With decreasing critical dimensions (CD) on lithography masks, increasing demands on CD metrology techniques come along. Already today the results of the three standard methods for CD measurements currently used, atomic force microscopy (AFM), scanning electron microscopy (SEM), and optical microscopy, typically do not yield the same results. This is because of, e.g. incomplete knowledge of the material parameters, insufficient modelling accuracy or-
especially in the case of optical microscopy-insufficient resolution. With decreasing CDs these systematic differences
increase. The need on new cross-calibration strategies arises.
Non-imaging metrology methods like scatterometry as non destructive, non diffraction limited, fast optical methods offer access to the geometrical parameters of periodic structures like e.g. top and bottom CD, pitch, side-wall angle, line height, or roughness. Therefore, these methods provide independently achieved additional information that can be used for cross-calibration.
At the PTB two scatterometers are in use (VIS @ λ=633 nm and EUV @ λ=13.55 nm). A third device (DUV @ λ=193 nm) is under construction. It will offer a wide spectrum of measuring principles like scatterometry, ellipsometric scatterometry, reflectometry, polarisation reflectometry, and using a broadband light source spectroscopic ellipsometry and spectroscopic reflectometry.
For simulation and modelling of the intensity distribution of the diffraction pattern two programs based on the rigorous
coupled-wave analysis (RCWA) method and a finite element method (FEM), respectively are used. The program features will be illustrated. A comparison of the simulations with the results achieved with the scatterometers on different types of lithography masks (chrome on glass, EUV-masks) will be presented as well.