Local Structural Studies of TiSi 2 and ZrSi 2 Thin Films on Si(111) Surfaces
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Y. DAO, A.M. EDWARDS, D.E. SAYERS AND R.J. NEMANICH North Carolina State University, Department of Physics, Raleigh, NC 27695-8202, USA ABSTRACT This study explores the local structures of TiSi2 and ZrSi2 films on Si. The strain effect on the surface morphologies and phase stability of the films is investigated. Titanium silicide and zirconium silicide films were fabricated on atomically clean Si(1 11) substrates in UHV using electron gun evaporation followed by in situ annealing. A quantitative X-ray absorption spectroscopy analysis was performed to study the formation of epitaxial TiSi2 and ZrSi2 films on the Si surfaces, as well as the phase transition of TiSi2 from the metastable C49 structure to the stable C54 structure. The surface morphologies and the surface roughness of the films were studied using atomic force microscopy (AFM). The strains of these films were determined by comparing the calculated bond lengths with the measured bond lengths using the extended X-ray absorption fine structure (EXAFS) technique. The final stable C54 structure of TiSi2 has less strain than the initially formed metastable C49 phase. While the surface and interface free energies and the bulk free energy are critical to the structural formations and phase changes of the films, the energy associated with the lattice strain is another important factor. INTRODUCTION Refractory metal silicides are often considered for contacts and interconnections in Very Large Scale Integrated (VLSI) circuits. Compared to other metal silicides, TiSi2 is an optimal choice because of its low resistivity and high temperature stability. The formation of TiSi2 on Si by thin film reaction is often considered for source or drain contacts on Si MOS device structures [1, 2]. Since Zr is in the group IVb as Ti is, it has very similar chemical properties. Films of ZrSi2 also show very good electrical and thermal properties. However, very little work has been done on the Zr/Si system [3, 4]. The surface and interface free energies, the bulk free energy and the strain energy all contribute to the total free energy of the film. It is widely accepted that the growth modes, phase changes and surface morphologies are determined by the total free energy of the thin films. TiSi2 exhibits two different phases, a stable C54 phase with an orthorhombic face centered structure and a metastable C49 phase with an orthorhombic base centered structure [5, 6]. Previous work shows that the Ti atoms react with the Si substrate and initially form the C49 structure, which has a lower surface and interface free energy. After annealing at higher temperatures, the TiSi2 transforms to the C54 phase, which has a lower bulk free energy [6, 7]. The phase transition has been modeled as a competition between the surface and interface free energies and the bulk free energy of the film. On the other hand, ZrSi2 has only one stable structure, the C49 phase [8, 9]. The strain of the films, which may have a strong effect on the initial growth and the phase transition, has not been studied previ
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