The role of oxygen in the development of Hf-base high-k/metal gate stacks for CMOS technologies

Abstract

The flatband and/or threshold voltage in HfO2/metal gate field effect transistors is found to dependent strongly on the annealing conditions used for device fabrication. This processing dependence arises in a large part from the formation of positively charged defects (oxygen vacancy) in the HfO2 layer during high temperature (≥ 500°C) process steps. At lower temperatures, oxygen stored in parts of the transistor structure such as the metal gate or residual oxygen in the ambient of processing tools can diffuse back into the HfO2 layer, having a profound effect on the threshold voltage by modulating the concentration of charged defects. In absence of any oxygen sources, the threshold voltage is dictated by the temperature alone suggesting a thermal equilibrium between the vacancy concentration in the HfO2 layer and Si or metal oxidation. The extreme sensitivity of Vt to the external oxygen pressure can be used to tune the threshold voltage of pFET devices in advanced CMOS technologies. ©The Electrochemical Society.