Development of nanotwins in electroplated copper and its effect on shear strength of tin/copper joint

Highlights: • Electroplated Cu film is fabricated using a specific plating formula. • Polyethylene glycol (PEG) and chloride ion (Cl⁻) are additives as suppressor. • Nanotwinned lamellae of Cu form with decreasing plating current density. • Impurity incorporation is suppressed with the formation of twinning structure. • Shear strength of Sn/Cu joint is improved by suppressing interfacial void formation. Cost-effective copper (Cu) electroplating is the primary technique used to fabricate wires/interconnects in microelectronics. Grain microstructure and impurity incorporation in electroplated Cu plays an important role in its mechanical, electrical, and thermal properties. In this study, a specific plating formula with a basic electrolyte and a suppressor, polyethylene glycol (PEG) and chloride ion (Cl⁻), is used to fabricate the Cu layers. An adjustment of plating current density dramatically alters the grain microstructure and impurity incorporation in the electroplated Cu. Scanning electron microscopy (SEM) and coincidence site lattice (CSL) analyses show that the Cu deposit plated at a low current density (< 0.43 A/dm²) exhibits a lamellar nanotwinning structure with a large fraction of Σ3 (60° rotation at 111) twin boundary. Time-of-flight secondary ion mass spectrometer (TOF-SIMS) analysis shows that the impurity incorporation level in electroplated Cu is significantly reduced with the formation of twinning structure. The Sn/Cu joint benefits greatly from the formation of nanotwins in the electroplated Cu layer due to effective suppression of interfacial voids, which exhibits an improved shear strength as compared with that without the formation of nanotwins.