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The impact of TiN capping layer on dual work functions of Ni, Co, and Co-Ni fully silicided (FUSI) metal gates was investigated. It was found that the TiN capping layer significantly altered the distribution of both n- and p-dopants during the FUSI process, which in turn changed the work functions of both As-doped and B-doped in the three FUSI metal gate systems. The work function tuning was found to have a linear relationship with the change of dopant level at the silicides/dielectric interface after adding TiN capping layer. The investigation of TiN capping layer on FUSI provided some insights on work function tuning mechanism in FUSI systems. This work also suggested a new methodology for optimizing the nMOS and pMOS work functions for CMOS device applications.