Interfacial reaction pathways and kinetics during annealing of 111-textured Al/TiN bilayers: A synchrotron x-ray diffraction and transmission electron microscopy study

Abstract

Growth of TiN layers in most diffusion-barrier applications is limited to deposition temperatures Ts≲500 °C. We have grown polycrystalline TiN layers, 160 nm thick with a N/Ti ratio of 1.02±0.03 and a 111 texture, at Ts=450 °C on SiO2 by ultrahigh vacuum reactive magnetron sputter deposition in pure N2. Al overlayers, 160 nm thick with inherited 111 preferred orientation, were then deposited at Ts=100 °C without breaking vacuum. The as-deposited TiN layer is underdense due to the low deposition temperature (Ts/Tm≃0.23 in which Tm is the melting point) resulting in kinetically limited adatom mobilities leading to atomic shadowing which, in turn, results in a columnar microstructure with both inter- and intracolumnar voids. The Al overlayer is fully dense. Synchrotron x-ray diffraction was used to follow interfacial reaction kinetics during postdeposition annealing of the 111-textured Al/TiN bilayers as a function of time (ta=12–1200 s) and temperature (Ta=440–550 °C). Changes in bilayer microstructure and microchemistry were investigated using transmission electron microscopy (TEM) and scanning TEM to obtain compositional maps of plan-view and cross-sectional specimens. Interfacial reaction during annealing is initiated at the Al/TiN interface. Al diffuses rapidly into TiN voids during anneals at temperatures ⪝480 °C. In contrast, anneals at higher temperatures lead to the formation of a continuous nanocrystalline AlN layer which blocks Al penetration into TiN. At all annealing temperatures, Ti atoms released during AlN formation react with Al to form tetragonal Al3Ti at the interface. Al3Ti exhibits a relatively planar growth front extending toward the Al free surface. Analyses of time-dependent x-ray diffraction peak intensities during isothermal annealing as a function of temperature show that Al3Ti growth kinetics are, for the entire temperature range investigated, diffusion limited with an activation energy of 1.5±0.2 eV.