Inelastic Deformation of Copper Nanoparticle Based Joints and Bonds

Abstract

The inelastic deformation properties of sintered metal nanoparticle joints are complicated by the inherent nanocrystalline and nanoporous structures as well as by dislocation networks formed in sintering or under cyclic loading. Creep rates of sintered nanocopper structures were found to be dominated by the diffusion of individual atoms or vacancies, while dislocation motion remained negligible up to stresses far above those of practical interest. Rapid sintering of one material led to unstable structures the creep of which could be strongly reduced by subsequent annealing or aging. Longer sintering of another material led to more stable structures, but creep rates could still be strongly enhanced by subsequent work hardening in mild cycling.

Copper-Based Nanomaterials for Fine-Pitch Interconnects in Microelectronics

“…The propensity to voiding becomes even more worrisome in today’s 2.5/3D fine pitch assemblies in the packaging industry, that are a result of a solder-joint volume shrinkage by a factor of 1000 or more. − This size-reduction significantly impacts the joints’ reliability by generating issues such as early electromigration failures, transformation of the entire solder joint to a complete intermetallic joint, and enhancement of the sporadic Kirkendall voiding. , Our recent work confirmed much higher voiding propensity at scales typical of 2.5/3D miniaturized assemblies and revealed important differences in systematic trends in comparison with larger joints. , To address the latter problem, researchers have explored a variety of alternative techniques for solder-free interconnection such as direct Cu–Cu bonding and sintering of metal nanoparticles or nanostructured Cu onto copper pillars. , In the earlier stages of these studies, Cu nanoparticulate pastes had been shown to form joints after sintering at relatively low temperatures. − …”

“…As seen in Figure , the sintered joints feature porosity due to the evaporation and partial evacuation of organic binding material during sintering, that leads to inferior mechanical properties. Overall, said binders are used in the pastes to hold together the otherwise non-self-supported nanoparticles, yet not allowing them to agglomerate below the sintering temperature of 300 °C. ,, …”

“…Another common disadvantage of nanoparticulate Cu pastes is the formation of copper oxides at low temperatures and low pressure. ,, Separately, work to attach Cu pillars to pads by sintering of a nanoparticulate Cu paste has also been ongoing in IBM. To solve the reliability issues associated with void formation in the solder interconnects, IBM researchers considered Cu–Cu bonding as an alternative interconnect material with applicability in most chips and PCBs. , The physical limitation of all-Cu interconnects is the high melting temperature of (1085 °C).…”

“…Overall, said binders are used in the pastes to hold together the otherwise non-self-supported nanoparticles, yet not allowing them to agglomerate below the sintering temperature of 300 °C. 24,27,30 Another common disadvantage of nanoparticulate Cu pastes is the formation of copper oxides at low temperatures and low pressure. 24,25,30 Separately, work to attach Cu pillars to pads by sintering of a nanoparticulate Cu paste has also been ongoing in IBM.…”

New generation copper-based interconnection from nanoporous CuSn alloy film sintered at low temperatures

“…10–13 Further investigations have shown that np-Cu holds a significant advantage as a joint precursor over nano-Cu paste, mostly due to the latter's tendency to form oxides and incorporate non-conductive bonding chemicals during the sintering process. 9,14…”