A small and fast SCPEM-based ellipsometer

Abstract

We propose a small and fast ellipsometer with a basic layout similar to that of conventional ellipsometers using photo-elastic modulators (PEM) oscillating with 50 kHz. A conventional PEM is rather large, ~10×20×100mm, since it consists of one piece of glass and an actuator. Both parts are carefully adjusted to the desired frequency and then glued together. We replace such a standard modulator by a 127 kHz Single Crystal Photo-Elastic Modulator (SCPEM), a LiTaO₃-crystal with a size of 20.6×7.5×5mm. The polarization of light that travels through this crystal is strongly modulated. The modulated light is reflected from the sample, passes a polarizer and hits a detector. Its signal is split into the dc-value and the amplitudes of the 1st and 2nd harmonic of the modulation frequency. These values lead via simple formulas to the ellipsometric parameters. Usually a Lock-In-Amplifier is used here, whereas we propose an automated digital processing based on a fast analog to digital converter controlled by a highly flexible Field Programmable Gate Array (FPGA). This and the extremely compact and efficient polarization modulation allow fast ellipsometric measurements as needed in high volume manufacturing of optics.