On the Impact of the Gate Metal Work-Function on the Charge Trapping Component of BTI

We investigate BTI charge trapping trends in high-k metal gate (HKMG) stacks with a variety of work function metals. Most BTI models suggest charge trapping in oxide defects is modulated by the applied oxide electric field, which controls the energy barrier for the capture process, irrespective of the metal work function. However, experimental data show enhanced or reduced charge trapping at constant oxide electric field for different work function metal stacks. We ascribe this to a different chemical interaction of the metal stack with the dielectric, yielding different defect profiles depending on the process thermal budget. Furthermore, by employing the imec/T.U. Wien physics-based BTI simulation framework “Comphy”, we also show that different metal work functions within a typical range of relevance (4.35-4.75eV) can yield a different charge state of the deep high-k defects, and can therefore have an impact on charge trapping kinetics during BTI stress, particularly in nMOSFETs.