Ellipsometry and reflectometry at photosensitive surfaces

Abstract

The surface of a photodetector can be the site of reflectance and ellipsometric monitoring of deposited thin films. The detector surface is partially specularly reflecting and intercepts a polarization-modulated monochromatic beam at oblique incidence. In its simplest form, polarization modulation is obtained by a synchronously rotating polarizer in the incident beam, and the ac/dc signal ratio of the monitor (or witness) detector is recorded. This modulation depth determines the thickness of a growing thin film of known refractive index. If a second detector absorbs the reflected light and its modulation depth is measured, the refractive index and thickness of a transparent thin film can be determined. If the second detector is also specular and intercepts the light beam reflected from the monitor detector at oblique incidence and with a new plane of incidence, which is rotated (preferably by 45°) with respect to the first, a complete ellipsometer-reflectometer system is realized. In this case, the incident light beam is modulated by a rotating quarterwave retarder that follows a fixed polarizer. By Fourier analysis of the periodic output currents of the two detectors, the absolute reflectance, ellipsometric parameters, and photoelectric responsivity of the monitor detector are all determined. This permits more complete characterization of the deposited thin films.