Abstract
A new null ellipsometer is described that uses photoelastic modulator (PEM). The phase modulation adds a good signal-to-noise ratio, high sensitivity, and linearity near null positions to the traditional high-precision nulling system. The ellipsometric angles Delta and psi are obtained by azimuth measurement of the analyzer and the polarizer-PEM system, for which the first and second harmonics of modulator frequency cross the zeros. We show that the null system is insensitive to ellipsometer misadjustment and component imperfections and modulator calibration is not needed. In addition, a fast ellipsometer mode for fine changes measurement of ellipsometric angles is proposed.
Highlights
Null ellipsometry [1,2,3] is a classical ellipsometric technique based on measurement of azimuth angles of polarizer, compensator, and analyzer for which the detected intensity is extinguished
Zone averaging, i.e. averaging of obtained ellipsometric angles for different sets of azimuth angles corresponding to nulls, gives measurement insensitive to azimuths misadjustment and polarizing component imperfections
A method with Faraday magnetooptical cells was proposed by Winterbottom [4] and realized afterwords [5,6,7] A reason why the azimuth modulation null ellipsometry has not been widespread and commercialized comes probably from relatively low modulation amplitude of the Faraday cells, interference from stray ac magnetic fields, and spurious reflections in the many-component system
Summary
Null ellipsometry [1,2,3] is a classical ellipsometric technique based on measurement of azimuth angles of polarizer, compensator, and analyzer for which the detected intensity is extinguished. Zone averaging, i.e. averaging of obtained ellipsometric angles for different sets of azimuth angles corresponding to nulls, gives measurement insensitive to azimuths misadjustment and polarizing component imperfections. Problems of this method come from a weak signal near nulls and parabolic intensity dependence near null during a polarizer scan due to the Malus’ law. The null ellipsometer with phase modulation takes advantages of both configurations: (i) very high precision, insensitivity to component adjustment, and measurement almost free of systematic errors characteristic for null ellipsometry and (ii) high sensitivity, strong linear signal near nulls, and lock-in detection as advantages of modulation ellipsometric methods.
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