Fermi pinning-induced thermal instability of metal-gate work functions

The dependence of the metal-gate work function on the annealing temperature is experimentally studied. We observe increased Fermi-level pinning of the metal-gate work function with increased annealing temperature. This effect is more significant for SiO2 than for HfO2 gate dielectric. A metal-dielectric interface model that takes the role of extrinsic states into account is proposed to explain the work function thermal instability. This letter provides new understanding on work function control for metal-gate transistors and on metal-dielectric interfaces.