ag portion in the Cu crystals can be concluded from both diffraction
[33] and resistivity [36] data.
The functionality of the Cu(Ag) deposits is closely related to the element distribution in the as-received and annealed deposits. The composition depth profile of
the as-deposited samples shows two Ag peaks: one at the substrate/deposit boundary
and another one at the external surface of the sample [33, 36]. Upon annealing, the
bulk Ag concentration decreases while the surface peak becomes more intense. This
indicated that, beside the segregation of Ag to the grain boundary, a significant diffusion towards the external surface also takes place [36, 38]. The latter trend explains
why Ag-containing Cu deposits are much more oxidation resistant than pure Cu
coatings. These features of the electroplated samples are in good agreement with
those of sputtered metastable Cu(Ag) alloys [41]. The segregation trend of sulphur
in Cu(Ag) alloys is completely different from that of silver. If sulphur is present in
the deposit as a result of the decomposition of a bath additive [42], it accumulates at
the substrate/deposit interface upon annealing.
7.1.5 Miscellaneous Composites Obtained with Metal
Codeposition
Similarly to the binary systems discussed above, Ag and Ni also make an immiscible metal pair whose codeposition leads to various structures. The motivation of
the codeposition of Ag with Ni stemmed from the successful preparation of their
metastable alloys by physical deposition techniques. The common experience was
that some complexing agent was necessary for the formation of a compact deposit,
even though the complexing agent applied had little influence on the difference of
the onset potential of the deposition of the two metals. From electrochemical point
of view, the Ag deposition took place in the background of Ni deposition as the
current density increased, which was accompanied by a decrease of the Ag content
225
Pure Cu deposits are notorious of self-annealing also at room temperature that
leads to an increase in the grain size, and, consequently, the decrease in the specific
resistivity due to the diminished accumulated volume fraction of the grain boundary
zones. Silver doping of copper deposits can be used for the elimination of this selfannealing process by reducing the grain boundary diffusivity [33]. The inhibition of
the self-annealing of Cu can be observed at as low as 1.2 at.% Ag content [32] and
above.
Annealing at higher temperature is a core question in the relevant studies. An
increase in grain size was reported in all annealing-related studies. As the temperature dependence of the resistivity of the as-deposited samples shows, annealing up to
200 °C results in a small relative resistivity decrease due to an impurity redistribution
process. A further temperature increase up to 500 °C is required for silver precipitation and grain growth of the Cu matrix [35]. Nevertheless, one short annealing cycle
even up to 500 °C does not lead to the complete segregation of silver. The presence
of a residual Ag portion in the Cu crystals can be concluded from both diffraction
[33] and resistivity [36] data.
The functionality of the Cu(Ag) deposits is closely related to the element distribution in the as-received and annealed deposits. The composition depth profile of
the as-deposited samples shows two Ag peaks: one at the substrate/deposit boundary
and another one at the external surface of the sample [33, 36]. Upon annealing, the
bulk Ag concentration decreases while the surface peak becomes more intense. This
indicated that, beside the segregation of Ag to the grain boundary, a significant diffusion towards the external surface also takes place [36, 38]. The latter trend explains
why Ag-containing Cu deposits are much more oxidation resistant than pure Cu
coatings. These features of the electroplated samples are in good agreement with
those of sputtered metastable Cu(Ag) alloys [41]. The segregation trend of sulphur
in Cu(Ag) alloys is completely different from that of silver. If sulphur is present in
the deposit as a result of the decomposition of a bath additive [42], it accumulates at
the substrate/deposit interface upon annealing.
7.1.5 Miscellaneous Composites Obtained with Metal
Codeposition
Similarly to the binary systems discussed above, Ag and Ni also make an immiscible metal pair whose codeposition leads to various structures. The motivation of
the codeposition of Ag with Ni stemmed from the successful preparation of their
metastable alloys by physical deposition techniques. The common experience was
that some complexing agent was necessary for the formation of a compact deposit,
even though the complexing agent applied had little influence on the difference of
the onset potential of the deposition of the two metals. From electrochemical point
of view, the Ag deposition took place in the background of Ni deposition as the
current density increased, which was accompanied by a decrease of the Ag content