A Tuneable Metal Gate Work Function Using Solid State Diffusion of Nitrogen

Abstract

The work function of a metal gate electrode has been adjusted by the incorporation of nitrogen at the metaldielectric interface. The nitrogen was introduced using solid state diffusion from an over-stoichiometric TiN1+X layer. RBS measurements demonstrate that up to 35 at% nitrogen can be diffused into a 10nm Ta layer from the TiN layer and that the diffused concentration increases with anneal temperature. The effects of nitrogen incorporation upon the gate work function were studied for tantalum on Al2O3 and for molybdenum on SiO2 and on Al2O3. The work function of tantalum is slightly affected by the presence of the TiN exhibiting a change of -0.08±0.05eV, comparable to the work function change attributed to the thermal processing. Molybdenum is shown to be extremely sensitive with a work function change of -0.52±0.04eV for Mo/SiO2 and by -1.05±0.12eV for Mo/Al2O3. The absolute work functions for the Mo/SiO2 system are approaching the requirements of n- and p-MOSFET gate electrodes and there is evidence from the RBS measurements that there is a large process window for further optimisation.