Impact of metal gates on remote phonon scattering in titanium nitride/hafnium dioxide n-channel metal–oxide–semiconductor field effect transistors–low temperature electron mobility study

We report low temperature (40–300 K) electron mobility measurements on aggressively scaled [equivalent oxide thickness (EOT)=1 nm] n-channel metal–oxide–semiconductor field effect transistors (nMOSFETs) with HfO₂ gate dielectrics and metal gate electrodes (TiN). A comparison is made with conventional nMOSFETs containing HfO₂ with polycrystalline Si (poly-Si) gate electrodes. No substantial change in the temperature acceleration factor is observed when poly-Si is replaced with a metal gate, showing that soft optical phonons are not significantly screened by metal gates. A qualitative argument based on an analogy between remote phonon scattering and high-resolution electron energy-loss spectroscopy (HREELS) is provided to explain the underlying physics of the observed phenomenon. It is also shown that soft optical phonon scattering is strongly damped by thin SiO₂ interface layers, such that room temperature electron mobility values at EOT=1 nm become competitive with values measured in nMOSFETs with SiON gate dielectrics used in current high performance processors.