A Standard Reference Material (SRM) has been designed and fabricated and will be certified for thickness and refractive index using a highly accurate ellipsometer. The SRM consists of a three-inch diameter silicon wafer with a silicon dioxide film of uniform thickness. The design and preparation of the SRM are discussed and the ellipsometric measurement results and their comparisons with stylus profilometry are presented, along with the precision of the measurements. The ellipsometric accuracy depends upon the wafer oxide film, the model that represents the film-interface-substrate system, and the methods used to make the measurements. When the optical thickness, as determined by the ellipsometer, is compared with the mechanical thickness, as determined by the stylus profilometer, both the correct sample preparation and correct model are important in order to obtain high accuracy. This SRM will be available initially in three nominal oxide film thicknesses of 50, 100, and 200 nm. The SRM can be used to calibrate many different optical and mechanical thickness monitoring instruments as well as ellipsometers for which it was specifically designed.