Time dependent failure of thin films

Citation

Srinivas, Mullahalli V. (1994) Time dependent failure of thin films. Engineer's thesis, California Institute of Technology. doi:10.7907/n3rb-w197. https://resolver.caltech.edu/CaltechTHESIS:09282010-150048718

Abstract

The prediction of time dependent failure at the interface of a thin film bonded to a substrate is considered in terms of a thermorheologically simple viscoelastic material. An initial investigation of stress distribution at the interface is carried out to understand the possible location and the mechanism of failure. Subsequently, a detailed analysis is developed to obtain not only the longitudinal stress component but also the lateral and shear stress components at the interface. Viscoelastic material behavior is included in the model to account for the stress relaxation behavior of the film. The proposed model satisfies both the edge free boundary conditions as well as the displacement and traction continuity at the interface. Using the model, the magnitude of the stresses and their relaxation behavior has been studied for the case of a polyimide film bonded to a silicon substrate. The analysis is capable of incorporating the material property at different temperatures. Using this capability, the stress evolution in a polyimide film during a typical curing process is obtained. With a view towards reducing the residual stress developed in the film, the effect of cooling rate and cooling cycle on final built in residual stresses during the curing process is studied. Based on the stress analysis and also from experimental observations, the most likely type of failure mechanism appears to be edge decohesion. So, the problem of edge decohesion along the interface of a thin viscoelastic film bonded to an elastic substrate under tensile residual stresses is considered in the next chapter. An analytical model is developed to predict the crack growth along the interface and its velocity. The tensile residual stress in the film is replaced by a combination of edge loads and an explicit relation of strain energy with respect to time is obtained by simple beam analysis. The strain energy function is computed at small time intervals and the energy release rate is calculated using Griffith's energy balance approach; the discretized time step is assumed to be very small so that the dissipation effects over a given time step are neglected. Crack growth along the interface is computed based on a fracture criteria. The validity of the assumptions in the analytical results is checked by performing a finite element analysis.

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