Chapter 12 Opto-Acoustic Technique to Investigate Interface of Thin-Film Systems Sanichiro Yoshida, David Didie, Daniel Didie, Sushovit Adhikari, and Ik-Keun Park Abstract The adhesion strength of nano-scale thin-film systems is evaluated with an opto-acoustic technique. The thin-film specimens are oscillated with an acoustic transducer at a range of audible frequency and the resultant film-surface displacement is detected with an optical interferometer. For each film material, a pair of specimens is prepared; one is coated on a silicon substrate after the surface is treated with plasma bombardment, and the other is coated on an identical silicon substrate without a treatment. For comparison, a bare silicon specimen of the same dimension is tested. All coated specimen show greater film-surface displacement than the bare silicon specimen in the entire frequency range, confirming that the detected oscillation represents the differential displacement at the interface. In some cases, the specimen coated on the plasma treated substrate shows greater film-surface oscillation than the one coated on the untreated substrate. The frequency dependence of the oscillation indicates resonance-like behavior at a frequency orders-of-magnitude lower than the resonance estimated from the bonding strength. Keywords Thin-film systems • Opto-acoustic technique • Adhesion strength • Metal-silicon interface • Michelson interferometer 12.1 Introduction Thin film coating is used in a wide variety of engineering systems ranging from semiconductor products to artificial joints, for various purposes such as provision of better lubrication, prevention from erosion, and enhancement of wear resistance. The recent trend is that the film thickness is reduced for better performance of the system. In some cases, the film thickness is a few tens of nanometers for an application where the film surface is exposed to vigorous rubbing motion. With these advancements in applications, the quality control of the film adhesion strength and endurance has become more important than ever. Although the mass percentage of the film itself is low, often a minute damage on the coating ends the life of the entire system. As a good example, slight degradation of the coating of an artificial knee joint can not only shorten the life of the joint but also cause excruciating pain and force replacement of the entire joint. Prevailing techniques employed for evaluation of adhesion strength can be classified into destructive methods and nondestructive methods. The former includes the pull-off test [1], the bending test [2], the nano-scratch test [3], and the nanoindention test [4]. These methods measure the ultimate strength of coating adhesion. The main stream of the techniques classified as the nondestructive methods is the ultrasonic technology [5–7]. These techniques either detect abnormality in the interface such as partial delamination and defects, or measure the elastic constant of the interface through examination of changes in the acoustic velocity. Apparently, the destructive methods provide accurate and useful information regarding the ultimate adhesion-strength. As for the evaluation of endurance, however, the information provided by these methods is not necessarily accurate. It is rather rare that external force is applied to the film-surface at a level comparable to the ultimate stress. Usually, the coating is S. Yoshida (*) • D. Didie • D. Didie • S. Adhikari • I.-K. Park Department of Chemistry and Physics, Southeastern Louisiana University, SLU 10878, Hammond, LA 70402, USA Seoul National University of Science and Technology, Seoul, Republic of Korea e-mail: syoshida@selu.edu; sanichiro.yoshida@selu.edu H. Jin et al. (eds.), Advancement of Optical Methods in Experimental Mechanics, Volume 3: Proceedings of the 2014 Annual Conference on Experimental and Applied Mechanics, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-06986-9_12, #The Society for Experimental Mechanics, Inc. 2015 117
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