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Optical scatterometry has matured to become a routine technique in semiconductor and submicron metrology. Due to its rather simple instrumentation, the spectral approach is the preferred solution in submicrometrology. Several years ago, Fourier (backplane) scatterometry was proposed taking advantage of a strongly focused light beam in order to simultaneously illuminate the sample from various directions. The tiny focal spot enables to address a much smaller area of interest as compared to state-of-the-art OCD that operates with metrology boxes of 20 to 50 microns side length. We propose a scanning Coherent Fourier Scatterometry approach where the wave front diffracted by a non-periodic sample is recorded by means of a Shack-Hartmann sensor. It can be observed that measuring the wave front distortion rather than the intensity distribution is more sensitive to slightest profile (or scan position) variations. Several measurements on periodic as well as non-periodic sub-resolution patterns are presented. It is demonstrated that our approach is highly sensitive even when scanning a red laser spot across non-periodic profile features as small as 100 nm and below. In similarity to standard OCD, the sample profile has to be reconstructed by solving the inverse problem. To this end, we developed a rigorous model based on modal diffraction methods such as RCWA and C-method in combination with a physical optics ray tracing model. Our simulations show good agreement to the measurements. We believe that our approach may have the potential to help abandon the usage of reference patterns and instead pave the way to direct in-die measurement. Particularly, the measurement of non-periodic features will become possible by scanning the spot across the sample profile. The application of Zernike coefficients is suggested to reduce the complexity of the approach with regard to the solution of the inverse problem. First experiments show that only a few (mostly low order) coefficients are very sensitive whereas other show only small change during scanning.
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