Texture and phase transformation of sputter-deposited metastable Ta films and Ta/Cu multilayers

Two different structures of Ta are known: the bulk bcc structure of alpha-Ta (a=0.33058 nm) and the metastable tetragonal beta-Ta phase (a=0.534 nm, c=0.994 nm). The metastable phase is mainly observed in sputter-deposited thin films and a dependence on film thickness and deposition conditions have been reported. In our study we investigated the structure and the crystallographic orientation of d.c.-magnetron sputtered Ta films using wide-angle X-ray diffraction after preparation and after annealing. Ta films were deposited onto different underlayer materials (Al, SiO2 and Cu). For Ta/Cu multilayers the single layer thickness and the sputter parameters are varied. It is observed that the Ta structure after deposition mainly depends on the substrate or underlayer material. beta-Ta is always observed after sputtering onto SiO2 or (111)-textured Cu, with the orientation of the Ta grains depending on the Ta-layer thickness, whereas alpha-Ta is observed on an (111)Al underlayer. Therefore, the bulk equilibrium phase only seems to be forced by an epitaxial relation. During annealing up to 700 degrees C the (100)-oriented grains of beta-Ta change their orientation and the metastable beta-phase transforms into the equilibrium alpha-phase with (110) texture. The temperature for structural transformation decreases with increasing Ta layer thickness.