The Ti layer deposited by ionized metal plasma deposition technique and the reacted region between Ti and Al-0.5%Cu, in Al-0.5%Cu/Ti/SiO2 /Si structure, were characterized by scanning transmission electron microscopy (STEM) and Rutherford backscattering spectroscopy (RBS). The results indicate that the Ti layer with a column-like structure grew epitaxially in the close-packed [001] direction. This growth mechanism resulted in a high-density Ti layer in Al-0.5%Cu/Ti/SiO2/Si structure. This epitaxially grown Ti layer has anisotropic diffusion properties that can retard the diffusion of Al across the Ti layer in the initial stage of high temperature Al sputtering or reflow processes. But the column-like Ti crystals recrystallized during further processing, altered the [001] alignment of the crystals to cause the interdiffusion of Ti and Al. The reactions between Ti and Al took place subsequently and formed the intemetallic Al3Ti, Al5Ti2, TiAl, and Ti3Al layers from the surface inwards. As the diffusion of Al in Ti, Ti3Al, and TiAl decreased in order, the formation of Ti3Al and TiAl layers further retarded the diffusion of Al into the SiO2 layer. It can be concluded that the intermetallic layers play an important role of diffusion barrier that impede the migration of Al across the films.