Investigation Of Buckling And Failure Mechanism Of Nanoscale Thin Films On Stiff Substrate

Residual stresses and interfacial strength are both important factors offilm/substrate system. The large residual stresses appearing after deposition may leadto undesirable damages between thin film and substrate. The finite element method(FEM) based on the thin shell theory in addition to micro scanning x-ray diffraction(mSXRD) has been used to determine stress maps on the same buckling patternswhich appear in metallic films. Stress distributions obtained from the two techniquesover gold blisters and tungsten wrinkles are equivalent and show a strong stressrelaxation from the adherent part to the top of the buckling, validating the hypothesisdone for both techniques.The evolution from straight-sided to telephone-cord wrinkles is investigated usingoptical microscopy. Finite element modeling of instability is performed for analyzingthe secondary buckling of straight-sided wrinkles. Imperfections based on a randomcombination of buckling eigenmodes are introduced in the finite element analysis totake into account secondary bifurcation. This random and small perturbation of theimperfection is a more realistic approach for taking into account the factors such asroughness and flatness of thin films. Moreover, the perturbation method can beapplied more widely in some complex patterns of telephone-cord wrinkles.The interfacial adhesion energy between metal films and stiff substrates is animportant parameter for the reliability study of the film/substrate system. Thus, themorphology of straight-sided buckling and the relationship between the fractureenergy and buckling width are deduced based on energy method. An image entropybased focus measure is introduced into (Shape form Focus) SFF to measure the3Dmorphology of the spontaneous telephone-cord buckling in micro scale. Thetelephone-cord geometry of thin film can be used to calculate interfacial adhesionenergy. The instability of the FEM model is introduced to fit the buckling morphologyobtained by SFF. The adhesion energies and phase angle at the interface are extractedbased on the straight-sided model and the pinned circular model.Delamination growth driven by thin film buckling is evaluated based on theinternal stress of thin film at the edge of telephone-cord buckling. For the mixed modes I and II, the steady-state propagation over the edge of telephone-cord bucklingis used. Experimental results were fitted with computations of the damage evolutioncriterion through the plotting of the total critical strain energy release rate, GC, versusthe GII/GTmodal ratio. The estimated parameters of B-K criterion is in introduced intothe FEM simulation. The FEM simulation is performed to investigate the bucklingpropagation of telephone-cord wrinkles started from an imperfection. A nonlinearplate deformation and a cohesive zone model are used to investigate the propagationof the telephone cord wrinkle.