Metastable Nanocrystalline Copper for Effective Copper-to-Copper Bonding

Direct copper-to-copper (Cu-Cu) bonding is critical for advanced electronic packaging. However, high bonding temperatures and prolonged bonding times present challenges for heterogeneous integration. To lower the thermal budget, researchers explored grain engineering with nanotwinned Cu (NT-Cu) and nanocrystalline Cu (NC-Cu). While NT-Cu requires bonding temperatures above 250 °C for better bonding interfaces, NC-Cu's self-annealing behavior limits its practicality in real manufacturing scenarios due to extended processing times after plating and before bonding. This work fabricates metastable nanocrystalline copper (MS-Cu), engineered with coherent nanotwin boundaries, stacking faults, and 9R phase structures to stabilize grain boundaries, ensuring structural stability during room-temperature processing while enabling substantial grain growth at elevated bonding temperatures. This unique Cu material, with minimal self-annealing over 14 days, facilitates high-quality bonding at a lower temperature and a shorter bonding time of 200 °C for 30 min, offering a promising and practical approach for Cu-Cu bonding.