Interface Engineering Suppresses Self‐Annealing in Electroplated Nanograined Copper for Low‐Temperature Copper‐to‐Copper Bonding

ABSTRACT Nanograined copper (ng‐Cu) attracts interest in advanced electronic packaging due to its grain boundary–mediated fast diffusion, enabling low‐temperature copper‐to‐copper (Cu‐Cu) bonding. However, after electroplating ng‐Cu, self‐grain growth at room temperature (self‐annealing) during prolonged processing time limits its practical use in manufacturing. This work proposes an ng‐Cu layer atop a (111)‐oriented nanotwinned Cu substrate (ng/nt‐Cu) that resists self‐annealing. The Cu(111) interface between the ng and nt layers resists grain boundary migration due to uniform atomic energy, which inhibits atom adsorption. Moreover, electroplating on the nt‐Cu substrate promotes the formation of a high density of Σ3 twin boundaries and stacking faults in the ng layer, relaxing internal stress and suppressing grain boundary motion. As a result, the ng/nt‐Cu retains its microstructure for over 30 days and enables high‐quality Cu‐Cu bonding at a low thermal budget. This study provides crystallographic insights of electroplated copper and advances the low‐temperature Cu─Cu bonding technique in electronic packaging.