Method for the determination of layer thicknesses

A method for the determination of layer thicknesses and optical parameters of a number (N) of layers of a specimen, the method comprising:

• introducing the specimen into a measurement arrangement and measuring the reflectance spectrum of the specimen in a defined wavelength region;
• smoothing the measured reflectance spectrum by a reduction equivalent to noise caused predominantly by external influences;
• selecting a set (S1) of a number (M) of wavelengths (λ1,i), where i=1, . . . , M, arranged in order of size, each wavelength (λ1,i) in the set (S1) corresponding to a respective local extreme in the smoothed reflectance spectrum, and the selection being performed under the condition that two adjacent extremes differ by at least one defined contrast criterion, and that one of the two extremes is a minimum and the other a maximum;
• adapting a modeled reflectance spectrum in stepwise fashion to the smoothed reflectance spectrum for the number (N) of layers, using a model, in which layer thicknesses, or layer thicknesses and optical parameters, are defined as variable magnitudes, in each adaptation step
• selecting a set (S2) of a number (M) of wavelengths (λ2,j), where j=1, . . . , M, arranged in order in the same fashion as in the set (S1), each wavelength (λ2,j) in the set (S2) corresponding respectively to a local extreme in the modeled reflectance spectrum, and the selection being performed under the condition that of two adjacent extremes, the one is a minimum and the other a maximum;
• determining an optimization criterion, the best adaptation corresponding to a minimum of the optimization criterion so that the actual layer thickness can substantially be determined;
• and
• determining the optimization criterion by the totality of the absolute values of the wavelength differences of all pairs of wavelengths (λ1,i, λ2,i), where i=1, . . . , M.