Physical and electrical properties of metal gate electrodes on HfO2 gate dielectrics

Abstract

As the metal–oxide–semiconductor field-effect transistor (MOSFET) gate lengths scale down to 50 nm and below, the expected increase in gate leakage will be countered by the use of a high dielectric constant (high-k) gate oxide. The series capacitance from polysilicon gate electrode depletion significantly reduces the gate capacitance as the dielectric thickness is scaled to 10 Å equivalent oxide thickness (EOT) or below. Metal gates promise to solve this problem and address other gate stack scaling concerns like boron penetration and elevated gate resistance. Extensive simulations have shown that the optimal gate work functions for the sub-50 nm channel lengths should be 0.2 eV below (above) the conduction (valence) band edge of silicon for n-type MOSFETs (p-type MOSFETs). This study summarizes the evaluations of TiN, Ta–Si–N, WN, TaN, TaSi, Ir, and IrO2 as candidate metals for dual-metal gate complementary metal–oxide semiconductor using HfO2 as the gate dielectric. The gate work function was determined by fabricating metal–oxide–semiconductor capacitors with varying dielectric thicknesses and different post-gate anneals. The metal–dielectric compatibility was studied by annealing the gate stacks at different temperatures. The gate stacks were characterized using transmission electron microscopy, secondary ion mass spectroscopy, Rutherford backscattering spectroscopy, atomic force microscopy, and x-ray diffraction. Based on work functions and thermal stability, Ta–Si–N and TaN show the most promise as metal electrodes for HfO2 n-MOSFETs.