CMOS devices fabricated in a strained Si layer grown epitaxially on a strain‐relaxed SiGe virtual substrate exhibit enhanced carrier mobility compared to that of devices fabricated in bulk Si. We demonstrate that the thickness and strain state of the Si layer, and the alloy composition and strain state of the SiGe “virtual substrate” are critical parameters for process monitoring. The strengths and limitations of several characterization methods including x‐ray diffraction, Raman spectroscopy and spectroscopic ellipsometry for characterization of these layer structures are discussed. © 2003 American Institute of Physics