Electroless-deposition of epitaxial (111)-oriented silver on nanotwinned copper microbumps for metal direct bonding

A large lattice mismatch between Ag and Cu commonly results in incoherently polycrystalline deposited features. It is very challenging to epitaxially deposit an Ag film on Cu. In this study, an epitaxial Ag film was electroless-deposited on nanotwinned Cu (NT-Cu) microbumps. Although the Ag/Cu large lattice mismatch, the Ag films replicated the NT-Cu structure and epitaxially grew on the NT-Cu microbumps. Fast Fourier transform (FFT) in high resolution transmission microscopy (HRTEM) was employed to confirm the epitaxial growth. Such Ag films also played a vital role in inhibiting oxidation during the bonding processes of the microbumps. Possessing (111)-preferred orientation and high surface diffusivities, the epitaxial Ag films were thus beneficial for the metal direct bonding at a low temperature below 100 °C in air. HRTEM FFT analysis indicates excellent bonding interfaces with (111) Ag on (111) Cu. Interdiffusion of Cu and Ag atoms may occur during the bonding process to form Cu–Ag alloys to strengthen the bonding interface.