Patent ID: 7307723

Claim:
A method for optical characterization of at least one layer of material in an interval A of values taken by a function α of an optical wavelength λ, when λ varies in an interval of wavelengths, the at least one layer being created on a substrate, the method comprising: 1) carrying out a set of reflectometry and/or ellipsometry measurements over the interval A with ellipsometric and/or reflectometric devices and a spectrometer, the set of measurements leading to a measured spectrum, marked ψ, and choosing methods for calculating associated with a nature of the measurements and with a type of layer to be characterized; 2) choosing m initial values α 1 . . . α m of the function α, belonging to the interval A, m being a whole number at least equal to 1, and defining an interval B as being the set of points α of the interval ranging from the smallest to the biggest number among α 1 . . . , α m , when m is greater than 1, and as being the interval A when m equals 1; 3) choosing m complex initial values of a complex refraction index n*=n+jk for the m points α i , i ranging from 1 to m; 4) when m is not 1, choosing an interpolation law that allows calculating the refraction index n(α) of the material over the interval B, from the points (α i , n i ), with n i =n(α i ), i ranging from 1 to m, and when m equals 1, n(α) is taken equal to the number n 1 (α 1 ) over the entire interval B; 5) choosing M variable parameters, M being less than or equal to 2 m+1; 6) choosing an error function Er (ψ, ψ ) that characterizes the difference between a measured spectrum ψ and a theoretical spectrum ψ ; 7) using a minimizing function of Er (ψ, ψ ) with M parameter, performing: a) by applying the interpolation law of (α i , n i ) over the interval B, deducing n(α), α belonging to B; b) by using n(α) and the thickness ε of the layer, and methods for calculating spectrums, calculating a theoretical spectrum ψ (n(α),ε); c) comparing ψ and ψ by using Er(ψ, ψ ) and, if Er(ψ, ψ ) is less than a predetermined value e , or is minimal, going to sub-step e), otherwise going to sub-step d); d) making the M variable parameters vary so as to tend to the minimum of Er(ψ, ψ ), and returning to sub-step a); e) if Er(ψ, ψ ) is less than e, then obtaining a set of M variable parameters, for which Er(ψ, ψ (n(α,M),ε)) is minimal and the refraction index is then taken equal to the last one obtained, and if Er(ψ, ψ ) is greater or equal to e going to step 8); 8) increasing the number m of initial values of the function α and returning to step 2).