Grain boundary plays an important role in determining the physical properties and chemical stability of the materials. In particular, the structures of grain boundaries in atomic layer deposited TiN film may be one of the main factors to dominate the reliability and performance of ULSI devices with multilayer structure of Cu-based interconnects. In this work, the characteristics of grain boundaries in the atomic layer deposited (ALD) ultralthin TiN films are investigated in detail. The results show that the small angle boundaries frequently appear in (200) epitaxy growth areas while the large angle grain boundaries are often observed in the areas with (I 11) close-packed stacking configuration. The tilt and twist boundaries are the two main structures of large angle grain boundaries. It is of interest to note that some large angle grain boundaries in the ALD TiN film are in an atomic scale. Such small and large angle boundaries in atomic scale lead to the pinhole free, high density and homogeneity ultrathin polycrystalline ALD TiN film. (c) 2005 Elsevier B.V All rights reserved.