Electro-mechanical performance of thin gold films on polyimide
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Electro-mechanical performance of thin gold films on polyimide Barbara Putz1*, Oleksandr Glushko1, Vera M. Marx2, Christoph Kirchlechner1,2, Daniel Toebbens3, Megan J. Cordill1 1 Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, and Department of Materials Physics, Montanuniversität Leoben, Jahnstrasse 12, Leoben 8700, Austria 2 Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Str.1, 40237 Düsseldorf, Germany 3 Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Str. 15, 12489 Berlin, Germany *corresponding author: [email protected] ABSTRACT Thin metal films on compliant polymer substrates are of major interest for flexible electronic technologies. The suitability of a film system for flexible applications is based on the electromechanical performance of the metal film/polymer substrate couple. This study demonstrates how a 10 nm Cr interlayer deteriorates the electro-mechanical performance of 50 nm Au films on polyimide substrates by inducing the formation of cracks in the ductile layer. Combined in-situ measurements of the film lattice strains with x-ray diffraction and electrical resistance with four point probe of the Au-Cr and Au layers during uniaxial straining confirmed different electro-mechanical behaviours. For Au films with a Cr interlayer the film stress decreases rapidly as cracking initiates and reaches a plateau as the saturation crack spacing is reached. Crack formation and stress drop correspond to a rapid increase in the film resistance. Without the interlayer the Au film stress reaches a maximum around 2% engineering strain and remains constant throughout the experiment. The film resistance is unaffected by the applied elongation up to a maximum strain of 15%, giving no sign of cracking in the metal layer. The outstanding electro-mechanical performance of the gold film indicates that adhesion layers, like Cr, may not be necessary to improve the performance of ductile films on polymers. INTRODUCTION Fragmentation testing is a method commonly used to examine the electrical or mechanical behaviour of metal or ceramic films on polymer substrates for flexible electronic applications [1–3]. During fragmentation testing the film-substrate couple is strained under uniaxial tension to determine the film’s crack onset strain, εf, and saturation crack spacing, λs. These two values can be measured in-situ (during straining) with optical microscopy [4], scanning electron microscopy (SEM) [5] or atomic force microscopy (AFM) [6]. These methods when performed in-situ, can provide the crack spacing, λ, as a function of applied strain, ε. Another method used to determine the fracture strain is to use in-situ 4-point-probe (4PP) resistance measurements. Here the electrical resistance is measured during straining and fracture is described when the resistance ratio (R/R0) deviates from a constant volume approximation (R/R0 = (1+ε)²) when through thickness cracks (TTCs) form in the film [7–9]. This relation is applicable as long as there are no changes
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