We demonstrate that the work function of a metal gate can be varied by inserting a very thin metal layer ("metal A") between a thick metal ("metal B") and the gate dielectric. The flat band voltage (VFB) of the MOS (metal-oxide-semiconductor) capacitor structure can be controlled within the range bounded by metal A and metal B individually, as demonstrated with various stacked bi-metal layers. For continuous thin layers, we speculate that the work function tunability may be due to the drastic change of the electron density in the thin continuous metal layer in direct contact with a bulk metal. This drastic change of electron density results in a larger junction depth than that expected for a bulk metal. Non-uniform thin layers also appear effective for work function tuning as well, and the observed VFB shift is attributed to the metal island formation at the dielectric/metal A interface.
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