91 Chapter 10 Interfacial Strength of Thin Film Measurement by Laser-Spallation Leila Seyed Faraji, Dale Teeters, and Michel W. Keller Abstract Improving adhesion properties and strength of thin films used in wide range of important application is the key factor in successfully manufacturing micro-electro-mechanical devices, multilayer micro-electronic and optical devices. Mechanical performance and reliably of thin film on a substrate depends on strength of interfacial adhesion. Understanding the role of process parameters like deposition condition and design parameters such as substrate thickness and film thickness, on mechanical performance of the layer is essential in optimizing manufacturing. However, there are limited techniques for characterizing intrinsic interfacial strength of thin films. The use of laser-generated stress pulses has shown the ability to provide quantitative evaluation of the adhesion strength of thin films. In the current work, we use the laser-spallation approach for the investigation of the adhesion strength of thin tin oxide films to alumina substrates. Keywords Laser-spallation • Interfacial strength • Aluminum thin film • Tin oxide thin film • X-ray diffraction 10.1 Introduction The mechanical strength of thin films used in electronic component such as electrodes for batteries, solar cells, or batteries is dependent on many factors. Critical issues include deposition technique and the chemical and electrical environment in which the films exist. Probing the mechanical strength of thin films and their interfaces is challenging and frequently requires complex micromachining and processing to produce samples. In order to reduce the time required for iterative designs that may eventually result in an acceptable structure, fundamental investigations of the thin film properties and interfaces are essential. These studies are expected to improve the efficiency of the design process and speed the optimization of the thin film process conditions. A critical point in any multi-layer structure is the interface region between two dissimilar materials. This interface is commonly a location for mechanical stress concentration, which can lead to failure of the interface and eventually the device [1, 2]. Due to lattice mismatch or thermal expansion differences between the layers, there are also residual stresses at interface. Depending on the magnitude and sign of the residual stress at the interface, this location may become a preferred site of the film debonding. Many approaches have been adopted to probe these interfaces, such as scratching by sharp tip [3, 4], AFM nano indentation [5], and laser spallation [6, 7]. All mentioned interfacial strength measurement methods are destructive tests. For example in scratching method, spalling and buckling failure modes occur as a result of the compressive stress field preceding the moving tip scratching the surface [8]. There are many other failure modes of films that depend on the measurement method. Debonding, spallation, cracking, channeling are some of these failure modes that could be used for attachment strength of a film [9]. One application for this technique is in the understanding of adhesion in solid state battery systems. There is high demand for advanced rechargeable lithium-ion batteries with high energy densities, small dimensions and ultra-light weight for portable electronic devices. Fabrication of small-scale solid state batteries requires optimization of fabrication method, microstructure, crystal structure and electrochemical properties of both anode and cathode thin films. The interfaces in a solid state battery system consisting of the current collector/anode/solid electrolyte/cathode/current collector system are all susceptible to severe stress during charge/discharge cycles. One promising anode material for these batteries is tin oxide, SnO2 [10, 11]. L.S. Faraji • M.W. Keller (*) Department of Mechanical Engineering, The University of Tulsa, Tulsa, OK, USA e-mail: michael-keller@utulsa.edu D. Teeters College of Engineering & Natural Sciences, Chemistry and Biochemistry, The University of Tulsa, Tulsa, OK, USA © The Society for Experimental Mechanics, Inc. 2017 G.L. Cloud et al. (eds.), Joining Technologies for Composites and Dissimilar Materials, Volume 10 Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-42426-2_10
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