Co, In, and Co–In alloyed Cu₆Sn₅ interconnects: Microstructural and mechanical characteristics

The mechanical reliability of the future miniaturized interconnects is mainly governed by the intermetallic compounds such as Cu₆Sn₅. Alloyed Cu₆Sn₅ with various elements, including Co and In, have been introduced and attracted attention for different reasons, such as the enhancing mechanical reliability and lowering the bonding temperature. Hence, this work aimed to evaluate the microstructural and mechanical properties of Cu₆Sn₅-, Cu₆(Sn,In)₅-, (Cu,Co)₆Sn₅-, and (Cu,Co)₆(Sn,In)₅-interconnects. The grain size, grain orientation, and crystal structure of the pure and alloyed Cu₆Sn₅ phases were analyzed using electron backscatter diffraction. The results revealed that all the joints contained monoclinic and hexagonal crystal structures arbitrarily formed across the bond-line. Furthermore, the Cu₆Sn₅ grains exhibited random grain orientation and there was no discernible difference between the pure and alloyed Cu₆Sn₅ interconnects other than Cu₆(Sn,In)₅ grains elongated along the perpendicular direction to the bonding interface. However, it was found that alloying elements altered the grain sizes. In alloying refined and elongated the Cu₆Sn₅ grains while the Co alloying enlarged the Cu₆Sn₅ grains. The mechanical properties of the interconnects were examined using nanoindentation test. The results indicated that the hardness (H) and Young's modulus (E_i) values of Cu₆Sn₅ is increased with the alloying elements. (Cu,Co)₆(Sn,In)₅ showed the highest E_i/H value which indicates its highest plasticity.