Microstructure Evaluation of Engineered Cu for Low-Temperature Cu-Cu Hybrid Bonding

The demand for low-temperature, reliable$\text{Cu}-\text{Cu}$hybrid bonding is increasing with the advancement of three-dimensional integrated circuits (3D ICs), where high thermal budgets risk damaging temperature-sensitive components. This study investigated the microstructural stability and oxidation behavior of nanocrystalline Cu (NC-Cu) compared with standard electrodeposited Cu (STD Cu). Cu films were deposited by direct current electroplating under different additive conditions and characterized by SEM, AFM, XRD, t-EBSD, XPS, and TDS, with electrical properties monitored during natural aging and post-annealing. Results showed that NC-Cu exhibited stable nano grains for over 30 days, with suggested grain boundary pinned by impurities suppressing self-annealing, while STD Cu underwent rapid grain growth after 7 days of electroplating. NC-Cu demonstrated stable resistivity over time (natural aging before bonding), and oxidation resistance compared with STD Cu, providing thinner oxides for removal during CMP and bonding. After annealing at 200° C, NC-Cu achieved enhanced grain growth and lower resistivity, supporting its suitability for die-to-wafer hybrid bonding.