${\rm TiCl}_{4}$ Soak on NiSi: Selective Formation of Pure Sub-Nanometer NiTi to Reduce Contact Resistance

A titanium tetrachloride (TiCl₄) soak, which is a TiCl₄ gas flow without any radio frequency power, was used prior to the use of plasma-enhanced chemical vapor deposition (PECVD) using TiCl₄ at 450°C to form titanium and titanium nitride (Ti/TiN) as the contact-barrier films between nickel silicide (NiSi) and a tungsten plug. We found a 15% reduction in the contact resistance and its uniformity in the wafer was also improved compared with those without the TiCl₄ soak. The Ti/NiSi interface and the NiSi surface were characterized by using transmission electron microscopy along with electron energy loss spectroscopy, X-ray fluorescence, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy, where the selective formation of a pure nickel-titanium (NiTi) alloy on NiSi using a TiCl₄ soak was revealed. The results indicate that the TiCl₄ gas selectively decomposed on the NiSi surface, accompanied by the oxide removal on the NiSi and the formation of sub-nanometer-thick NiTi, although it does not kinetically decompose. The reduced contact-resistance regardless of the increase in the total thickness of the barriers was attributed to the improvement of the integrity at the NiTi/NiSi interface compared with the conventional scheme prepared by using chemical dry-cleaning and TiCl₄ PECVD.