Initial Bonding Process, Shear Strength and Interfacial Characterization Results of Copper Nanofoams Die-attach Interconnections
Research Report Highlight
Cu nanofoam was demonstrated to be a promising low-cost die-attach technology for wide bandgap power modules operating at temperatures > 250°C, with improved manufacturability, scalability and design flexibility compared to standard nanopaste approaches.
This report presents the first demonstration of nanocopper foam as a novel low-temperature die-attach joining technique for high-power, high-temperature applications. Nanoporous metals, here referred to as nanofoams, are proposed as a low-cost replacement of nano-sintering pastes with the following benefits: (i) synthesis by electrochemical dealloying, compatible with standard lithography processes; (ii) no organic content to minimize risks of voiding and corrosion; and (iii) controllable physical properties post sintering through tailorable initial nanostructure and morphology. As a first proof-of-concept, nano-Cu foams with a 25-50nm feature size and ~60% relative density were fabricated by dealloying of Cu-Si thin films. The nano-Cu foams were then sintered on bulk Cu metalizations at temperatures of 200-250ºC for 5-15min with an applied pressure of 6-9MPa, in reducing atmosphere. A maximum shear strength of 4.2kgf was achieved and analysis of the fracture profiles showed failure through the sintered joints, confirming strong metallurgical bonding to bulk Cu. Cross-sections of joints formed at 200ºC and 250ºC – 15min observed by SEM showed relative density as high as 85%, achieved for the first time with sintered copper.