Improved Low Temperature Sinter Bonding Using Silver Nanocube Superlattices

We examined the potential of silver nanocubes to achieve sintered conductive junctions for applications in microelectronics and die-attach bonding devices. For such applications, it remains a challenge to achieve dense and robust joints, sintered at low temperature (<200 °C). The minimum sintering temperature is strongly dependent on the particle shape, interparticle distance, and surface energy. In this respect, nanocubes offer two advantages over nanospheres: a higher proportion of surface atoms and the ability to assemble into denser 3D colloidal packings. Here, we used a colloidal approach to create joints using 3D close-packed arrays of silver nanocubes of different edge lengths (from 20 to 60 nm). Starting from monodisperse nanocubes, we assembled them into close-packed supercrystals by drop-casting and investigated their thermal stability and electrical properties on silicon nitride substrates. Spectroscopic measurements allowed a correlation of the plasmonic signature with the length and the degree of corner curvature of the cubes. Using electron microscopy and electrical measurements, we studied the impact of the nanocube size on the sintering temperature and electric properties of the self-organized arrays. The small gaps between the building units yielded uniform sintered patterns possessing high electrical conductivity. More broadly, our approach shows how Ag particles featuring a high density of low-coordination surface atoms, separated by small gaps, increase the efficiency of sintered Ag technology for microelectronics.