Determination of Thin Film Interfacial Properties by Laser Generated Stress Waves

University Of Illinois At Urbana-Champaign, Urbana IL

Investigators

Abstract

An SGER award supports development and feasibility testing of a new technique to investigate interfacial failures in thin films. The technique is based on using a laser pulse- induced shear wave. Laser pulse absorption will generate high amplitude, short duration stress wave pulses that will be deflected to load the interface between film and substrate in shear. Failure of the film is related to the known dynamic stress state which allows the strength of the interface to be inferred. This adaptation of established methods of high amplitude dynamic tensile loading of interfaces to the case of pure shear loading will later be extended to the mixed mode case, because thin film interfaces fail under mixed mode conditions in most commercial applications. A short term, exploratory investigation will establish the feasibility of the technique and answer several key questions regarding the design of this novel test method. A careful series of experiments combined with rigorous theoretical analysis will be carried out for Al films on a Si substrate. This material system is chosen for ease of fabrication and low cost. A shear wave will be generated with amplitude large enough to fail the film interface. The resulting shear stress at or just prior to film failure will be calculated from high speed interferometric displacement measurements using elastic wave propagation theory. Samples will be carefully examined using an optical microscope and SEM to assess the nature and extent of interfacial failure. The interfacial strength and failure mode determined for the Al film loaded by a shear wave will be compared with that obtained for the same film loaded by a tensile wave. Initial set-up of the method is complex, but once established, this novel technique is expected to have direct impact on mechanical testing and consequent design of thin films and interfaces, with significance for a wide variety of applications in multi-layered electronic and optical structures.