Directed self-assembly of nanoparticles for novel electrical interconnects

Future 3D chip stacks and high bandwidth flip-chip-onboard applications require advanced packaging processes that support high density electrical interconnects at a high reliability. This paper discusses the directed self-assembly of silver nanoparticles by capillary-bridging, resulting in electrical joints — so called “necks”. The necks are achieved by the evaporation of an injected nanosuspension between the chip and a carrier. A subsequent annealing step achieves high electrical conductivity and mechanical integrity. The evolution of capillary-bridges was observed in an array of copper pillars with partially depopulated areas. The meniscus penetrates these zones first, as they allow for larger curvature radii compared to fully populated zones. Initial individual necks form once all liquid has evaporated in the depopulated areas. The electrical resistance and morphology of silver nanoparticle agglomerations was studied at annealing temperatures of 60°C to 200°C by Kelvin probes, atomic force microscopy and X-ray diffractometry. A significant improvement in electrical conductivity of the film was observed above annealing temperatures of 150°C for 80 min and correlated with an increased average grain size. An in-situ characterization method, to investigate the evolution of individual capillary-bridges and the formation of electrical conducting necks is presented. A dedicated Kelvin probe measurement could track the neck formation process and shows a drop of electrical resistance of more than five orders of magnitude within 10 min from evaporation onset. Finally, silver interconnects were formed at low temperature between copper pillars and respective pads. Their shear strength identified by die shear testing was 16 MPa.