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e-A-Seul Shin, Ji-Hoon Lee, and Tae-Youl Yang Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Korea

Thomas Haas and Patric Gruber Institute for Applied Materials, Karlsruhe Institute of Technology, Karlsruhe 76021, Germany

In-Suk Choi High Temperature Energy Materials Research Center, Korea Institute of Science and Technology, Seoul 136-791, Korea

Oliver Krafta) Institute for Applied Materials, Karlsruhe Institute of Technology, Karlsruhe 76021, Germany

Young-Chang Joob) Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Korea (Received 16 March 2014; accepted 8 October 2014)

The thickness dependence of the electrical stability under monotonic and cyclic tensile loading is investigated for Cu films on polymer substrates. As for monotonic tensile deformation, thicker films show better stability than thinner films due to their higher ductility and the larger capability of strain accommodation. For the fatigue resistance, however, a more complex behavior was observed depending on the amount of the applied strain. For low strain amplitude in the high cycle fatigue (HCF) regime, thinner films exhibit longer fatigue life because the larger strength of thinner films suppresses dislocation movement and damage nucleation. However, for high strain amplitudes in the low cycle fatigue (LCF) regime, the fatigue life for thinner films is drastically reduced compared to thicker films. It is shown that fatigue coefficients in the LCF regime can be obtained when applying the Coffin–Manson relationship.

I. INTRODUCTION

Flexible electronic devices with high portability, light weight, and high durability to mechanical impact have been developed for various applications such as displays,1 batteries,2,3 solar cells,4 and electronic skins.5 With the flexible devices approaching the market, strong requirements are being placed on the mechanical and electrical performance during the long-term usage of the devices. In typical flexible device systems, consisting of various organic and inorganic materials, one of the weakest parts against mechanical deformation is the metal layer, which is used as an electrode or an interconnect. The stability of metal films under mechanical deformation needs to be addressed for monotonic tensile loading as well as for fatigue failure. When a large strain is applied, the mechanical and electrical failure of a metal electrode on a flexible substrate occurs due to crack

Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2014.339 J. Mater. Res., Vol. 29, No. 23, Dec 14, 2014

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formation,6,7 and a large effort has been made to improve the stretchability of thin metal films by improving the adhesion 8 or the geometry of the electrodes.9–11 Moreover, when small strains below the fracture strain are applied in a cyclic manner, the mechanical and electrical properties are found to degrade gradually due to fatigue damage,