Nondestructive thickness determination of 2-layer ultrathin dielectric stacks was investigated with spectroscopic ellipsometry. The high-k dielectric HfO 2 film (∼40Å) was formed by e-beam evaporation followed by thermal oxidation at different temperatures. The interfacial layer, assumed to be SiO 2, was inevitably formed in this process. The ellipsometric ψ and Δ data for the 4-layer structure, air/HfO 2/SiO 2/Si, were accurately calculated and fitted to the experimental data acquired at incident angles 70° and 75° with the thickness of each dielectric layer as fitting parameters. The effect of optical constant on the fitted thickness was also taken into account. The ellipsometric thickness was examined with subsequent TEM, and the agreement between TEM and ellipsometry was excellent with an error of less than 10%. The ellipsometric thickness was also consistent with the thickness from C-V measurements.