Thermal stability of transition metal nitrides as NMOS gate electrodes

Refractory metals and their nitrides are being considered as gate electrodes for scaled CMOS devices. The advantages of metal gates over doped polysilicon include the reduction of parasitic gate depletion, the elimination of the need for doping of the electrode, the potential compatibility with high-k dielectrics and reduced resistivity of the electrode. Simulations of scaled devices indicate a dual metal approach is necessary to achieve low and symmetric threshold voltages while maintaining gate control over the channel. The development of an NMOS compatible metal gate is especially challenging since the metals with appropriate work functions (∼4eV) tend to be thermodynamically unstable on SiO₂ and promising high-K dielectrics.; The thermal stability of a refractory metal is improved with the incorporation of nitrogen. The effect of nitrogen on the thermal stability of the metal/dielectric interface was the focus of this work. Ta_{1 − x}N_x films were deposited via reactive sputtering. The nitrogen content of the electrode was varied by controlling the partial pressure of nitrogen in the sputtering ambient. The incorporation of nitrogen into the dielectric during gate electrode deposition and the redistribution of nitrogen during subsequent thermal treatments is expected to limit the use of metal nitrides as gate electrodes. Reactions in the Ta_{1 − x}N_x/SiO _{1 − y}N_y materials system were investigated through electrical and analytical characterization of MOS devices subjected to rapid thermal anneals at temperatures ranging from 400°C to 1000°C.