Effect of [Zn2+]/[Cu2+] ratio of the bath on the composition and property of Cu–Zn alloy micropillars prepared using microanode-guided electroplating

Cu–Zn alloy micropillars were fabricated using microanode-guided electroplating (MAGE) at a constant voltage of 4.6 V under an initial inter-electrode gap of 30 µm in pyrophosphate baths. Energy-dispersive spectroscopy and X-ray diffractometry analysis revealed that the micropillar alloys exhibit an increase in the Zn content from 8.05 at.% (in α phase) to 54.60 at.% (in β, and β' phases) with an increase in the ratio of [Zn2+]/[Cu2+] in the bath from 8/1 to 128/1. Transmission electron microscopy revealed that the Cu–Zn alloy micropillars were composed of nano-grains with different sizes depending on the bath employed. Nano-indentation test indicated that the micropillar containing 40.51 at.% Zn (in presence of β-phase) exhibits the maximum hardness (at 5.30 GPa) and highest Young's modulus (at 113.64 GPa). Simulation of the electric field using the commercial software COMSOL 5.2 revealed that the highest field strength centralized at the cylindrical top displayed only a slight increase (from 152.2 to 152.5 kV/m) with an increase in the ratio of [Zn2+]/ [Cu2+] in the bath. Potentiodynamic cathodic polarization is useful for understanding the mechanism of microanode-guided electroplating. Electrochemical impedance spectroscopy performed at characteristic potentials confirms the mechanism extremely well.