Using first-principles total-energy calculations, we investigated the atomic structure of a very thin Ti film on Si(001) and their intermixing. Our calculations show that the Ti film forms a pseudomorphic body-centered-tetragonal structure on Si(001). We found that intermixing of Ti and Si atoms energetically favors the formation of aC49-TiSi2phase. We propose a plausible explanation of how theC49-TiSi2phase and the interfacial structure between C49 phase and Si(001) are formed. We discuss why the transformation of the C49 phase into the C54 phase is inhibited when theTiSi2dimensions reach the submicron region. ©1998 American Institute of Physics.