Determination of Thin Film Interfacial Properties by Laser Generated Stress Waves

SGER奖支持研究薄膜界面失效的新技术的开发和可行性测试。该技术是基于使用激光脉冲诱导的剪切波。激光脉冲吸收会产生高振幅、短持续时间的应力波脉冲，这些应力波脉冲会发生偏转，在剪切过程中加载薄膜与衬底之间的界面。薄膜的破坏与已知的动应力状态有关，从而可以推断界面的强度。这种将已建立的界面高振幅动态拉伸加载方法适用于纯剪切加载情况的适应性将随后扩展到混合模式情况，因为在大多数商业应用中，薄膜界面在混合模式条件下失效。短期的探索性调查将确定该技术的可行性，并回答有关这种新测试方法设计的几个关键问题。将对硅衬底上的铝薄膜进行一系列细致的实验，并结合严格的理论分析。选择这种材料体系是为了便于制造和低成本。将产生一个振幅大到足以破坏薄膜界面的剪切波。利用弹性波传播理论，从高速干涉位移测量中计算出薄膜破坏时或破坏前的剪应力。将使用光学显微镜和扫描电镜仔细检查样品，以评估界面破坏的性质和程度。将剪切波载荷下得到的界面强度和破坏模式与拉伸波载荷下得到的界面强度和破坏模式进行比较。该方法的初始设置是复杂的，但一旦建立，这种新技术有望对薄膜和界面的机械测试和后续设计产生直接影响，对多层电子和光学结构的各种应用具有重要意义。

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.