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Mode-dependent Thin Film Interfacial Property Measurement by Laser Induced Stress Waves

Junlan WangDepartment of Mechanical Engineering,
University of California, Riverside, CA AbstractThin films are crucial components in a wide range of multilayer microelectronic and optical devices. They are also desirable candidates for micro-actuators in micro-electro-mechanical systems. Due to the dissimilar nature of the constituents, large residual stress can be induced in the film during the fabrication process which leads to the subsequent failure of the thin film devices. Among the many properties, interfacial adhesion between the thin film and substrate is one of the key parameters influencing the overall reliability and durability of the integrated thin film devices. However, due to the critical dimension of thin films, conventional techniques face challenges to reliably evaluate the thin film interfacial properties.
To address the above challenge, we developed a unique set of laser-induced stress wave techniques to quantitatively investigate the intrinsic strength of a planar thin film/substrate interface. High-amplitude short-duration stress wave pulses generated by laser-pulse absorption are used to delaminate a thin film/substrate interface and the corresponding interfacial stress is calculated from the transient high-speed interferometric displacement measurement using wave mechanics. Depending on the geometry of the substrate, the thin film interfaces can be subjected to a variety of loading modes including tensile, mixed-mode and pure-shear. Systematic studies of similar interfaces failed under different loading conditions reveals that the thin film interfacial failure as well as the adhesion is highly mode-dependent. Significant wrinkling and tearing of the films happens under mixed-mode and pure-shear loading, in great contrast to blister patterns observed in similar films failed under tensile loading. This technique has been further developed to investigate the interfacial adhesion of various thin film/substrate interfaces interesting to semiconductor industry and biomedical applications as well as those under high strain-rate loading for defense applications.
Biosketch Junlan Wang received her Ph.D. in Theoretical and Applied Mechanics from the University of Illinois at Urbana-Champaign in 2002. She joined the faculty in the Department of Mechanical Engineering at the University of California, Riverside in 2003 after finishing one year post-doctoral research in the Solid Mechanics and Structures group at Brown University. Her research interest is in the mechanics of thin films and coatings, high strain rate materials behavior, size-dependent mechanical behavior of surface micro and nanostructures, and mechanical reliability of multifunctional nanoporous materials. Her recent awards include the SEM Hetenyi Award in 2004, UC Regents Faculty Fellowship in 2004, Faculty Development Award in 2006, UCR College of Engineering Excellence in Teaching Award in 2007, and ASEE Beer and Johnston, Jr. Outstanding New Mechanics Educator Award in 2007.

Location: Staufer Science Lecture (SLH) Rm 102

Audiences: Everyone Is Invited

Contact: April Mundy