Effects of a Ta Interlayer on the Titanium Silicide Reaction: C40 Formation and Higher Scalability of the TiSi 2 Process
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Effects of a Ta Interlayer on the Titanium Silicide Reaction: C40 Formation and Higher Scalability of the TiSi2 Process. F. La Via CNR-IMETEM, Stradale Primosole 50, Catania, Italy.
S. Privitera, F. Mammoliti, M.G. Grimaldi INFM and Physics Department, Corso Italia 57, Catania, Italy.
ABSTRACT When a Ta layer is deposited at the Si/Ti interface a new phase has been detected, i.e. the TiSi2 C40. The C40-C54 transformation kinetics and the film morphology are consistent with an increase of the nucleation density with respect to the C49-C54 transition. The activation energies for the nucleation rate (4.2±0.3 eV) and the growth velocity (4.0±0.4 eV) have been obtained from the in situ sheet resistance and the Transmission Electron Microscopy results. These results show that the process with a Ta layer at the Ti/Si interface has a greater scalability with respect to the standard TiSi2 process.
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 two crystalline phases appear, and the metastabile C49-TiSi2 usually forms before the final C54TiSi2 phasei. The transformation from C49 to C54 phase becomes more difficult in submicron Si lines because of the low densityii of nucleation sites. In recent papers,iii,iv it was shown that the presence of Mo, Ta or Nb impurities at the metal-silicon interface lowers the formation temperature of the C54 phase with respect to pure Ti/Si bi-layers. In a first attempt this reduction was attributed to the suppression of the intermediate C49 phasev,vi. Later measurementsvii by high resolution transmission electron microscopy indicated the formation of a ternary compound (TiTa)Si2 C40 at the Ti/Si interface that acts as a seed for the successive growth of the TiSi2 C40 phase. The C54 phase starts to form after the formation of the C40 phase. In a recent paperviii it has been observed that the transition from the C40 to the C54 is nucleation limited. Finally we reportedix that during the reaction of the Si/Ta/Ti system the C40 TiSi2 is observed to growth over the C40 TaSi2. This phase is the only detected phase before the formation of the final C54 TiSi2 and the experimentally
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determined lattice parameters of the new phase agree quite well with that ones obtained by ab-initio calculations. In this work the C40-C54 TiSi2 transformation kinetics has been monitored by in situ sheet resistance measurements and by ex-situ Transmission Electron Microscopy (TEM) and BraggBentano X-Ray Diffraction (XRD). The experimental results have been fitted by a Montecarlo code that allowed an independent evaluation of the activation energies for the nucleation rate and the growth velocity. Using these results we simulated the transition kinetics to the C54 phase in narrow lines and it comes out that the process with a Ta layer at the Ti/Si interface has a greater scalability with respect to the standard TiSi2 process.
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