Al2O3/HfO2 Multilayer High‐k Dielectric Stacks for Charge Trapping Flash Memories

Dielectric and electrical properties of Al2O3/HfO2 multilayer stacks deposited by atomic layer deposition (ALD) in dependence on the thickness of Al2О3 and HfO2 layers and the post‐deposition annealing (PDA) in different ambient (O2, N2, air) are investigated in terms of their application as trapping layers in emerging charge‐trapping non‐volatile flash memories. Differentiation is made between different processes giving rise to hysteresis effects: trapping of electrons and holes under positive and negative gate bias, respectively (as useful processes which define the memory window), and the generation of positive charge under high electric field stress (which degrades the stacks and results in permanent damage and ultimately in breakdown). Dependence of these processes on HfO2 and Al2O3 thickness as well as annealing ambient is analyzed. It is established that electrically active defects and trapping phenomena are most strongly affected by the annealing ambient. Oxygen annealing is favorable in terms of charge storage ability as it increases electron trapping and improves the electric field stress stability of Al2О3/HfO2 multilayer stacks. The charge trapping properties of nanolaminated HfO2/Al2O3 stack structures are presented in terms of their application in future non‐volatile flash memories. The dependence of the memory window on HfO2 and Al2O3 thickness as well as on the annealing in different ambients (air, N2, and O2) is analyzed. The achievable memory windows are significantly enhanced by a post‐deposition O2 treatment.