Studies of the interfacial layer in high-k/metal gate transistors

Abstract

HfO2 layers, 25-Å-thick, were grown by cyclic Hf sputter deposition and room temperature oxidation steps on chemically oxidized Si(001). Subsequent in situ annealing and TiN deposition yields a high- gate stack for which the original 8-Å-thick SiO2 layer is eliminated, as confirmed by transmission electron microscopy. Transistors fabricated with this gate stack achieve an equivalent oxide thickness in inversion Tinv = 9.7 Å, with a gate leakage Jg = 0.8 A/cm2. Devices fabricated without in-situ annealing of the HfO2 layer yield a Tinv which increases from 10.8 to 11.2 Å as the oxidation time during each HfO2 growth cycle increases from 10 to 120 s, also causing a decrease in Jg from 0.95 to 0.60 A/cm2, and an increase in the transistor threshold voltage from 272 to 294 mV. The in-situ annealed devices have a 1.5 Å (`10%) lower Tinv but 0.1 A/cm2 (~30%) higher gate leakage, as well as a 60 mV reduction in Vt . These positive effects are attributed mainly to the oxygen-deficiency of the as-deposited HfO2, which facilitates both the reduction of an interfacial SiO2 layer as well as a partial phase transition to a high- cubic or tetragonal HfO2 phase.