Dual-metal-gate transistors with symmetrical threshold voltages using work-function-tuned Ta/Mo bilayer metal gates

We have investigated dual-work-function metal gates based on Mo and stacked Ta/Mo fabricated by an interdiffusion method that does not require a metal etching process from the gate dielectrics. The diffusion of Ta into the Mo layer provides a markedly reduced work function for the Ta/Mo gate (4.42 eV) in comparison with that for the single Mo gate (5.04 eV). The metal-oxide- semiconductor field-effect transistors (MOSFETs) with the Mo and Ta/Mo gates on 4-nm-thick SiO₂ were fabricated by the gate-first process with dopant activation annealing at 850°C. The Ta/Mo-gate n-MOSFET with the reduced gate work function and the Mo-gate p-MOSFET exhibited almost symmetrical threshold voltages (+0.53/-0.45 V). It was also confirmed that the Ta/Mo interdiffusion process did not degrade the gate dielectric integrity and electron mobility in contrast to the case of the single Mo gate.