The W deposition step serves as the primary gap-fill process to form conductive contact plugs, bridging the underlying silicon source/drain or gate regions with the subsequent first metal layer (MET0) P4.Prior to this step, a Ti/TiN stack is deposited to provide an ohmic contact interface and act as a diffusion barrier P1.Without this conformal TiN barrier, the highly reactive tungsten hexafluoride (WF6) precursor used in W deposition would aggressively attack the underlying silicon or silicide,