Low temperature nanojoining of silver-copper nanopaste as die attach material for high temperature packaging

Low temperature nanojoining has been identified as a challenging nanotechnology in the construction of nanoscale structures as well as microelectronic devices. Herein, three kinds of Ag-Cu nanopastes have been designed by mixing different organics with the Ag-Cu nanocrystalline particles synthesized in our previous work. The mechanical properties of sintered Ag-Cu nanopaste including hardness, Young's modulus and shear strength were investigated and analyzed. It turned out that the sintered Ag-Cu nanopaste-B with 20 wt% addition of polyethylene glycol (PEG) has lower hardness and Young's modulus, but a much higher shear strength than Ag-Cu nanopaste-A (1,2-propanediol) and -C (terpineol), which is superior to be considered as die-attach material for high temperature packaging material for wide band gap semiconductors. The uniform and dense sintered nanopaste-B layer presents a feature of eutectic microstructure comprised of Ag-rich and Cu-rich phases. It was also interesting to find that remarkable nanostructural solid solubility arose in Ag-rich phase (36 At% Cu) and Cu-rich phase (44 At% Ag). Moreover, the atomic inter-diffusion at interface could eventually interlock between the sintered nanopaste and substrate, which allows the Cu chip firmly attaching onto the Cu substrate. This work not only breaks through the synthesis bottleneck of Ag-Cu alloy NPs and a new bonding material, but also has the potential to affect a broad fields range.