Role of ultra-thin tungsten interlayer in blocking nitrogen diffusion and reducing specific contact resistivity in titanium/n⁺-Si ohmic contacts

In advanced Complementary Metal Oxide Semiconductor (CMOS) fabrication processes, titanium (Ti) and titanium nitride (TiN) are typically deposited sequentially to reduce the Schottky barrier height and to form Ohmic contacts in the source and drain regions. In this study, an ultrathin tungsten (W) layer was inserted between 50 Å Ti and 50 Å TiN films to attempt to reduce the specific contact resistivity (ρ_c). Compared to TiN/TiSi_x/n⁺-Si contacts, TiN/W/TiSi_x/n⁺-Si contacts exhibited a significantly lower ρ_c at annealing temperatures below 600 °C. The ultrathin W layer effectively acted as a diffusion barrier, preventing N in TiN penetrating into Ti. However, at higher annealing temperatures, the W barrier began to degrade, which leads to the diffusion of N into Ti layer. As a result, the deterioration of the ρ_c occurs. For silicidation at 550 °C/60 s, the ρ_c of the TiN/W/TiSi_x/n⁺-Si contact is 1 × 10^–8 Ω·cm², representing a 47% reduction compared to 2.09 × 10^–8 Ω·cm² for TiN/TiSi_x/n⁺-Si contact.