Design of a scanning ellipsometer by synchronous rotation of the polarizer and analyzer

We have designed and constructed a new type of spectroscopic ellipsometer to study the optical properties of materials in the 3500-8000-Å wavelength range. In the system, the analyzer and polarizer are driven 10(4) steps/revolution by two stepping motors that have hollow shafts and rotate synchronously with a speed ratio of 2:1, i.e., A = 2P. Both the polarizer and analyzer are mounted directly on the shafts to avoid mechanical transmission and vibration problems entirely and make the system simple and reliable. An additional source polarizer was placed in the optical path to reduce the slight polarization effects of the light source. The light intensity finally received by the detector contained five components, one dc and four ac, with frequencies of ω(0), 2ω(0), 3ω(0), and 4ω(0), respectively. One can independently obtain the ellipsometric parameters of ψ and Δ as well as the optical constants by calculating any one of the two sets of ac signals, with a raw data self-consistency of better than 0.5%. The incident angle, aligned precisely by a laser beam, was continuously variable through a mechanical system with a computercontrolled resolution of 0.001° or a visual resolution of 0.005°. The system operations, including data acquisition and reduction, high-voltage control of the photomultiplier, incident angle, as well as wavelength setting and scanning, were fully and automatically controlled by a 386-based microcomputer. We self-calibrated the system by adjusting and setting precisely the initial azimuthal angles of the prisms. The results from the measured spectra of the complex refractive index for a gold-film sample are presented, and we show that the data obtained at three different incident angles of 65°, 70°, and 75° are remarkably consistent with one another. A comparison of the two results from the ellipsometry and reflectance measurements is given. The experimental skills and system error reduction are discussed in detail.