Characterization of Nanostructured Metals and Metal Nanowires for Ultra-High Density Chip-to-Package Interconnections
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Nanocrystalline materials are being explored as potential off-chip interconnects materials for next generation microelectronics packaging. Mechanical behavior and deformation mechanisms in nanocrystalline copper and nickel have been explored. Nanostructured copper interconnections exhibit better fatigue life as compared to microcrystalline copper interconnects at a pitch of 100 and #956;m and lower. Nanocrystalline copper is quite stable upto 100 oC whereas nickel is stable even up to 400 oC. Grain boundary (GB) diffusion along with grain rotation and coalescence has been identified as the grain growth mechanism. Ultimate tensile and yield strength of nanocrystalline (nc) Cu and Ni are atleast 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. Activation energies for creep are close to GB diffusion activation energies indicating GB diffusion creep. Creep rupture at 45o to the loading axis and fracture surface shows lot of voiding and ductile kind of fracture. 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 high cycle fatigue life. However, a deteriorating effect of grain refinement has been observed on low cycle fatigue life. This is because of the ease of crack initiation in nanomaterials. Persistent slip bands (PSBs) at an angle of 45o to 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 oC/ 2 hrs for nc- Cu and 250 oC/ 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.