Fast, Self-nulling Spectroscopic Ellipsometer : Instrumentation and Application

The advantages of rapid spectral scanning have been combined with -the inherent accuracy of a compensating ellipsometer operated in the polarizer-compensator-sample-analyzer configuration. Wavelength is varied over the visible-UV ( 3 7 0 7 2 0 nm) at a maximum rate of 114 nm/s by rotating a continuously-variable interference filter. A three-reflection Fresnel rhomb serves as the achromatic quarter-wave compensator. A microcomputer is used to collect spectroscopic measurements, perform instrument calibrations, digital filtering and interpret data. Wavelength-independent parameters of multiple-film optical models have been determined by treacing measurements of delta and psi at different wavelengths as independent measurements. Experimental and predicted ellipsometer measurements are compared by use of statistical techniques for the determination of optimum values and confidence limits of model parameters. I. OPTICAL COMPONENTS Extensive modification of a self-compensating ellipsometer built previously (1) ha: made it possible to combine rapid spectral scanning with rhe inherent accuracy ( 2 ) of compensating measurements. The ellipsometer employs the polarizer-compensatorsample-analyzer configuration. Faraday cells produce a magneto-optic rotation of Lhe plane of polarization which is addicive to the mechanical rotation of polarizer and analyzer prisms and is electronically controlled. Spectral scanning is achieved by rotating a continuously-variable interference filter, positioned between a white light-source (75 W, current stabilized highpressure Xe arc) and the collimating telescope of the polarizer. The maximum scan rate was 114 nm per second (1 pass through the spectral range every 3 seconds); Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19831011 C 10-38 JOURNAL DE PHYSIQUE lower noise levels can be achieved by use of a lower scan rate (23 nm per second or 1 pass through the spectral range every 15 seconds). As the filter revolves, the wavelength passed varies linearly from approximately 370 to 720 nm during the first half turn rotation, and then decreases to 370 nm during the second half turn of rotation. A digital incremental encoder is used to measure angular position and, thus, wavelength. The dependence of the specific rotation (effective Verdet coefficient, rotation per unit solenoid current) of the Faraday cells on wavelength has been determined experimentally. Rotation of the analyzer and polarizer azimuths is linearly proportional to solenoid current over the entire dynamic range of the Faraday cells, which is 18" at 700 nm and + 70" at 400 nm for the SF-6 glass cores. A three-reflection Fresnel rhomb with extremely Low axial skew (c0.01 deg., Continental Optical Co.) has been used as the achromatic quarter-wave compensator.