An in situ X-ray diffraction study of phase separation in Cu-Ta alloy thin films

In this work a systematic in situ annealing study of sputtered Cu-Ta alloys is reported. Under equilibrium conditions Cu and Ta are completely immiscible, the metastable Cu-Ta alloys hence undergo phase separation upon annealing. Using in situ X-ray diffraction the phase evolution at temperatures < 650 °C is studied over the entire composition range and the data is assembled in a phase map. It is shown that the presence of a surface oxide layer influences the β to α phase transformation temperature of Ta and thus determines the crystal structure of the Ta-rich phase formed upon phase separation. Surface segregation of Cu necessitates the use of an alumina capping layer on the Cu-Ta/SiNx/SiO2/Si samples.