Fermi-level pinning induced thermal instability in the effective work function of TaN in TaN/SiO2 gate stack

In this letter, we demonstrate for the first time that the Fermi-level pinning caused by the formation of Ta(N)-Si bonds at the TaN/SiO₂ interface is responsible for the thermal instability of the effective work function of TaN in TaN/SiO₂ devices after high temperature rapid thermal annealing (RTA). Because of weak charge transfer between Hf and Ta(N) and hence negligible pinning effect at the TaN/HfO₂ interface, the effective work function of TaN is significantly more thermally stable on HfO₂ than on SiO₂ dielectric during RTA. This finding provides a guideline for the work function tuning and the integration of metal gate with high-κ dielectric for advanced CMOS devices.