Thermal stability of a Cu/Ta multilayer: An intriguing interfacial reaction

When a Cu/Ta multilayer is formed by sputtering, it is found that the Cu has the equilibrium f.c.c. structure whereas the Ta is in a metastable tetragonal (β-Ta) form. The transformation of the latter into the stable b.c.c. phase (α-Ta), in the presence of Cu, is the subject of the present study. We observed that for a Ta film alone without Cu, the phase transformation starts at about 800 °C. In a Cu/β-Ta multilayer, we found from X-fay diffraction analysis that a large amount of β-Ta already transformed into b.c.c.-Ta upon annealing at 700 °C, which is much lower than the transformation temperature of Ta by itself. Our transmission electron microscopy (TEM) analysis furthermore reveals that during annealing at around 600 °C, small α-Ta grains nucleate at the Cu/β-Ta interface and grow into Cu. Annealing at 800 °C results in a complete transformation of the multilayer into the agglomeration of separate Cu and α-Ta grains. Also, we observed that an amorphous layer about 2 nm thick forms upon annealing at 500 °C in the Cu/β-Ta system, which has a slightly positive heat of mixing. In energy dispersive spectroscopy (EDS) analysis, the Cu/β-Ta interface of as-deposited samples is found to be chemically discrete, while the Cu/β-Ta interface of a 600 °C annealed sample shows extensive interdiffusion and the amorphous layer formed at the interface is found to be a mixed layer of Cu and Ta. Upon annealing at a higher temperature (800 °C), no amorphous layer appears and the interface is found to be chemically abrupt. We conclude that the microstructural change of the multilayer is the process of the Ta phase transformation and this occurs through the interaction with Cu. We, furthermore, discuss the amorphization in the Cu-Ta system based on a calculated free energy diagram.