Epitaxial growth of seed layers: Achieving highly (111)-oriented nanotwinned copper

Electroplated copper with a (111)-oriented nanotwin structure (111-nt-Cu) exhibits high strength, low resistivity, and superior electromigration resistance. These properties render 111-nt-Cu appealing as a candidate for interconnect in three-dimensional integrated circuits (3D ICs). Conventional direct current (DC) electroplating of 111-nt-Cu inherently forms a polycrystalline transition layer (TL) between the seed layer and twin structure. The TL could compromise structural integrity and reliability. Mitigation of the TL formation during electroplating has been a critical challenge and rarely reported. Here, we demonstrate a strategy for TL-free 111-nt-Cu via epitaxial DC electrodeposition, enabled by the synergistic optimization of a tailored (111)-oriented seed layer and electroplating recipes. The epitaxial seed layer growth mechanism relies on minimal lattice mismatch between Ti(0002) and Cu(111). The 111-nt-Cu exhibits (111) preferred orientation along the vertical direction ( Z -axis) and (110), (112) preferred orientations along the horizontal direction. The Schmidt factor is significantly higher along the horizontal direction compared to the vertical direction, while the cross-sectional elastic modulus and hardness are lower than those of the surface normal. TL-free 111-nt-Cu holds promise for sub-micrometer Cu interconnect applications, addressing critical challenges in advanced 3D IC fabrication.