MULTIPLE MINIMA IN THE ELLIPSOMETRIC ERROR FUNCTION

A study of the multiple minima problem was performed for a simple case using simulations. The test case was static (usually ex-situ) spectro scopic ellipsometry of thick dielectric films on crystalline silicon s ubstrates taken at high sensitivity conditions near the s-wave antiref lection angle-of-incidence (AOI). Analysis of the data was performed u sing the Levenberg-Marquardt algorithm with the unbiased tan Psi and c os Delta estimator. A large number of minima were observed on both sid es of the main minimum, albeit with larger values of the error functio n. Moreover, data simulated at additional AOIs and/or at additional wa velengths showed an even higher number of minima. Three ways to overco me this problem are suggested: using another estimator, reducing the s ensitivity of the data or performing a grid search. (C) 1998 Elsevier Science S.A.