Cu Sintering for Cu Pillar Bonding: A Comparative Study Among Pressure-Less, Pressure-Assisted, and Transient Liquid Phase Sinter Pastes

The solder-based interconnections have been the backbone of microelectronics. However, the ever-growing trends toward ultrahigh-density interconnected systems with higher thermal and mechanical stability drove the solder to its limit. Alternatively, the solid-state copper (Cu)-based interconnects have gained momentum not only due to their compatibility with back-end-of-the-line and downscalability through the lithography process but also due to the unique characteristics of Cu (e.g., low resistivity, high-temperature stability, high electromigration resistance, as well as low cost). As an interconnect, Cu pillars favor ultrafine pitch applications, as the bump height and footprint can be well-controlled. However, the conventional direct (thermocompression) Cu pillar bonding involves high bonding temperature and pressure. The use of solder caps alleviated these requirements but at the cost of possible issues, such as thermal mismatch and brittle intermetallic compound. Therefore, a solder-free, all-Cu interconnect solution with reduced processing temperature and pressures is currently the holy grail in advanced microelectronics packaging. Accordingly, in this study, Cu-based interconnects consisting of Cu pillars and Cu microparticle (MP) sinter paste caps were fabricated and investigated as an alternative to direct Cu pillar bonding and solder caps. Here, recently developed Cu-based sinter-paste materials [i.e., pressure-less, pressure-assisted Cu sinter pastes, and Cu-based transient liquid phase sinter transient liquid phase sintering (TLPS) pastes] were assessed and applied to join Cu pillars. The electrical and mechanical properties as well as the long-term reliability of the bonded samples were characterized and compared. The bonded interface was also examined using 3-D-tomography analysis.