Comparative measurements on atomic layer deposited Al₂O ₃ thin films using ex situ table top and mapping ellipsometry, as well as X-ray and VUV reflectometry

In this study we compare the thicknesses and optical properties of atomic layer deposited (ALD) Al₂O₃ films measured using table top and mapping ellipsometry as well as X-ray and optical reflectometry. The thickness of the films is varied in the range of 1-50 nm. ALD samples are used as references with well-controlled composition and thickness, as well as with a good lateral homogeneity. The homogeneity is checked using mapping ellipsometry. Optical models of increasing complexity were developed to take into account both the top (surface roughness on the nanometer scale) and bottom interfaces (buried silicon oxide and interface roughness). The best ellipsometric model was the one using a single interface roughness layer. Since the techniques applied in this work do not measure in vacuum, organic surface contamination even in the sub-nanometer thickness range may cause an offset in the measured layer thicknesses that result in significant systematic errors. The amount of surface contamination is estimated by in situ reflectometry measurement during removal by UV radiation. Taking into account the surface contamination the total thicknesses determined by the different methods were consistent. The linearity of the total thickness with the number of atomic layer deposition cycles was good, with an offset of 1.5 nm, which is in good agreement with the sum of thicknesses of the interface layer, surface nanoroughness, and contamination layer.