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 [S.sub.4] of nulling angles can be less than 0.00025 deg.