Systematic study of pFET V/sub t/ with Hf-based gate stacks with poly-Si and FUSI gates

Abstract

The flatband/threshold voltages (V/sub fb//V/sub t/) in poly-Si gated pFETs with Hf-based gate dielectrics are shown to be set during poly-Si deposition and are found to remain virtually unchanged during gate implantation and activation, independent of the p-type dopant. The reaction of Si with HfO/sub 2/ at poly-Si deposition temperatures is identified as the root cause for the poor V/sub fb//V/sub t/ control. No improvement in V/sub t/ control is obtained by engineering physically closed Si/sub 3/N/sub 4/ barrier layers on HfO/sub 2/. It is furthermore shown for the first time that even when the gate is fully silicided (FUSI) large V/sub fb//V/sub t/ shifts are observed with HfO/sub 2/. Reduced pFET shifts are observed when Hf-silicates with low Hf content are used and further improvements are observed by using Al/sub 2/O/sub 3/ cap layers on silicates.