Size Effect on Crack Formation in Cu/Ta and Ta/Cu/Ta Thin Film Systems
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Size Effect on Crack Formation in Cu/Ta and Ta/Cu/Ta Thin Film Systems P.Gruber1, J. Böhm1, A. Wanner2, L. Sauter3, R. Spolenak3 and E.Arzt1, 3 Universität Stuttgart, Institut für Metallkunde, Stuttgart, Germany 2 Universität Karlsruhe, Institut für Werkstoffkunde I, Karlsruhe, Germany 3 Max-Planck-Institut für Metallforschung, Stuttgart, Germany 1
ABSTRACT 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.
INTRODUCTION The application of thin metallic films in micro- and nanoelectromechanical systems MEMS/NEMS, as well as microelectronics, is widespread. During fabrication and service, large mechanical stresses may develop, which can result from deposition, microstructural changes, thermal mismatch, or external loading. In metallic films, these stresses are partially relaxed by yielding or creep, both of which are strongly influenced by the dimensions of the components. For many years, materials research has therefore focused on the understanding of deformation mechanisms in thin metal films on stiff substrates [1-6]. Current semiconductor technology demands the use of low-resistivity metal layers for integrated circuit conduction lines and contact structures. A well established material system for the CMOS process is a Cu metallization on Si including a Ta diffusion barrier [7]. Recently, flexible electronic circuits have gained widespread interest for numerous applications including flexible displays [8] and wearable electronics [9]. Due to their low Young’s modulus, devices on polymeric substrates will have to endure high strains during application. Thus the maximum total strain of such a structure is limited by the extensibility of the metal film. A recently developed in situ synchrotron X-ray diffraction technique [10] allows for the measurement of isothermal stress-strain curves of ultrathin polycrystalline films on a polymeric substrate. Within the scope of this paper, we utilized this novel technique to investigate the deformation behavior and microstructural changes of different Cu/Ta and Ta/Cu/Ta thin film systems during tensile testing to total strains of 5% or higher.
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Ta Cu Ta
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