Work function control at metal–oxide interfaces in CMOS

Metal gate electrodes will replace poly-Si gates to reduce the depletion width contribution to the gate capacitance of MOSFETs. However, the choice of suitable gate metals is problematic. To understand the factors affecting the work function, we have calculated the Schottky barrier heights (SBH) of a range of metals at different configurations on HfO2 (1 0 0) and (1 1 0) surfaces. On (1 0 0) surfaces, different O-terminations are found to be able to shift the SBH by up to 1 eV. Metals of different work function from Hf to Au are found to be able to shift the SBH by over 1 eV. This is a key conclusion, which contrasts with the ‘Fermi level pinning’ on HfO2 found for some groups such as Schaeffer et al. On the non-polar (1 1 0) surface, the low work function metals like Al and Hf are found to bond to O sites, whereas high work function metals like Au bond to both Hf and O sites. Thus, many factors such a termination and stoichiometry control the SBH and the effective work function of a metal.