BACKGROUND

Metal-Oxide-Semiconductor (MOS) devices typically include metal gates, which are formed to solve poly-depletion effect in conventional polysilicon gates. The poly depletion effect occurs when the applied electrical fields sweep away carriers from gate regions close to gate dielectrics, forming depletion layers. In an n-doped polysilicon layer, the depletion layer includes ionized non-mobile donor sites, wherein in a p-doped polysilicon layer, the depletion layer includes ionized non-mobile acceptor sites. The depletion effect results in an increase in the effective gate dielectric thickness, making it more difficult for an inversion layer to be created at the surface of the semiconductor.

A metal gate may include a plurality of layers to meet the requirements of the NMOS devices and PMOS devices. The formation of metal gates typically involves depositing a plurality of metal layers, forming a filling metal region with tungsten, and then performing Chemical Mechanical Polish (CMP) to remove excess portions of the metal layers. The remaining portions of the metal layers form metal gates.

