提出了一种新型激光外差干涉椭偏测量术用于实现纳米级精度的薄膜厚度测量。采用偏振光p和s分量透射比、反射比、反射相移、透射相移共同表征非偏振分光镜(NPBS)的退偏效应，建立了相应的误差模型，从而研究了多层介质膜NPBS的退偏效应和方位角对椭偏参数误差的影响。研究结果表明，由环境温度、入射角和光束偏振态的变化引起的NPBS退偏参数的漂移对椭偏测量精度影响很大，且无法通过标定来降低。为实现纳米级测量精度，NPBS的对准误差需要控制在0.1°以内。相对而言，用于合光的NPBS方位角误差对测量精度影响较大，NPBS所导致的膜厚测量总误差约为1.8～2.5 nm，说明NPBS是马赫曾德尔干涉式椭偏仪的一个不可忽视的误差源。

A novel heterodyne interferometric ellipsometer was investigated to measure the thicknesses of thin films in the nanometer-accuracy. The depolarization effect of a Nonpolarizing Beam Splitter (NPBS) was characterized by the transmission-induced-retardance (TIR) between p and s components of a polarizing beam, reflectance ratio, transmittance ratio and reflection-induced-retardance (RIR),then a corresponding error model was established. The influence of the depolarization effect and misorientation of a multi-layer dielectric NPBS on the errors of ellipsometric parameters was investigated. Experimental results indicate that the fluctuation of the depolarization effect caused by the environmental temperatures, incident angles and the change of polarizating state has a major impact on the measurement accuracy, and it can not be eliminated by calibration. In order to implement the nanometer-accuracy, the misorientation angle must be smaller than 0.1°. Furthermore, it shows that the NPBS orientation angle errors for recombination of the measurement light and the reference light have more important contribution than another ones to the measurement accuracy. The thickness measurement error of thin film induced by NPBS is about 1.8-2.5 nm，which means that the NPBS is an assignable error source in the Mach-Zehnder interferometric ellipsometer.