Formation and Growth Mechanisms of Preferentially Oriented Roof-type CuGa2 Grains on (111) Nanotwinned Cu

Ga-based liquid metals are widely used in flexible electronic micro-interconnections due to their unique combination of metallic and fluidic properties. (111) -Oriented nanotwinned Cu (nt-Cu) is a newly developed under-bump metallization (UBM) for three-dimensional (3D) packaging technology. This study investigates the formation and growth mechanisms of interfacial CuGa 2 intermetallic compound (IMC) on both (111) -oriented nt-Cu and conventional polycrystalline Cu substrates at 150°C. Compared to polycrystalline Cu, (111) -oriented nt-Cu substrate significantly retards the growth kinetics of interfacial CuGa 2. Distinct roof-type CuGa 2 grains with a strong (102) preferred orientation form on (111) -oriented nt-Cu, whereas randomly oriented brick-type CuGa 2 grains form on polycrystalline Cu. The initial (102) preferential orientation of CuGa 2 grains on (111) -oriented nt-Cu remains stable even after an extended reaction time of 20 min. The smallest atomic misfit (10. 17 %) occurs between the direction on the 111 cu plane and the direction on the 102 CuGa2 plane. The preferred orientation relationships between the roof-type CuGa 2 grains and the (111) -oriented nt-Cu are identified as follows: 102 CuGa2 ||111 Cu and CuGa2 || Cu ; 102 CuGa2 ||111 Cu and CuGa2 || Cu. This study provides fundamental insights into interfacial reactions at (111) -oriented nt-Cu UBMs for Ga-based liquid metal flexible micro-interconnects.