Real-time in situ ellipsometry was used to investigate the etching of SiO2/silicon wafers with a high concentration of Cl-2. We monitored the temporal trajectory of the ellipsometric parameter Delta and then selected several points for ex situ study using atomic force microscopy (AFM). There was a clearly observable transition period in the trajectory near the endpoint of the SiO2/Si interface. We studied the relationship between the ellipsometric parameter Delta and the same point in the AFM ex situ measurements. Three stages, thin film, the interface layer, and the substrate, were analyzed in this work. (C) 2010 Elsevier B.V. All rights reserved.