Shallow trench isolation (STI) has emerged as one of the primary techniques for device isolation in complementary metal oxide semiconductor technologies. This device isolation technology has become extremely important to satisfy the high-density requirements of modern integrated circuits. It is of great importance to measure the critical dimensions and sidewall features of the STI structure. Currently used critical-dimension scanning electron microscopes (CD-SEMs) cannot identify the rounding typically present at the bottom or top of the profile, and it is difficult to differentiate between the top and bottom line-width values. Cross-sectional SEMs that can give the profile information require the destruction of the wafer. In this article, we present the measurement of STI by using the optical critical dimension (OCD) technique. This technique measures line or trench profiles using normal incidence polarized reflectometry with sensitivity to sub-50 nm grating lines. In the OCD technique, a broadband polarized light beam is focused onto the grating surface, and the reflected zeroth order is measured as a function of wavelength. The data obtained by measuring the grating structure give a signature of the profile, which is analyzed in real time using rigorous coupled wave analysis. Since the data are fitted in real time, there is no requirement for library generation, which makes the analysis simpler and easier to extend to other structures without the need for lengthy regeneration of a new library of profile data. Data from STI wafers before and after oxide fill will be presented. Both simulation and experimental results show great sensitivity to features such as CD, depth, sidewall angle, as well as the quality of oxide fill. (C) 2004 American Vacuum Society.