Thermally Unstable Ruthenium Oxide Gate Electrodes in Metal/High-k Gate Stacks

Abstract

RuO2 films were investigated as a work-function-determining metal in metal/high-k gate stacks required for scaled complementary metal–oxide–semiconductor (CMOS) devices. After a low-thermal-budget processing below 400 °C, RuO2 on HfSiON exhibits a high effective work function of about 5.5 eV, which is applicable in p-channel field-effect transistors. However, RuO2 is easily reduced to Ru metal in metal/high-k gate stacks by annealing above 400 °C. Consequently, the high effective work function is unintentionally lowered. This is because RuO2 is thermodynamically unstable when in contact with W and polycrystalline silicon, which are both common components of a metal gate electrode structure. The insertion of TiN and TaN between RuO2 and W does not suppress the reduction. It is concluded that RuO2 is difficult to employ as a metal gate in metal/high-k gate stacks.