Layered structures of Cu and Ta thin films on silicon are well established for many technological applications in microelectronics. Electronic circuits used for flexible displays or wearable electronics are becoming increasingly popular. For such applications, the Cu/Ta system must be transferred to flexible substrates, incorporating a design rule for several percent of total strain. We have investigated the deformation behaviour of different Cu/Ta and Ta/Cu/Ta thin film systems on a flexible polyimide substrate subjected to total strains of more than 5%. A novel synchrotron X-ray diffraction technique allowed us to characterize the evolution of mechanical stress in very thin metallic films during isothermal tensile tests. We found that samples with a Cu film thickness below 300 nm showed a sudden stress decrease at a total strain of about 2.5%. This stress drop was attributed to fracture of the entire film system, initiated by cracks in the Ta layers.
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The authors would like to thank the beamline team of the MPI-MF-Surface-Diffraction-Beamline at ANKA (R. Weigel, N. Kaspar, A. Rühm and A. Stierle) for their excellent technical support as well as T. Wagner and F. Thiele of the MPI Thin Film Laboratory for sample preparation.
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Gruber, P., Böhm, J., Wanner, A. et al. Size Effect on Crack Formation in Cu/Ta and Ta/Cu/Ta Thin Film Systems. MRS Online Proceedings Library 821, 72–78 (2004). https://doi.org/10.1557/PROC-821-P2.7