THERMAL STABILITY OF A Cu/Ta MULTILAYER; AN INTRIGUING INTERFACIAL REACTION

摘要

When a Cu/Ta multilayer is formed by sputtering, it is found that the Cu has the equilibrium fc.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 multi- layer, we found from X-ray diffraction analysis that a large amount of β-Ta already transformed into b.c.c.-Ta upon annealing at 700℃, 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℃, small α-Ta grains nucleate at the Cu/β-Ta interface and grow into Cu. Annealing at 800℃ 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℃ 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℃ 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℃), no amorphous layer appears and the interface is found to be chemically abrupt. We conclude that the microstructural change of the multilayer i