Interdiffusion studies of single crystal TiN/NbN superlattice thin films

The interdiffusion of TiN and NbN in superlattice structures has been inves tigated by studying the evolution of superlattice satellite peaks in x-ray diffraction; as well as by cross-sectional transmission electron microscopy . Single crystal TiN/NbN superlattices with composition modulation periods of 4.4 and 12.3 nm were deposited, by reactive dual-cathode unbalanced magn etron sputtering in an Ar/N-2 discharge, onto MgO(001) substrates held at a temperature of 700 degrees C. Isothermal annealings (in the range of 750-8 75 degrees C for 20 min) as well as a ramped annealing (3 degrees C s(-1) u p to 1200 degrees C) were performed, and in situ x-ray diffraction spectra were continuously recorded using synchrotron light and a linear detector. T he results pointed to a nonlinear diffusion in TiN-NbN couples. The structu re maintained abrupt interfaces throughout annealing, while the position of the interfaces was continuously shifted into the TiN layers. A model is pr oposed where Ti diffuses into the NbN layer to form a NbTiN alloy, while th e diffusion of Nb in the opposite direction is restricted. Within the tempe rature range from 750 to 875 degrees C, activation energies for metal inter diffusion were limited to 1.2 eV for the lower temperature end, and 2.5 eV for the higher temperature end. The expected lifetime against alloying has been determined using the random walk theory, and a TiN/NbN superlattice wi th a period of 4.4 nm is expected to sustain a layered structure for simila r to 10 h at 750 degrees C and similar to 2 h at 850 degrees C. (C) 1999 Am erican Vacuum Society. [S0734-2101(99)03805-1].