Theoretical And Experimental Study On The Heterodyne Interferometric Ellipsometry

The films with thickness of nanometers have wide applications in the fields of microelectronics, materials and chemical engineering, optical and laser technology. For aiming at the disadvantages of spectroscopic ellipsometers and the requirements of in-situ and real time measurement, in this dissertation the heterodyne inteferometric ellipsometry are studied systematically. The experimental system consists of heterodyne light source, frequency stabilization system, heterodyne signal processing, phase-differences measurement and the design of optical system, the analysis of nonlinear frequency mixing error are attached more importance. The experimental results fit well with the theoretical analysis.For the first time, the application of heterodyne interferometric ellipsometry to nanometer films'measurement is studied systematically at the base of fundamental ellipsometry equations and the heterodyne interferometry with high anti-interference performance. For a specific model of nanometer films, the variation rules of the complex sensitivities of ellipsometric parameters are analyzed and calculated theoretically. For reaching nanometer accuracy, the measurement requirements of the amplitudes and phase differences of heterodyne signals are given.By adopting longitudinal Zeeman laser and two acousto-optical modulators respectively, the optical system of reflection and transmission ellipsometers are designed and analyzed by Jones Vector method. It shows the frequency-separation and common-path configuration is helpful to improve the anti-interference performance. Without any rotational mechanical parts, the measurement system is expected to operate at higher speed and stability.Laser frequency stabilization is realized by light intensity comparison, thermal compensated method and analogue PID controller, the longitudinal Zeeman laser located in weak magnetic field outputs a beat signal with 40KHz. The resolution of phase detection reaches to 0.12°by using direct comparison and counting method. The analog signal processing, computer interface and the programs of ellipsometry equation's inversion are also built up. The films measurement with nanometer accuracy by heterodyne interferometric ellipsometry is realized for the first time under the disturbed environment without constant temperature control. The influence of the errors of incident angle, beat frequency's stability, phase measuring technique and the environmental disturb on measurement are studied.By using Jones Vector method, the mechanism of nonlinear frequency mixing error and its influence on measurement accuracy are studied synthetically, it is mainly caused by the imperfection of polarizing beam splitters (PBS), the elliptical polarization and non-orthogonality of light beams. As the main obstacle to the practical application of the heterodyne interferometric ellipsometry, it results in an error up to several nanometers of the thickness measurement of thin films. Comparably speaking, the ellipsometric parameterΔshould have the priority for the inversion algorithm because |ρ| is affect greatly by the errors. The laser source and wave plate with high quality are prerequisite to reach nano-accuracy because the elliptical polarization and non-orthogonality have major effect on the measurement results. The definitions of the error evaluation-factors are defined to pre-estimate the error magnitude of ellipsometric parameters. The analysis results are used as foundation for optimum design of the heterodyne interferometric ellipsometer.