The demand for accurate and highly reliable in-fab characterization of thin layers included in advanced CMOS and MEMS stacks has placed stringent requests on in-line optical metrology methodology and protocols. This work investigates the capability of systematic combination of X-ray reflectometry (XRR) and spectroscopic ellipsometry (SE) to decrease the correlation concerns that sometimes affect the reliability of SE analysis. (C) 2010 Elsevier B.V. All rights reserved.