An investigation of the work function of metal gate electrodes for advanced CMOS applications
Scaling the gate length and oxide thickness of the metal oxide semiconductor field effect transistor (MOSFET) offers great potential to improve device performance and circuit density. The use of metals for the gate electrode eliminates problems associated with conventional polycrystalline silicon, and shows better compatibility with high-k gate dielectrics. In order to optimize transistor performance, metals with appropriate work functions for both NMOS and PMOS must be identified and integrated into the conventional CMOS process flow.
In this work, both single metal and two-component metal gate systems were investigated to develop a fundamental understanding of the factors that influence the metal gate electrode work function. The work function of tungsten was found to be higher in evaporated electrodes than in sputter-deposited films, which may be related to differences in roughness, density, grain size and as-grown oxygen content observed by physical characterization.
In the two-component systems, metal gates were fabricated using a stacked bilayer structure. In previous work, it was found that the work function can be controlled by the thickness of the underlayer metal (the layer closest to the oxide). Depending on the amount of diffusion, the influence of the overlayer on the atomic concentration at the dielectric interface varies. Three two-component systems were investigated: Nb-W, Ti-W and Pt-Ti. By selecting materials systems that exhibit differences in diffusion and phase formation (as predicted by their phase diagrams), the change in work function due to underlayer thickness and composition for all three metal pairs is elucidated. The diffusion behavior in bilayer metal gates was investigated using x-ray reflectivity of multilayer films and was also modeled to quantify the differences between these three metal-metal systems. The effect of composition on the work function was directly probed by fabricating alloy metal gate electrodes by co-sputtering. A non-linear behavior was observed where the work function is dominated by the lower work function constituent. Structural information on the alloy films was obtained using x-ray diffraction. Thermal stability of all three systems was demonstrated during extended anneals and the observed time-dependent diffusion behavior is proposed to be related to diffusion through the grain boundaries.
0794: Materials science