We performed X-ray reflectivity (XRR) and spectroscopic ellipsometry (SE) analysis of several ONO stacks with different deposition conditions. TEM was also used as reference technique. In the energy range from 1.5 to 6.5 eV, SE analysis shows a correlation between bottom and top oxide thickness. The small electron density difference between Si and SiO2 makes the SiO2/ Si interface determination difficult to estimate by XRR. By etch profiling we observed a relatively large extension of the oxide/ nitride interfaces. Comparing the results of both techniques, we have observed major differences in thickness estimation. Most of these differences can be attributed to an estimation and weighting of the interfaces among the different techniques are considered in this paper. (C) 2003 Elsevier B.V. All rights reserved.