Electrochemical Co-deposition of Binary Alloys

Electrochemical deposition (ECD) is widely utilized in the fabrication of micro- and nano- scale structures, including Cu interconnects in semiconductor chips and metallic components in micro- electro- mechanical systems (MEMS). This technique is easy to implement and cost effective; in addition it can provide close control of the morphology and microstructure of the metal by simply tuning electrolyte chemistry, applied potential or current density. ECD of alloys could extend the fabrication opportunities offered by the pure metals, providing materials having better and more tunable properties. However, this process is complicated by the need to achieve uniformity of both film thickness and composition. A better understanding of the processes involved in the ECD of alloys is needed in order to control and predict the properties of the deposited materials.
This dissertation has focused on a simplified description of the alloy ECD process based on the energetics of the overall transformation from the ions in the solution to the atoms in the alloy lattice. In particular, I investigated how thermodynamics can be used to control and predict composition and crystal structures of electrodeposited binary alloys by exploiting the underpotential co-deposition (UPCD) phenomenon, whereby an active metal is deposited at potentials more positive than its redox potential by co-deposition with a noble metal, due to the negative enthalpy of mixing. Several alloy systems with different alloying behaviors have been investigated.

Au-Cu and Fe-Pt, both having a negative enthalpy of mixing, have been studied by using various electrolyte chemistries, aiming to the production of high quality films. Both systems follow the UPCD predictions; composition is well controlled by the applied deposition potential and solid solutions are formed. Smooth films with uniform composition are obtained with complexing solutions, while in some cases non-complexing solutions lead to compositional inhomogeneity and porous films.

UPCD predicts that the formation of solid solutions for systems with positive enthalpy of mixing (that do not tend to mix in the bulk) should be accompanied by a polarization of the deposition potential. We study Ag-Cu and Au-Ni, two systems with positive enthalpy of mixing, differing however in the fact that the first shows a tendency for unlike pairs not to form, while the second shows the tendency for Au-Ni pairs to form preferentially with respect to homologous pairs. Our work shows a distinct behavior for Ag-Cu with respect to Au-Ni. Ag-Cu shows practically no miscibility when deposited from non-complexing solutions: the two metal ions are reduced independently and form separate phases; in contrast, deposition from thiourea complexing solutions leads to solid solution formation as evidenced by XRD. This contrasting behavior is tentatively explained by the influence of the Potential of Zero Charge and by thiourea complexation in the latter solution. Au-Ni on the other hand forms a continuous series of solid solutions as observed by XRD. However, XRD is insufficient in clarifying the homogeneity of nanostructured alloy, and XPS was carried out on these samples at ESRF in Grenoble (France). XPS data also suggest alloying between Au and Ni. HRTEM clarifies this issue by showing a heterogeneous microstructure; samples with an average Ni fraction of 40 at% consist of ~5nm Au-Ni crystalline grains with less Ni than the overall composition, and ~2nm grain boundaries having poor crystallinity and being richer in Ni, indicating nano-scale phase separation.

Finally, a study of the Ag-Ni system with positive enthalpy of mixing and immiscibility extending well into the liquid phase shows that UPCD is not obeyed in this case. Ag-rich solid solutions are detected by XRD. HRTEM shows an evolution of the microstructure with increasing Ni content: Ag rich films (<1at.% of Ni) show uniform grain structures ~20nm; grain size decreases and grain boundaries with poor crystallinity or amorphous structure take up a larger portion of the films; regions of pure Ni, pure Ag and Ag-Ni alloy are all found for Ni rich films, indicating a multiphasic structure. Annealing of the films at 600ºC led to the decomposition into separate phases, confirming the formation of a metastable structure in the as-deposited films.