The application of the multiple-angle incident ellipsometry to measure the ultrathin (< 100 A) oxide has been studied in this paper. First, four interfacial models are investigated by using a fitting scheme to fit ellipsometric data (h, ~) at various incident angles, and the abrupt model is found to be the most appropriate model to model the transition region of the SiO2-Si interface. The sensitivities on the incident angle and the errors induced by the ellipsometric parameter variations are also analyzed. Ellipsometry is applied to measure the native oxides of Si wafers after they are treated with different cleaning processes, and it is also applied to measure refractive indexes and thicknesses of ultrathin thermally grown SiO2. It is believed that these are the most accurately measured results on the refractive indexes of ultrathin oxides. Also, an empirical formula for thermal oxide growth in dry 02 is obtained.Thin dielectrics find many applications in metal oxide semiconductor (MOSFET) devices, especially as the gate insulators of MOSFET. As very large scale integrated (VLSI) technology continues to advance, the gate insulators, usually oxides, are getting thinner and thinner. It is predicted that, at the year of 2000, the thickness of the gate oxide will be 40 A (1). Much research work has been devoted to studying the preparation, physics, and device applications of thin dielectrics (2, 3). In these studies, it is important to measure precisely the thickness of thin dielectric. For example, one of the needs is to measure accurately the thickness of thin oxides in studying their oxidation kinetics, especially at the initial oxidation stage (4-6), in order to accurately model the growth process. Another example is that, in studying the carrier conduction mechanism in tunneling oxides for their applications in the electrically erasable programmable read-only memory (EEPROM), it is required to obtain the exact thickness of oxides in order to properly model the carrier conduction process, since several angstroms of error in the oxide thickness could lead to one order of magnitude deviation in calculating the I-V characteristics (7, 8).To measure the oxide thickness, ellipsometry is usually employed because of its simplicity, nondestructiveness, and easy sample preparation (9). As early as 1957, this method had been used to study the oxide growth kinetics of silicon and germanium when they were exposed to the room air (10). Besides measuring thickness, it can also measure the refractive index of the film. Today, ellipsometers, either the nulling type or the rotary type, have been manufactured as automatic instruments to characterize thin films and the measurement results can be obtained within 30 s for one measurement (9). However, when it is applied to measure ultrathin dielectric films, serious errors could occur due to measurement random errors and the instrumental systematic errors (11,12). It has been shown (11) that for the Si-SiO2 system, to measure SiO~ of the thicknesses less than 200 A, errors in the ob...