Nano-Scale Conductive Films with Low Temperature Sintering for High Performance Fine Pitch Interconnect
In this paper, a novel nano-scale conductive film which combines the advantages of both traditional anisotropic conductive adhesives/films (ACAs/ACFs) and nonconductive adhesives/films (NCAs/NCFs) is introduced and developed for next generation high performance ultra-fine pitch packaging applications. This novel interconnect film possesses the properties of electrical conduction along the z-direction with relatively low bonding pressure (ACF-like) and the ultra-fine pitch (< 100 nm) capability (NCF-like). Unlike typical ACF which requires 1-5 vol% of conductive fillers, the novel nano-scale conductive film only needs less than 0.1 vol% conductive fillers to achieve good electrical conductance in the z direction. The nano-scale conductive film also allows a lower bonding pressure than NCF to achieve a much lower joint resistance (over two orders of magnitude lower than typical ACF joints) and higher current carrying capability. With low temperature sintering of nano-silver fillers, the joint resistance of the nano-scale conductive film could be as low as 10<sup>-5</sup> Ohm, even lower than the NCF and lead-free solder joints. The insertion loss of nano-scale joints are almost the same as the standard ACF or NCF joints, suggesting that the nano-ACF joints are suitable for reliable high frequency adhesive joints in microelectronics packaging. The reliability of the nano-scale conductive film after high temperature and humidity test (85degC/85%RH) was also improved compared to the NCF joints. In order to reduce the silver migration and maintain a good insulation/dielectric property in the x-y plane for the nano-scale conductive film, self-assembled molecular wires (SAM) are used to passivate/protect the silver nano fillers. The protection of silver nano particles with molecular monolayers reduced the silver migration dramatically and no migration was observed upon application of high voltages (up to 500 V) due to the formation of surface chelating compounds between- the SAM and nano silver fillers. The migration behavior of SAM passivated nano-Ag conductive adhesives was investigated by analyzing the results with the migration model.