Characterization of nanostructured metals and metal nanowires for chip-to-package interconnections

The Packaging Research Center at Georgia Tech is proposing nano-interconnections as a new interconnect paradigm for potential low-cost, highest performance and reliability. The idea is to use nanocrystalline (nc) metals and metal nanowires as potential interconnect materials with good mechanical properties and shortest electrical interconnection.; The goal of the present work is to determine, as closely as possible, the intrinsic electrical and mechanical behavior of nc- metals and metal nanowires to assess the suitability of these materials for off-chip interconnections. In this study, the microstructural stability, creep, fatigue and fracture properties of nanocrystalline copper and nickel (grain size ~ 50 nm) have been reported, in such depth, for the first time to the best of our knowledge. Fatigue life of nanostructured interconnects has also been computed through finite element models, and a clear advantage of using such a material has been demonstrated.; Nanostructured copper interconnections exhibit better fatigue life as compared to microcrystalline copper interconnects at a pitch of 100 mum and lower. Nanocrystalline copper is quite stable upto 100°C whereas nickel is stable even upto 400°C. The activation energy of grain growth is 33.427 kJ/mol and 53.056 kJ/mol for ECAE nanocrystalline copper and nickel respectively. GB diffusion along with grain rotation and coalescence has been identified as the grain growth mechanism. Ultimate tensile and yield strength of nc- copper are 454 MPa and 438 MPa, respectively. These for nc- nickel are 898 MPa and 867 MPa, respectively. These values are at least 5 times higher than microcrystalline counterparts. Considerable amount of plastic deformation has been observed and the fracture is ductile in nature. Fracture surfaces show dimples much larger than grain size and stretching between dimples indicates localized plastic deformation. Nanoindentation hardness for nc- copper and nickel is 2.33 GPa and 3.92 GPa, respectively. Young's moduli are 100 GPa and 206 GPa for nc- copper and nickel, respectively. Young's modulus values are lower than mc- copper and nickel because of the presence of a higher volume fraction of GBs. Activation energies for creep are close to GB diffusion activation energies, indicating GB diffusion creep. Presence of a threshold stress implies that GBs in nanomaterials are not perfect sources/sinks of vacancies. Creep rupture at 45° to the loading axis and fracture surface shows lot of voiding and ductile kind of fracture. Again, grain rotation and coalescence along direction of maximum resolved shear stress plays an important role during creep. Grain refinement enhances the endurance limit and hence HCF life. However, a deteriorating effect of grain refinement has been observed on LCF life. This is because of the ease of crack initiation in nanomaterials. Persistent slip bands at an angle of 45° to the loading axis are observed at higher strain ranges (> 1% for nc- Cu) with a width of about 50 nm. No relationship has been observed between PSBs and crack initiation. A non-recrystallization annealing treatment, 100°C/2 hrs for nc- Cu and 250°C/2 hrs for nc- Ni has been shown to improve the LCF life without lowering the strength much. Fatigue crack growth resistance is higher in nc- Cu and Ni compared to their microcrystalline counterparts. This is due to crack deflection at GBs leading to a tortuous crack path. Nanomaterials exhibit higher threshold stress intensity factors and effective threshold stress intensity is proportional to the elastic modulus of the material.; Based on our study, we would strongly recommend stable nanocrystalline microstructures as potential candidates for next-generation interconnect technologies. However, there are challenges of developing and maintaining these stable structures which need to be addressed in the future.