Comparison of FDTD excitation models for scatterometry of periodic reticles

Complex looped FDTD algorithms originally proposed for periodic waveguiding structures are adapted for scatterometry applications. To this end, two mechanisms of launching the plane wave illumination and extracting the scattering parameters are proposed. The first one is based on a classical connecting surface, imposed however on both real and imaginary FDTD grids to generate a single propagating plane wave. It is operated jointly with the near-to-far transformation in the frequency domain. The other mechanism uses the concept of waveguide mode templates and S-parameter extraction procedures borrowed from microwave circuit analysis. Results from both techniques are mutually consistent, and have also been validated against the rigorous coupled wave analysis for simplified scenarios. Both techniques may be implemented into other periodic FDTD formulations.