Bending Fatigue Behavior and Reliability Improvement of Metal Electrode for Flexible Devices

Abstract

Degradation of the electrical property of metal electrodes on flexible substrates during repeated deformations is considered to be an inevitable reliability issue for the realization of flexible devices. This stability problem is induced by the fatigue failure of metal thin films during repeated deformations. In this study, fatigue failure of metal electrodes was investigated under repeated bending/sliding deformations, which can mimic the real deformation motion of flexible devices. In addition, the failure cycle, which was defined by an electrical resistance change, is a meaningful practical meaningful approach to characterizing the reliability problem of flexible and electrical devices. The bending fatigue characteristics of metal electrode were studied in single-layer, multi-layer (under-layer or over-layer), and nanohole structures. Fatigue failure of Cu on a polyimide (PI) substrate occurs as a result of damage evolution, such as crack and protrusion (extrusion and intrusion) formation. This fatigue damages evolved as a results of dislocation motion during repeated deformations. The accumulated dislocations generated extrusions and intrusions on the surface and these protrusions acted as stress concentration sites. Finally, cracks were formed and the electrical resistance of the metal electrodes increased as a result of electron scattering. Fatigue damage formation is dependent on the bending strain. At a larger bending strain, the fatigue life cycle, which was defined by a 1% electrical resistance increase, was shortened. The applied strain and the failure cycle followed the Coffin-Manson relation. The fatigue resistance was improved in thinner films because of the constraint effect of small grains. During the bending and sliding motions, the sliding distance defined the damage zone. Fatigue damage occurred at the edge of the bending where bending and unbending were repeated, but no damage was observed at the center of the bending because the metal electrode was always in the bent state and under a uniform stress. Fatigue failure cycles shortened exponentially as the sliding distance (damage zone) increased. This phenomenon is attributable to the probability of fatigue damage initiation, which increased exponentially for large sliding distances. The effect of multi-layer structures on fatigue damage was investigated in NiCr under-layer and Al over-layer structures under repeated tensile or compressive bending deformations. The electrical stabilities depended on the multi-layer structure significantly. NiCr under-layers deteriorated under tensile bending fatigue as a results of mode I crack propagation, but Al over-layers improved the bending fatigue resistance of Cu by blocking crack formation on the surface. To exclude the fatigue damage, which is an inevitable issue in conventional metal films, we adopted a nanohole structure on the Cu electrodes using nanorod PI substrates. Nanohole electrodes showed an extremely low electrical resistance change (<10%) after 500,000 bending cycles, where as conventional metal films had a resistance increase of over 300%. This improvement is ascribed to the nanoholes, which suppressed fatigue damage initiation by dislocation annihilation and damage propagation by crack tip blunting. This study investigated the physics of fatigue failure of metal electrodes. Based on an understanding of this, the criteria for the design of multi-layer metal electrodes are discussed and a new concept of nanohole electrodes is suggested for improving the stability of flexible devices.