Effective Work Function Engineering of $\hbox{Ta}_{x}\hbox{C}_{y}$ Metal Gate on Hf-Based Dielectrics

This letter reports the engineering of effective work function (EWF) for tantalum carbide (TaC) metal gate on high-$k$ gate dielectrics. The dependence of EWF on Si concentration in HfSiO as well as nitridation techniques is revealed. The EWF was extracted by both terraced oxide and terraced high-$k$ techniques with the bulk and interface charges taken into account. The incorporation of Si in Hf-based dielectrics results in an increase of EWF, while the presence of N tends to decrease the EWF. Plasma nitridation is found to be more effective in lowering the EWF than a thermal nitridation. The phenomena can be explained by the modification of TaC/high-$k$ interface dipole moment, which arises from the electronegativity difference for various interface bonds. Based on the above findings, we proposed a novel approach to reduce the EWF of TaC on HfSiON by using a thin $\hbox{HfO}_{2}$ cap layer after optimizing the nitridation. The MOSFET results show that this technique is able to achieve a lower $V_{t}$ without degrading the device performance.