Mechanical behavior and environmental stability of GLAD-based Cu nanospring films
Mechanics of compliant thin film interfaces;Mechanical behavior and environmental stability of nanospring films;Copper (Cu) nanostructures;Nanowires;Nanothermal interfaces;Compliant interfaces;Nanostructured thin films;Glancing angle deposition;Oblique angle deposition;Elasticity models for spring modeling;Experimental nanomechanics;Kirkendall effect;Recrystallization of copper;Compressive behavior of nanosprings;Shear behavior of nanosprings;Oxidation of nanowires;Copper (Cu) nanowires;MEMS-based testing;Digital Image Correlation;Nanotechnology;Thin film optimization;Nanotechnology;Microelectromechanical systems (MEMS);Multifunctional interfaces;Electronic packaging;Copper nanowires
Glancing Angle Deposition (GLAD) based Cu nanospring films are a promising form of film that combines high electrical and thermal conductivities with robust mechanical performance. This dissertation research focuses on quantifying the mechanical performance and environmental stability of GLAD-based materials via film-level and individual nanowire experiments. The compressive behavior of GLAD-based Cu nanospring films was quantified at the forest level by axial compression experiments performed on films with heights varying from 2.9 μm to 10.6 μm that resulted in 2.5-10 coil turns. Individual springs had coil diameters in the range of 2.5-4.1 µm and wire radii of 120-230 nm. For compressive stresses up to 70 MPa, the normal moduli of the Cu nanospring films were in the range of 35-315 MPa exhibiting a decreasing trend with increasing nanospring layer height due to the diminishing effect of boundary conditions. Irrecoverable compression took place at high compressive stresses of the order of 110 MPa, which amounted to 50% strain and resulted in a permanent set of 0.6-1.1 µm within the nanospring layer, which was less than 15% of the initial nanospring layer height.The shear response of Cu nanospring films was also investigated. The shear moduli were in the range of 65-190 MPa retaining the inversely proportional trend of the compressive moduli with respect to the nanospring layer height for films with identical coil/wire diameters and helix angles. An inversely proportional correlation was also established between the shear strength values and the nanospring layer height. The former dropped from 62.6±3.1 MPa to 42.7±3.9 MPa as the nanospring layer height increased from 2.9 µm to 10.6 µm. Finally, films with larger nanowire diameter and smaller helix angle exhibited higher shear strength reaching 74.8±3.3 MPa. The experimental results were compared with elasticity models with increasing fidelity with respect to the actual boundary conditions of the test specimens. A formulation based on a kinetic symmetry assumption, nulling of all degrees of freedom except the extensional one at the upper end of a helical spring, captured most accurately the compressive response of nanospring films. An uncertainty analysis emphasized the contribution of coil radius and wire diameter to the value of the compressive modulus of nanospring films. In the case of shear response, a formulation developed by strain energy minimization, assuming zero angular rotation at both ends of a helical spring, was the most effective in capturing the experimental trends in shear response. Unlike the case of compressive modulus, the coil diameter was not an influential geometric parameter on the shear response, whereas the wire diameter again emerged as the key design parameter.Since monolithic thin films of Cu have been shown to be prone to thermal oxidation even at low temperatures, the complex effects of near ambient temperature exposure, i.e. 20-150°C, on the chemical stability and the mechanical properties of pure Cu nanostructures were investigated by means of thermal solution grown faceted Cu nanowires, whose effective diameters were similar to the coil diameters of the Cu nanosprings. The mechanical behavior was quantified with experiments on individual Cu nanowires using a MEMS-based method for nanoscale mechanical property studies. The elastic modulus of pristine Cu nanowires with diameters in the range of 300-550 nm was 117±1.2 GPa which agreed very well with polycrystalline bulk Cu, while the ultimate tensile strength was more than three times higher than bulk Cu, averaging 683±55 MPa. Remarkably, annealing at only 50°C resulted in marked strengthening by almost 100% while the elastic modulus remained unchanged. Heat treatment in ambient air distinguished three regimes of oxidation: (a) formation of a thin passivation oxide for temperatures up to 50°C, (b) formation of thermal oxide obeying an Arrhenius type process for Cu+ migration at temperatures higher than 70°C, which was accelerated by grain boundary diffusion resulting in activation energies of 0.17-0.23 eV, and (c) complete oxidation following the Kirkendall effect at temperatures higher than 150°C and for prolonged exposure times, which did not obey an Arrhenius law. Notably, the formation of a weaker and more compliant thermal Cu2O did not compromise the effective strength and elastic modulus of oxidized Cu nanowires: experiments in Ar environment at temperatures higher than 70°C showed mechanical strengthening by ~50% and ultimate stiffening to about 190 GPa that is close to the upper limit for the elastic modulus of single crystal Cu in the <111> direction.
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Mechanical behavior and environmental stability of GLAD-based Cu nanospring films
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