Patent ID: 6985237

Claim:
A method for determining layer thickness ranges of a plurality of layers (S i ) of a specimen, the method comprises the steps of: generating for each of the layers (S i ), the wavelength-dependent refractive index and absorption functions n i (λ) and k i (λ), as well as starting values (T i min,0 , T i 0 , T i max,0 ) for a minimum, nominal, and maximum layer thickness; measuring a reflection spectrum of the specimen in a wavelength range (λ min , λ max ) smoothing the reflection spectrum by diminishing noise caused predominantly by external influences and thereby generating a smoothed reflection spectrum; determining the number of extremes in the smoothed reflection spectrum, wherein two adjacent extremes, of which the one must be a minimum and the other a maximum, being evaluated only if they differ at least by a specified contrast criterion; determining the layer thickness ranges based on the number of extremes, such that for each layer (S i ) the thickness (T i ) is varied between (T i min,0 ) and (T i max,0 ) in steps having a predetermined increment (dT i ), a reflection spectrum being modeled within the wavelength range (λ min , λ max ), and the number of extremes is determined and stored and compared to the number of extremes in the smoothed reflection spectrum, specifying the wavelength range (λ min , λ max ) and the increments (dT i ) in self-consistent fashion by means of a sensitivity criterion, such that for each layer S i for a starting increment (dT i 0 ) and in a wavelength range (λ 0 min , λ 0 max ), a reflection modulation spectrum is modeled, the absolute value thereof is determined, and the resulting spectrum is then smoothed; limiting the wavelength range to a continuous range (λ i min , λ i max ), wherein (λ i min ) is the smallest and (λ i max ) is largest wavelength, for which the modeled spectrum still exhibits a value above or equal to a specified threshold value and the modeled spectrum substantially exhibits, for values between those wavelengths, values above the threshold value; selecting (λ min ) as the minimum of all (λ i min ), and selecting (λ max ) as the maximum of all (λ i max ); calculating the increments (dT i ) for the wavelength range (λ min , λ max ) and are in turn used to model a reflection modulation spectrum for each layer (S i ); and repeating until the wavelengths (λ min ) and (λ max ) change, between two repetitions, only by less than a specified self-consistency criterion, in which case the wavelengths (λ min ), (λ max ) and increments (dT i ) are used for the further method steps.