We review the characterization of optical properties of high‐K gate dielectric films and film stacks. Modern high‐K dielectrics typically incorporate an ultra‐thin oxide interfacial layer designed preserve the channel mobility. High‐K filmstack physical characterization data is presented and correlated to the optical response of the material determined via spectroscopic ellipsometry. In‐line measurement of film physical thickness relies on the optical models used for fitting the dielectric function. Optimal optical models for use in process control are discussed, as well as limitations of spectroscopic ellipsometry to characterize high‐K dielectric/interfacial oxide stacks. We also discuss the application of spectroscopic ellipsometry to characterize high‐K dielectrics on silicon‐on‐insulator substrates. © 2003 American Institute of Physics