2-CHANNEL SPECTROSCOPIC POLARIZATION MODULATION ELLIPSOMETRY - A NEW TECHNIQUE FOR THE ANALYSIS OF THIN SIO2-FILMS

A new spectroscopic ellipsometer is described, where the incoming light is dynamically polarized using a photoelastic modulator. and the reflected light is separated into orthogonally polarized beams using a Wollaston prism. Both beams are detected using photomultiplier tubes whose bias voltage is dynamically controlled for constant d.c. All three of the associated ellipsometry parameters (N = cos 2-psi, S = sin 2-psi sin-DELTA, C = sin 2-psi cos-DELTA) can be determined simultaneously in a single scan (240-840 nm or 5.16 1.47 eV). This instrument was used to study the optical properties of thin SiO2 films from 3 to 325 nm in thickness. Using a biased estimator fitting technique. the raw ellipsometric data can be fitted to an air-SiO2-interface-Si model, where the optical functions of the SiO2 layer and the interface region are approximated using one-term Sellmeier approximation, and a 50% void, 50% SiO2 Bruggeman effective medium approximation respectively. The refractive index of the SiO2 layer is dependent on film thickness, increasing with decreasing film thickness, but always being greater than that of fused quartz. The thickness of the interfacial region increases with increasing film thickness, being less than 0.2 nm for film thicknesses less than 20 nm.