Structural Transitions in Titanium/Amorphous-Silicon Multilayers
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STRUCTURAL TRANSITIONS IN TITANIUM/AMORPHOUS-SILICON MULTILAYERS E. Ma, L.A. ClevengerI and C.V. Thompson Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA02139 R.R. DeAvillez Departmento de Ciencia dos Materiais e Metalurgia, Catolica, 22452-Rio de Janeiro, RJ-Brazil
Pontificia Universidade
K.N. Tu IBM T.J. Watson Research Center, Yorktown Heights,
NY 10598
1 Present address: IBM T.J. Watson Research Center, Yorktown Heights, NY 10598 We report a quantitative investigation of silicidation in Ti/amorphousSi thin-films using Differential Scanning Calorimetry (DSC), thin-film X-ray diffraction and Cross-sectional Transmission Electron Microscopy (XTEM). Multilayered thin films were used to facilitate calorimetric observation of the heat released or absorbed at many reacting interfaces. It is shown that calorimetric analysis, combined with structural analysis using X-ray diffraction and XTEM, is effective in providing both kinetic and thermodynamic information about interdiffusion reactions in thin films. The present paper describes experimental results for multilayers with an atomic concentration ratio of 1 Ti to 2 Si and modulation periods ranging from 10 to 60 nm. A thin amorphous titanium silicide layer was found to exist between the as-deposited Ti and a-Si layers. Heating the multilayer film caused the amorphous Tisilicide to grow over a broad temperature range by an exothermic reaction. An endothermic relaxation occurs during the late stage of amorphous silicide growth. Heating to temperatures over 800K causes C49-TiSi 2 to form at the asilicide/a-Si interface. Temperatures at which all the above structural transitions occur vary with modulation period. Analysis of the DSC data indicates an activation energy of 3.1 eV for the formation of C49-TiSi2, which is attributed to both the nucleation and the early growth of the silicide. The heat of formation for C49-TiSi 2 from a reaction of a-silicide and a-Si was found to be -3015KJ/mol. Nucleation appears to be the controlling step in C49-TiSi 2 formation. INTRODUCTION Titanium/a-Si thin-film reactions are of importance due to the application of Ti-silicides in integrated circuits [1]. Recent discovery of the growth of a metastable amorphous Ti-silicide has generated further interest in this system [2]. Previous investigations of silicidation processes during thin-film reactions typically rely on analytical tools such as Rutherford Backscattering Spectrometry (RBS), X-ray diffraction, and Transmission Electron Microscopy (TEM). These tools are sensitive only to either compositional changes or structural changes, and generally a large number of separate experiments are required to follow reaction kinetics over a wide temperature range. In our recent work [3-5], it has been demonstrated that Differential Scanning Calorimetry (DSC) is a valuable technique for study of the nucleation and growth of metastable and equilibrium silicides in multilayered thin films. With DSC, which is sensitive to both compositional and
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