Fracture and Delamination of Chromium Thin Films on Polymer Substrates

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OVER the past few years, metal and ceramic films on polymer substrates have led to the emergence of deformable electronics, such as foldable displays, sensor skins, and electronic textiles. The typical route to create deformable devices on polymer substrates is to fabricate stiff islands, commonly made with silicon-nitride, where active cells (thin film transistors) are fabricated and connected by metal lines.[1–4] These interconnect lines must deform with the substrate and survive strains greater than 10 pct while maintaining electrical conductivity.[1,2,5] Several studies have examined the tensile strength of blanket copper (Cu), aluminum, and gold films on polymer substrates[6–11] as well as gold lines.[4,12] Further research on ceramic coatings on polymers for barrier gas coatings has also been completed.[13–15] Compared to bulk metals, these films on polymer substrates can have small rupture strains (less than 2 pct) but high strengths;[7,16] however, when the film is well bonded to the substrate, it will have high rupture strains due to the lack of localized deformation that is M.J. CORDILL, Postdoctoral Researcher, A. TAYLOR, Ph.D. Candidate, and G. DEHM, Professor and Head, are with the Erich Schmid Institute of Materials Science of the Austrian Academy of Sciences and the Department of Materials Physics at the University of Leoben, Jahnstrasse 12, A-8700, Leoben, Austria. Contact e-mail: [email protected] J. SCHALKO is with the Research Unit for Integrated Sensor Systems of the Austrian Academy of Sciences, Viktor Kaplan Strasse 2, A-2700 Wiener Neustadt, Austria. This article is based on a presentation given in the symposium entitled ‘‘Mechanical Behavior of Nanostructured Materials,’’ which occurred during the TMS Spring Meeting in San Francisco, CA, February 15–19, 2009, under the auspices of TMS, the TMS Electronic, Magnetic, and Photonic Materials Division, the TMS Materials Processing and Manufacturing Division, the TMS Structural Materials Division, the TMS Nanomechanical Materials Behavior Committee, the TMS Chemistry and Physics of Materials Committee, and the TMS/ASM Mechanical Behavior of Materials Committee. Article published online September 23, 2009 870—VOLUME 41A, APRIL 2010

suppressed by the strong adhesion.[3] When the film is brittle like silicon nitride, no plastic elongation takes place and fracture is caused by the breaking of an array of atomic bonds.[1] Failure of the films by cleavage fracture is similar to previous studies of ceramic films on metal substrates pulled in tension.[17–19] With the periodic cracking method that is based on a shear lag analysis,[17,18,20,21] the maximum shear traction that is supported by the interface can be calculated. The periodic cracking method was first developed to study the shear properties of metal-ceramic interfaces,[17] with similar periodic cracking of other material systems having also been studied.[22–25] With the metal-ceramic system, plastic deformation in the metal substrate is accommodated by elastic deformation of the ceramic film until t

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