Thermal annealing effects on the structural and electrical properties of HfO2/Al2O3 gate dielectric stacks grown by atomic layer deposition on Si substrates

Abstract

HfO 2 / Al 2 O 3 gate dielectric thin film stacks were deposited on Si wafers using the atomic layer deposition technique. A 3.3-nm-thick Al2O3 interlayer was grown at 400 °C using Al(CH3)3 and O3, and 2.5–3.5-nm-thick HfO2 films were grown at either 300 or 400 °C using HfCl4 and H2O. Thermal annealing of the dielectric film stack at temperatures ranging from 400 to 1000 °C under pure N2 atmosphere resulted in variation of the equivalent oxide thicknesses. The equivalent oxide thickness of the dielectric film stack showed a minimum after annealing at 650 °C irrespective of the HfO2 film growth temperature. High temperature (>800 °C) annealing induced the formation of SiO2 and intermixing between the HfO2 and Al2O3 layers, which resulted in an increase in the equivalent oxide thickness of the film stack. The structural changes in the stacked films as a function of the annealing temperature were compared with those of HfO2 and Al2O3 single layers. The film stack showed minimal hysteresis (<15 mV) behavior in the capacitance–voltage curve and a shift in flat-band voltage of 0.6–0.9 V by negative fixed charges at the Al2O3/SiO2 interface after annealing at temperature >500 °C. The variation in fixed charge density as a function of the annealing temperature was also investigated. A minimum equivalent oxide thickness of 1.3 nm with leakage current density of 8×10−6 A/cm2 at −1 V was obtained with the poly-Si electrode even after annealing at 1000 °C for 10 s. This leakage current density is seven orders of magnitude smaller than that of SiO2 with similar equivalent oxide thickness.