An enhanced sensitivity null ellipsometry technique for studying films on substrates: Application to silicon nitride on gallium arsenide

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Abstract
General considerations are applied to optimize the sensitivity of ellipsometric measurements for thin films on a substrate. s‐ or p‐wave suppression conditions are found to give maximum sensitivity. Approximate values of the optical parameters of the films and substrate are used to calculate discrete film thicknesses for the s‐ or p‐wave suppression to occur. For null fixed‐wavelength ellipsometry, these calculations are limited to experimentally available wavelengths, e.g., at strong emission lines from a Hg lamp. Films with thicknesses near the calculated ones are then deposited on the substrate. The ellipsometric parameters ψ and Δ are obtained at multiple angles of incidence and wavelengths, and a least‐squares procedure is used for the analysis. The method has been applied to silicon nitride films on GaAs. The problem of correlation between the calculated optical parameters of the system is addressed. It is shown that the multiple‐wavelength analysis decreases significantly the correlations as compared to single‐wavelength analysis.