Electrical Flexibility of Cu Thin Films for Applications in Flexible Electronics

Abstract

Great demands for flexible conducting electrodes are promoted considering the rapid development of flexible electronics. Conventional thin-film FCE, however, has limits of low conductivity, poor mechanical reliability and requires complicated fabrication process due to the use of nanowires and nanoparticles. In this work, Al-Cu bilayer thin films were grown on plastic flexible substrate by a DC magnetron sputtering at 75W power. Cu and Al target were used as source for the deposition Al-Cu bilayer. The deposition time for the Cu was 30 min and for Al it was varied (30s, 60s, 120s). The evolution of properties and stability of electrical resistance of Al-Cu bilayer on plastic flexible substrates was evaluated. The properties of resulting thin films were characterized by X-ray diffraction (XRD), and bending test. It was observed that the resistance of the Cu films significantly increased to as much as 40 times of the initial value after 400 bending cycles. The results indicate fast mechanical deterioration of the films due to initiation of cracks and their propagation, which impairs electrical conduction across the films. Compared to a single Cu layer, excellent electrical stability due to bending cycles was observed when a sputter-deposited thinner Al underlayer was used. The Al underlayers of deposition times of 30 and 60 s, shows that resistance remained the same up to 400 bending cycles. This is attributed to the enhanced mechanical integrity and better adhesion with the substrate. However, when a thicker Al underlayer was used, the resistance increased with bending cycles. The results suggest that there exist an optimum thickness of the underlayer which coupes well both with the substrate and the Cu over layer. The obtained results are quite relevant for flexible electronics in that an appropriate underlayer helps in electrical stability of Cu films, which might be used as bottom contact in flexible devices.