The time-temperature product during gate dielectric formation is very important in controlling the extent of vertical diffusion profiles in active devices as well as the extent of lateral diffusion in traditional isolation schemes. Rapid thermal oxidation (RTO) appears to be an attractive processes for use in the fabrication of ultrathin SiO2 films. Ultrathin film metrology requirements continue to increase as device features shrink below 0.35 mu m. Subangstrom 3 sigma-measurement performance on gate oxides is required for 64 Mbit to 1 Gbit dynamic random access memory and high performance microprocessors. Precision single wavelength ellipsometry (SWE) has been used to measure the values of Delta and Psi for ultrathin RTO films. We employ Fowler-Nordheim (FN) tunneling current oscillations as an independent method for determining ultrathin film thicknesses. Combining the Delta and Psi values from SWE and the thickness from FN, a value of the pseudorefractive index of 1.894 at a wavelength of 632.8 nm is used to determine oxide thickness in the ultrathin film regime. Multiple wavelength spectroscopic ellipsometry in the photon energy range of 2.0-5.0 eV using the Bruggeman effective medium approximation has shown that the best fit optical model is a two layer model : pure oxide and interface layer. A comparison of these techniques is discussed and presented in this article.