Low-temperature, Organics-free Sintering of Nanoporous Copper for Reliable, High-temperature and High-power Die-attach Interconnections
A novel die-attach joining technique based on low-temperature film sintering of nanoporous Cu is demonstrated. Nanoporous Cu films 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, thin films of nanoporous Cu with 25-50nm feature size and ~60% relative density were synthesized by dealloying of Cu-Si films. The nanoporous Cu films were then sintered on bulk Cu metallizations 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.