Methods for forming dual-metal gate CMOS transistors are described. An NMOS
and a PMOS active area of a semiconductor substrate are separated by isolation
regions. A metal layer is deposited over a gate dielectric layer in each active
area. Oxygen ions are implanted into the metal layer in one active area to form
an implanted metal layer which is oxidized to form a metal oxide layer. Thereafter,
the metal layer and the metal oxide layer are patterned to form a metal gate in
one active area and a metal oxide gate in the other active area wherein the active
area having the gate with the higher work function is the PMOS active area. Alternatively,
both gates may be metal oxide gates wherein the oxide concentrations of the two
gates differ. Alternatively, a dummy gate may be formed in each of the active areas
and covered with a dielectric layer. The dielectric layer is planarized thereby
exposing the dummy gates. The dummy gates are removed leaving gate openings to
the semiconductor substrate. A metal layer is deposited over a gate dielectric
layer within the gate openings to form metal gates. One or both of the gates are
oxygen implanted and oxidized. The PMOS gate has the higher work function.
