Abstract
•The (111)-oriented Cu seed layer promotes epitaxial growth of nanotwinned Cu, increasing tensile stress and inducing hillock formation through surface diffusion•The enhanced residual stress by the (111)-oriented Cu seed layer facilitates the generation of additional twin boundaries, resulting in reduced twin spacing and improved microstructural refinement•The seed layer effectively transforms stacking-fault defects into twin structures, reducing the transition layer thickness and lowering the overall film resistance without additional cost
During the electroplating of nanotwinned copper (nt-Cu), a transition layer with a non-negligible thickness is often formed. This commonly deteriorates the regularity of the Cu foil and consequently weakens its mechanical strength and electromigration resistance.
To address this issue, we introduce a two-step electroplating strategy where a thin (111)-oriented Cu seed layer is first deposited at a low current density (10 mA cm−2, ca. 0.1 µm). Then, it is followed by the high-rate nt-Cu growth in the same plating solution, which is a practically acceptable way.
The thin (111)-oriented Cu seed layer electroplated in the same bath prior to the main nt-Cu deposition effectively reduces the transition-layer thickness from ca. 2.6 μm to 1.22 μm and slightly decreases the average twin spacing (from 49.3 nm to 43.6 nm) of nt-Cu. This reveals an improvement in the microstructural quality of the subsequent deposit, potentially benefiting the electromigration resistance in the semiconductor interconnects and the mechanical strength of the negative electrode current collector of Li-ion batteries. However, the seed layer also slightly increases the surface roughness resulting from the formation of surface hillocks, probably detrimental to the electromigration performance.
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