Stress Evolution During the Formation and Transformation of Titanium Silicide
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V. Svilan,* J.M.E. 1larper, C. Cabral, Jr. and L.A. Clevenger, IBM Thomas .1.Watson Research Center, Yorktown tteights, NY 10598 *Massachusetts Institute of Technology, Cambridge, MA 02139 ABSTRACT
The evolution of stress vs. temperature was measured during the formation of TiSi 2 in the reaction of Ti with (100) Si and with polycrystalline Si, and during the phase transformation from C49 to C54 TiSi2. The formation of C49 TiSi 2 causes an increase in compressive stress, followed by relaxation before the transformation to C54 TiSi 2, which causes no significant stress change. C54 TiSi 2 is shown to be elastic in the temperature range of 750-860 'C. This difference in the deformation mechanisms of C49 and C54 TiSi 2 affects the morphological stability of TiSi2 in fine line structures. INTRODUCTION
Titanium disilicide is used for semiconductor device contacts and local interconnections. "l'iSi2 has two crystallographic phases, C49-TiSi 2, with a high resistivity (60-70 /A2cm), and C54-TiSi 2 , with a low resistivity (15-18 jtucm) which is needed in device applications. In the reaction of Ti with Si at 600-650 °C, the C49 phase forms first. Further annealing at temperatures of 700-800 'C is required to transform the C49 phase into the C54 phase. At these high temperatures, stresses induced by differences in thermal expansion may cause delamination, dislocation generation (1) or incomplete dopant activation (2). The purpose of this study is to quantify the evolution of stress vs. temperature during the reaction of Ti with Si. A significant difference in the deformation mechanisms of C49 and ('54 TiSi 2 was observed, which affects stress relaxation at high temperature. Stress vs. temperature was measured by Jongste et al. (3) during crystallization of C49 TiSi 2 from multilayers. An abrupt tensile stress increase occurred upon crystallization at about 300 'C, as has been observed in other silicides (4). Isothermal measurements showed relaxation from a tensile starting point with a time constant which decreased to several seconds as temperature was increased to 600 'C. It was proposed that a volume increase, hence a decrease in tensile stress, was caused by precipitation of excess Si in heating C49 TiSi2 . Chen et al. (5) measured stress vs. temperature during the reaction of Ti with (100) Si. They observed a stress change in the tensile direction as the C54 "fiSi2 phase was formed, and attributed this to grain growth in the C54 phase. Here, we show that the formation of C49 TiSi 2 causes a compressive stress increase, but is followed by stress relaxation before the transformation to C54 TiSi 2. The transformation causes no significant stress change, and the C54 phase remains elastic up to 860 'C. EXPERIMENTAL METHODS
The reaction of Ti with single crystal Si was studied using 125 mm diameter (100) Si wafers, and the reaction with polycrystalline Si was studied using 125 mm diameter oxidized (100) Si wafers coated with 200 nm undoped poly-Si. Wafer curvature was measured using a Flexus F2400S stress measurement system
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