Effect of a thin Ta layer on the C49-C54 transition
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Effect of a thin Ta layer on the C49-C54 transition. F. La Via*, F. Mammoliti#, M.G. Grimaldi#, S. Quilici° and F. Meinardi°. * CNR-IMETEM, Stradale Primosole 50, Catania, Italy. # INFM and Physics Department, Corso Italia 57, Catania, Italy. ° INFM and Material Science Department, Via Cozzi 53, Milano, Italy.
ABSTRACT The effect of a thin Ta layer at the Ti/Si interface on the kinetic of the C49-C54 transition will be shown in detail. The transformation kinetic has been monitored by in situ sheet resistance measurements that, coupled to structural characterisation, allowed to evidence the presence of an intermediate phase before the C54 formation. The temperature of the C54 phase formation decreases with a Ta concentration of 4.5·1015 cm-2 and µ-Raman images of partially transformed samples indicates that the density of C54 grains in presence of Ta is about one order of magnitude higher with respect to pure Ti/Si samples. INTRODUCTION TiSi2 is widely used as a self-aligned silicide (salicide) for the metallization of gates, sources/drains and local interconnects in Ultra Large Scale Integration (ULSI) devices and circuits. In thin films, there are two crystalline phases with the TiSi2 stechiometry: the high-resistivity (60-70 µΩ⋅cm) orthorhombic base-centred C49-TiSi2 which usually forms at 550-700 °C and the low resistivity (15-20 µΩ⋅cm) orthorhombic face-centred C54-TiSi2 phase, which usually forms at 700-850 °C [1]. The transformation from C49 to C54 phase becomes more difficult and requires annealing at high temperature in narrow (submicron) Si lines because of the low density [2] of nucleation sites. In recent papers it has been shown that a significant reduction of the formation temperature of the C54 phase occurs when Mo is added to the reacting film by ion implantation [3] and/or deposition [4] of a thin layer at the Si/Ti interface. A similar effect has been observed even with Ta [5] or Nb [6] impurities and it has been attributed to the avoidance of the intermediate C49 phase during annealing since this phase was not detected by X-ray diffraction. However, in spite of the large
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effort devoted to determine the effect of these metals on the C54 formation, a detailed kinetic study of the transformation is missing. In this paper the effect of a thin Ta layer at the Ti/Si interface on the kinetic of the C49-C54 transition will be shown in detail. The transformation kinetic has been monitored by in situ sheet resistance measurements in samples with different Ta content. Structural characterization by X-ray diffraction and µ-Raman spectroscopy allowed detection of an intermediate phase before the C54 formation. EXPERIMENTAL DETAILS Samples were prepared in a UHV chamber by e-gun deposition of a 150 nm amorphous Si layer on thermally oxidised 5 inches silicon wafers. After this deposition, without breaking the vacuum, thin layers of Ta and Ti were deposited sequentially with the same technique. The Ta and Ti layers were 1.9×1015 at/cm2 and 20 nm thick, respectively, as measured by Rutherford Backsca
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