Determination of Thin Film Interfacial Properties by Laser Generated Stress Waves

SGER奖支持一种新技术的开发和可行性测试，以研究薄膜中的界面失效。 该技术是基于使用激光脉冲诱导的剪切波。 激光脉冲吸收将产生高振幅、短持续时间的应力波脉冲，该应力波脉冲将被偏转以在剪切中加载膜和衬底之间的界面。 膜的失效与已知的动态应力状态有关，该动态应力状态允许推断界面的强度。 这种适应建立的方法的高振幅动态拉伸加载的界面的情况下，纯剪切载荷将被扩展到混合模式的情况下，因为薄膜界面失败的混合模式条件下，在大多数商业应用。 短期的探索性研究将确定该技术的可行性，并回答有关这种新型测试方法设计的几个关键问题。 一个仔细的一系列实验结合严格的理论分析将进行铝膜在硅衬底上。 选择这种材料系统是为了易于制造和低成本。 剪切波将产生足够大的振幅破坏膜界面。 在膜失效时或膜失效之前产生的剪切应力将通过使用弹性波传播理论的高速干涉位移测量来计算。 将使用光学显微镜和SEM仔细检查样品，以评估界面失效的性质和程度。 将由剪切波加载的Al膜确定的界面强度和失效模式与由拉伸波加载的相同膜获得的界面强度和失效模式进行比较。 该方法的初始设置是复杂的，但一旦建立，这种新技术预计将直接影响薄膜和界面的机械测试和随后的设计，具有广泛的多层电子和光学结构中的各种应用的意义。

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.