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

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.