(Nanshu Lu)

Flexible electronic devices such as flexible displays and solar cells draw more and more attention from the industry. In these devices, the functionality is delivered by small-scale structures consisting of thin metal lines and other materials that are integrated on a compliant (e.g. polymer) substrate to make the device flexible. A major concern is reliability, since the small-scale structures are highly fragile, but they are subjected to large thermo-mechanical loads during manufacturing and use. In this project, the influence of the polymer stiffness on the elasto-plastic material behavior of the adhered metal thin films was studied in detail by probing the stress-strain behavior of the metal-polymer stack upon loading/unloading using the plane-strain bulge test technique, which yielded some interesting insights in the dislocation mechanisms in the metal film.

Future projects will study the response of thin inorganic films (metallic and ceramic) on polymer substrates under monotonic and cyclic loads.  Specific issues include:

• Ductility of thin metal films on polymer substrates, with emphasis on using microstructures and adhesion to markedly increase the ductility of the metal films.

• Cracking in thin ceramic films on polymer substrates, with emphasis on developing an experimental method to determine crack driving force, and on studying crack initiation and the effects of inelastic deformation of the substrate on the cracks

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High Ductility of Nanometer Metal Films Adherent on Polymer Substrates (Nanshu Lu and Yong Xiang)

Flexible flat panel display is an emerging, commercially very promising technology. As an essential component, the development of deformable electronics requires high ductility of thin metal films serving as interconnect lines. We investigate the ductility of Cu films deposited on polymeric substrates, and demonstrate that the rupture strains of the metal films are sensitive to their adhesion to the substrates. Well-bonded 100 nm Cu films can sustain strains up to 10% without appreciable cracks, and up to 30% with discontinuous microcracks only. By contrast, poorly bonded Cu films form channel cracks at strains about 2%. The cracks form by a mixture of strain localization and intergranular fracture. The films rupture at large strains when the localization is retarded by the adherent substrates.

Publications:

N. Lu, Z. Suo, J. J. Vlassak, "The effect of film thickness on the failure strain of polymer-supported metal films", Acta Materialia 58, 1679-1687 (2010). (Download)

J.-Y. Sun, N. Lu, J. Yoon, K.-H. Oh, Z. Suo, J. J. Vlassak, "Inorganic islands on a highly stretchable polyimide substrate", J. Mater. Res. 24(11), 3338-3342 (2009). (Download)

N. Lu, X. Wang, Z. Suo, J. J. Vlassak, "Failure by simultaneous grain growth, strain localization, and interface debonding in metal films on polymer substrates",  J. Mater. Res. 24(2), 379-385 (2009). (Download)

N. Lu, X. Wang, Z. Suo, J. J. Vlassak, "Metal films on polymer substrates stretched beyond 50%", Appl. Phys. Lett. 91, 221909 (2007). (Download)

Y. Xiang, T. Li, Z. Suo and J. J. Vlassak, "High ductility of a metal film adherent on a polymer substrate", Appl. Phys. Lett. 87, 161910 (2005). (Download)