The Referenced Spectroscopic Ellipsometer (RSE) is an ellipsometer based reflectometer, designed for high speed thickness mapping in, e.g., quality control. It allows to accurately measure thicknesses from 0.1 nm - 10 µm. With 200 complete spectra recorded per second, a 100 mm x 100 mm area can be investigated in only 12 minutes while acquiring 67000 spectra.
The referenced spectroscopic ellipsometer combines the high sensitivity of an ellipsometer with the measurement speed of a reflectometer, and even exceeds it. In comparison to a laser ellipsometer, it includes the spectroscopic information between 450 and 900 nm. This is important in the event that more than one parameter of the processed layer is variable, for instance thickness and optical density. Basically, referenced methods are more sensitive than absolute methods. Therefore, the RSE method is superior to conventional ellipsometry when very thin layers are in focus. The advantage of increased sensitivity to thin films is even more evident when compared to reflectometry.
Ellipsometry is a very sensitive optical method which has been used for about a hundred years to derive information about surfaces. It makes use of the fact that the polarization state of light may change when the light beam is reflected from a surface. If the surface is covered by a thin film (or a stack of films), the entire optical system of film and substrate influences the change in polarization. Therefore, it is possible to deduce information about the film properties, especially the film thickness.
The RSE is a special type of ellipsometer, which compares the sample to a reference. In this way, the ellipsometric difference between sample and reference can be measured. Due to the orientation of the reference, none of the optical components needs to be moved or modulated during measurement, and the full high resolution spectrum can be obtained in a single-shot measurement. Thus, 200 spectra per second are acquired. The synchronized x-y stage enables acquisition of large field film thickness maps within a few minutes. As the reference compensated system is an ellipsometer, the measured data needs to be fitted to an optical model to obtain optical parameters like the complex refractive index and/or the film-thickness. To deal with the high data-rate, a look-up-table-fitting was implemented. Prior to the measurement, a look-up-table is calculated. The measured data can then be fitted in real-time and in high resolution.
