Formation and homogenization of Si interconnects by non-equilibrium self-propagating exothermic reaction

Si chips have been interconnected by the self-propagating exothermic reactions using Al-Ni NanoFoil and Au80Sn20 solder layers. The instantaneous localized heat across the entire solder presents great potential for the integration of MEMS and optoelectronic components requiring minimal thermal effects. However, the extremely rapid heating and cooling (10(5-7) degrees C/s) in the reaction can cause non-equilibrium interfacial reactions and metastable solidification in the transient bonding region. Segregations and metastable phases different from equilibrium ones are inevitably formed in the Si interconnects. In this study, numerical predictions on the temperature profile of the bonding process and the microstructural characterizations by in-situ TEM/SEM were carried out to reveal the formation and homogenization of the reactive bonded structures. The convective mass transportation, directional solidification and inverse segregation due to the great perpendicular temperature gradient in the mushy reacting zone are confirmed to cause the formation of nano-sized metastable phases. Significant segregation of high-temperature phases (Au-enriched phases) was observed, likely due to the convective flow and the inverse segregation induced by volume contraction. These non-equilibrium phases can be homogenized towards the equilibrium status in subsequently accelerated aging, which was supported by the evident inter-diffusion, intermixing and subsequent growth. (C) 2019 Elsevier B.V. All rights reserved.