The Formation of Silicon-Rich Silicides
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THE FORMATION OF SILICON-RICH SILICIDES MARIA RONAY and R.G. SCHAD IBM Research Division, Thomas .1.Watson Research Center, Yorktown Heights, New York, 10598
ABSTRACT Diffusion studies of mono and bilaycrs of transition-metal films on silicon showed that the formation of r' -Cu 3 Si lowers the formation temperature of subsequently forming ReSi 2 by 400'C. This is due to the creation of a large amount of silicon selfinterstitials accompanying the formation of the copper silicide, which lowers the activation energy for silicon diffusion. The generalization of this result - stating that the formation of all silicides, in which the volume density of silicon is much larger than in elementary silicon injects silicon self-interstitials into the silicon lattice - gives new insight into silicide formation, silicide-enhanccd dopant diffusion and the self-diffusion of silicon itself 6 .
INTRODUCTION The most important goal in silicon integrated circuit technology today is to increase the density of circuits per unit silicon area in order to enhance their performance. High density multi-level metal interconnects and self-aligned silicide contacts between the first metal layer and silicon are key areas of development. Currently the most favored silicides are the two disilicides TiSi 2 and CoSi 2. Among the most important questions concerning silicide contacts is their formation temperature and the nature of the moving species. While it is known that transition-metal disilicides form by silicon diffusion' in a temperature range between 450'C and 750'C, such relatively low formation temperatures are difficult to reconcile with the large activation energy for the self-diffusion2 of silicon (4.1-5.1 eV). We observed that the formation temperature of disilicides correlates with the size of the transition metal atom, suggesting that transition-metal diffusion may be a precursor to silicon diffusion, and may lower the formation enthalpy of mobile defects in silicon. Transition metals diffuse interstitially in silicon with an activation energy as low as 0.43 eV for the small copper ion and as high as 1.5 eV for the large titanium atom 3 If a fast-diffusing transition-metal lowers the formation enthalpy of a silicon defect, then this should decrease the formation temperature of a subsequently forming silicide of a slow-diffusing transition-metal. In order to investigate this suggestion we chose copper for a fast-diffusing transition-metal, which in thin film reactions forms only Cu 3Si. For a slow-diffusing transition-metal we chose rhenium, which in thin film reactions forms only ReSi 2 (ref.4). What we want to find out is whether the formation of Cu 3Si will introduce mobile silicon defects in silicon and thereby lower the formation tempcrature of ReSi 2.
Mat. Res. Soc. Symp. Proc. Vol. 163. t1990 Materials Research Society
574
EXPERIMENTAL We used silicon [100] n-type wafers of 8-12 fOcm resistivity. Following standard cleaning the wafers were dipped for 30 seconds in buffered HF to remove any oxide layer prior to metal film de
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