Improvement of surface carrier mobility of HfO₂ MOSFETs by high-temperature forming gas annealing

The surface electron mobility of HfO₂ NMOSFETs with a polysilicon gate electrode was studied in terms of the effects of high-temperature forming gas (FG) annealing. The high-temperature FG annealing significantly improved the drive current or the surface electron mobility of the NMOSFETs. Improvements were also observed in the subthreshold swings and the C-V characteristics, indicating a reduction in interfacial state density (D_it). The D_it reduction was quantitatively confirmed by charge pumping current measurements. The mobility enhancement was achieved without degrading the equivalent oxide thickness (EOT) or gate leakage current. Different surface preparations, such as NH₃ or NO annealing, were explored to examine their effects on the NMOSFET performance. Mobility enhancement due to high-temperature FG annealing was also observed on these samples. Whereas NH₃ surface nitridation was effective in scaling EOT, the NO-annealed sample exhibited the highest mobility. Similar improvements were also observed on HfO₂ PMOSFETs, in terms of subthreshold swings, drive current, and surface hole mobility.