Robustness of Cu/Gr multilayer interconnects for the application of flexible electronics and the roles of graphene

Abstract

Cu/Gr multilayer composite with potential application as an interconnect material is a very promising material system. Nanoscale metal/graphene nanolayered composite is known to have ultra-high strength as the graphene effectively blocks dislocations from penetrating through the metal/graphene interface. The same graphene interface, which has a strong sp2 bonding, can simultaneously serve as an effective interface for deflecting the fatigue cracks that are generated under cyclic bending cycles. In this dissertation, Cu/Gr composite with repeat layer spacing of 100 nm was tested for bending fatigue at 1.6\% and 3.1\% strain up to 1,000,000 cycles that showed for the first time a 5$\sim$6 times enhancement in fatigue resistance compared to the conventional Cu thin film. Fatigue cracks that are generated within the Cu layer were stopped by the graphene interface, which are evidenced by cross-sectional SEM and TEM images. Molecular dynamics simulations for uniaxial tension of Cu/Gr showed limited accumulation of dislocations at the film/substrate interface, which makes the fatigue crack formation and propagation through thickness of the film difficult in this materials system. In addition, much more efficient large scale fabrication of Cu/Gr composite using roll-based transfer of graphene and electrodeposition of copper is introduced. The nano-pillar compression tests are performed and compared with previous studies to confirm that such composite fabricated display similar strengthening effect.