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Scatterometry, the analysis of light diffracted from a periodic structure, is a versatile metrology tool for characterizing
periodic surface structures, regarding the critical dimension (CD) and other properties of the surface
profile. For extreme ultraviolet (EUV) masks, only EUV radiation provides direct information on the mask
performance comparable to the operating regime in an EUV lithography tool. With respect to the small feature
dimensions on EUV masks, the short wavelength of EUV is also advantageous since it provides a large number of
diffraction orders from the periodic structures irradiated. We present measurements at a prototype EUV mask
with large fields of periodic lines-space structures using an EUV reflectometer at the Berlin storage ring BESSY
II and discuss the corresponding reconstruction results with respect to their measurement uncertainties. As a
non-imaging indirect optical method scatterometry requires the solution of the inverse problem, i.e., the determination
of the geometry parameters describing the surface profile from the measured light diffraction patterns.
In the time-harmonic case the numerical simulation of the diffraction process for periodic 2D structures can be
realized by the finite element solution of the two-dimensional Helmholtz equation. Restricting the solutions to
a class of surface profiles and fixing the set of measurements, the inverse problem can be formulated as a nonlinear
operator equation in Euclidean space. The operator maps the profile parameters to special efficiencies of
diffracted plane wave modes. We employ a Gauss-Newton type iterative method to solve this operator equation,
i.e., we minimize the deviation of the calculated efficiencies from the measured ones by variation of the geometry
parameters. The uncertainties of the reconstructed geometry parameters depend on the uncertainties of the
input data and can be estimated by statistical methods like Monte Carlo or the covariance method applied to
the reconstruction algorithm. The input data of the reconstruction are very complex, i.e., they consists not only
of the measured efficiencies, but furthermore of fixed and presumed model parameters such as the widths of the
layers in the Mo/Si multilayer mirror beneath the line-space structure. Beside the impact of the uncertainties on
the measured efficiencies, we analyze the influence of deviations in the thickness and periodicity of the multilayer
stack on the measurement uncertainties of the critical dimensions.
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