Interfacial Diffusion in Scaled Cu/Sn Microbump Bonding

Interfacial Diffusion in Scaled Cu/Sn Microbump Bonding

As semiconductor integration advances toward high-density 2.5D and 3D architectures, Cu/Sn-based interconnects are undergoing rapid dimensional scaling, with microbump diameters evolving from approximately 50 μm to 20 μm, 10 μm, and ultimately 5 μm. This aggressive scaling enables higher interconnect density and finer-pitch heterogeneous integration, but it also fundamentally changes the materials and transport behavior of the bonding interface. As the bump dimensions decrease, the interfacial and surface diffusion of Cu and Sn becomes increasingly important, while the volume available for solder and the relative fraction of intermetallic compounds (IMCs) increase. Our research investigates how this scaling affects Cu/Sn interfacial reactions, IMC formation, and microstructural evolution, with the goal of establishing the fundamental mechanisms governing ultrafine-pitch microbump bonding.

A particular focus is the relationship between microbump size, diffusion kinetics, IMC growth, and Kirkendall void formation. Cu and Sn exhibit different diffusion rates, leading to asymmetric interdiffusion and the formation of Cu₆Sn₅ and Cu₃Sn during bonding and subsequent thermal exposure. As the bump size decreases, the accelerated role of surface and interface diffusion can substantially increase IMC formation and promote Kirkendall voids, potentially compromising the mechanical, electrical, and thermal reliability of the interconnect. By combining controlled microbump scaling, microstructural characterization, and diffusion analysis, we seek to develop scaling-aware design principles for reliable Cu/Sn bonding at the 5–10 μm scale, enabling robust interconnects for next-generation 3D semiconductor integration.

Publications

K. Wang et al., "Experimental Characterization of Thermal Transport in Cu/Sn–Diamond Microbumps," 2026 IEEE 76th Electronic Components and Technology Conference (ECTC), Orlando, FL, USA, 2026, pp. 1286-1293, doi: 10.1109/ECTC51846.2026.00213.
Wang, K.Y, Lyu, S.H., and Wei, T.W.*, A Novel Copper Microporous-Assisted Bonding Method for Fine-Pitch Cu/Sn Microbump 3D Interconnects, 2024 IEEE 74th Electronic Components and Technology Conference (ECTC), Denver, Colorado, May 28- May 31, 2024, pp. 563-570. IEEE, 2024. DOI: 10.1109/ECTC51529.2024.00095 [PDF]