Synthesis and Atomic Transport of CoSn₃ NanoIMC by In Situ TEM

In order to optimize the interfacial properties by adding Co to the bumps of copper pillars and to overcome the strong tendency of Co to oxidize, an intermetallic compound (IMC) "capsule" was developed for the purpose of transporting elements through the intermetallic compound. In this study, we present a comprehensive analysis of the transformation process of CoSn₂ nanoparticles into CoSn₃ at the nanoscale using in situ heating transmission electron microscopy (TEM). The experimental results reveal that CoSn₂ nanoparticle growth occurs through polymerization, whereas CoSn₃ nanoparticle formation relies on the reaction between CoSn₂ and Sn. During the initial stages of the reaction, Co dissolves and diffuses into Sn, leading to the nucleation and growth of CoSn₂ in Sn via Ostwald ripening. As the input energy increases, vacancies in CoSn₂ drive a reaction at the Sn/CoSn₂ interface, resulting in the generation of CoSn₃. In this process, Sn nanoparticles enter the CoSn₂ structure through the "Anti Structure Bridge (ASB) mechanism" to fill vacancies. Following the codeposition process, CoSn₃ nanoparticles were successfully plated within the Sn layer of the Cu-pillar bumps. Upon reflow heating, the CoSn₃ nanoparticles exhibited a preference for precipitating the vacant sites within the Sn layer. This process facilitated the release of Co atoms from CoSn₂, enabling their diffusion throughout the entire Sn layer.