In-situ Tensile Straining of Metal Films on Polymer Substrates under an AFM
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In-situ Tensile Straining of Metal Films on Polymer Substrates under an AFM M.J. Cordill1, V.M. Marx2 1 Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Jahnstrasse 12, Leoben, 8700, Austria 2
Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Straße 1, 40237 Düsseldorf, Germany
ABSTRACT Metal films on polymer substrates are commonly used in flexible electronic devices and as gas barrier coatings. One way to evaluate the fracture and adhesion properties of such film systems is the fragmentation test. In the fragmentation test a film-substrate system is strained in tension under an optical microscope or inside a scanning electron microscope to observe the cracking and delamination events in situ. The technique works very well for brittle metal and ceramic films. However, when ductile films are strained they deform plastically before cracks and buckles appear. Therefore, a tensile straining device was developed to fit under an AFM for in situ observation of ductile metal films on polymer substrates. With the new in situ device the first occurrence of plastic deformation in the form of localized thinning of the film and channel cracks are visible. These features can only be detected through a height difference in the AFM images and not with optical or scanning electron micrographs. A comparison to brittle Cr films on polymer substrates was performed. INTRODUCTION The reliability of flexible electronics depends upon the ability of metal thin films, both brittle and ductile, to bend and stretch with the supporting polymer substrate while maintaining mechanical and electrical integrity. Experiments to measure these behaviors are mostly based on fragmentation testing [1-3], initially employed to examine the fracture behavior of hard ceramic films and coatings on softer metal substrates (Al, Cu, steel) [2, 3]. This testing technique is now being used to examine metal and ceramic films on polymer substrates [4-9]. A key element to fragmentation testing is the detection (visibly or electrically) of the fracture strain or stress. This detection can be easily performed with an optical microscope (OM), scanning electron microscope (SEM), or 4 point probe (4PP) resistance measurement while straining (in situ). Brittle films or coatings will form channel cracks perpendicular to the straining direction at low strains and at higher strains, delamination, or buckles, can form in film fragments parallel to the straining direction. These channel cracks are easy to detect and thus failure is also easily determined. However, ductile films, such as Au, Cu, or Ag (all materials being used as conduction carrying elements in flexible electronics) plastically deform before channel cracks are observed. The localized plastic deformation, or necks (necking), are the first sign of yield in the film and are hard to observe with OM or SEM. With in situ 4PP the electrical resistance increases with the increased strain and deformation, thus making the detection of the initial yield in ductile films impossible. 4PP mea
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