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

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.