Thermal Stability of Transition Metal Nitrides as NMOS Gate Electrodes

Abstract

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.

This work focused on the impact of nitrogen on the Ta1-xNx/SiO2 system following rapid thermal anneals up to 1000°C. It was determined that the reaction in the Ta1-xNx/SiO2 system is fundamentally different than in the Ta/SiO2 system. Increasing the nitrogen content of the gate electrode improved the thermal stability of the metal/dielectric interface but did not change the work function of the electrode. The nitrogen content of the electrode was varied by controlling the partial pressure of nitrogen during the reactive sputtering of the films. 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 Ta1-xNx/SiO1-yNy materials system were investigated through electrical and analytical characterization of MOS devices subjected to rapid thermal anneals.