High-precision system for automatic null ellipsometric measurement.

A system for automatic null ellipsometric measurement, which uses a new kind of achromatic rhomb-type lambda/4 retarder, is designed and constructed. By use of terbium glass as the modulator's material, a magneto-optical modulator and its driving circuit are constructed. The polarized light is modulated by dc and ac. Combined with an ultra-low noise frequency-selective amplifier and a lock-in amplifier, the null time of the mechanical and photoelectric automatic null system can be less than 15 s. After the error of the mechanical parts is excluded, the repeatability standard deviation S4 of nulling angles can be less than 0.00025 deg.