Thickness determination of ultra-thin SiO2 films on Si by spectroscopic ellipsometry

For ultra-thin SiO2 films with thicknesses Less than 100 Angstrom on Si, the accurate determination of the thickness and the index of refraction is a technologically important issue because of the strict tolerances required for the fabrication of sub-micrometer integrated circuit (IC) devices. Ellipsometry is traditionally and commonly employed to determine these quantities. In this study, we use spectroscopic ellipsometry (SE) to measure a set of device quality SiO2 films ranging from 45 Angstrom to 2000 Angstrom in thickness. We demonstrate that a variety of models for the oxide index of refraction together with different sets of the silicon substrate dielectric functions fit the measured SE data comparably well. We show, however, that the resulting variations (relative error) in derived thickness and refractive index of oxide films thinner than 100 Angstrom can be as large as 23% and 13%, respectively.