In-Situ Studies of Silicide Formation in Ti-Ta Bilayer Thin Films on Poly-Si
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In-Situ Studies of Silicide Formation in Ti-Ta Bilayer Thin Films on Poly-Si A. S. Özcan and K. F. Ludwig, Jr. Boston University, Physics Department, Boston, MA 02215, C. Lavoie, C. Cabral, Jr., J. M. E. Harper IBM T.J. Watson Research Center, Yorktown Heights, NY 10598 ABSTRACT We have studied the formation of titanium silicides in the presence of an ultra-thin layer of Ta, interposed between Ti and Si. In-situ x-ray diffraction (XRD), resistance measurements and elastic light scattering were used to study the thin film reactions in real time during ramp anneals to 1000 °C. On poly-Si substrates the Ta thickness was varied from 0 to 1.5 nm while the Ti thickness was held constant at ~27 nm. The time-resolved XRD shows that the volume fraction of C40 and metal-rich silicide phases grows with increasing Ta layer thickness. Increased Ta layer thicknesses also delay the growth of the C49 disilicide phase to higher temperatures. Among the Ta thicknesses we examined, 0.3 nm is the most effective in lowering the C49-C54 transformation temperature. Films with Ta layers thicker than 0.5 nm do not completely transform into the C54 phase. The texture of the C54 phase is also sensitive to the Ta thickness. The C54 disilicide film is predominantly (010) textured for the Ti / 0.3 nm Ta sample. The final C54 texture is significantly different for Ta layers thinner or thicker than the optimal 0.3 nm. This suggests that the most effective thickness for lowering the C54 formation temperature is related to the development of a strong (010) texture. The possibility of a template effect by the C40 or metal-rich Ti5Si3 phases is also discussed on the basis of texture considerations. INTRODUCTION The formation of titanium disilicide (TiSi2) has been extensively studied due to its use in Si ultra-large-scale integrated (ULSI) circuits [1,2]. TiSi2 exists in two polymorphs, with crystallographic designations C49 and C54. The C54 phase has a significantly lower resistivity (15-20 µΩ-cm) than does the C49 structure (60-75 µΩ-cm) [3]. Although the technologically desired C54 phase is the equilibrium phase of the disilicide, under most conditions the C49 phase is kinetically favored. The transformation to the C54 phase can be difficult, particularly in submicron features. Interposing a thin layer of a refractory metal is one method for enhancing the C54 phase formation [4,5]. At least two major mechanisms have been suggested to explain this enhancement [6], but their validity remains unclear. The first is the creation of a template layer associated with the ternary C40 silicide phase and/or a metal-rich phase. The second possible mechanism, grain size effect, suggests that the addition of a refractory metal could decrease the average C49 grain size, thus creating more triple-junction grain boundaries where the C54 phase can nucleate. The majority of the previous studies on Ti-refractory metal bilayer structures have focused on Mo as the interlayer material [4,7]. Existing studies looking at the Ti-Ta system examined specific thicknesses of Ti
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