Interfaces in ALD very thin Al₂O₃/HfO₂ stacks studied by ellipsometry

Abstract

Stacks and laminates of high-k binary oxides as Al2O3/HfO2 deposited by Atomic Layer Deposition (ALD) recently are intensively investigated not only for optical applications and micro/nano- electronics devices but also for replacing the conventional single dielectric layers in charge trapping flash memories. The efforts are focused on low cycle numbers of HfO2 to Al2O3 ALD and the way the sublayers alternate as blocks. Although the growth of the both HfO2 and Al2O3 sublayers in ALD stacks is a result of fixed cycles and intended refractive index dispersions in combination whit fixed optical band gaps, the stack parameters frequently show some flexibility in regard to the thickness in case of low dimensions. Ellipsometry is a known optical method with powerful algorithms for experimental data interpretations. Here the ellipsometric data on very thin Al2O3/HfO2 stacks as active unites in charge trapping memory devices were processed by suitable algorithms in order to determine the individual thickness of HfO2 and Al2O3 sublayers, their variation in stack depth and deviations from the nominal thicknesses. Interface regions were determined in the stack beginning and in an initial growth of every block. Interfaces were identified as Al2O3-HfO2 mixtures with a changeable composition and thickness in the stack depth.