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This paper presents Finite Difference Time Domain (FDTD) method based on discretised Maxwell curl equations and
widely used in microwave circuit design - as a promising tool for new optical metrology purposes. We focus on periodic
FDTD formulations for scattering problems. The interest in efficient full-wave modelling of periodic structures has
arisen due to their increasing applications as slow wave transmission lines, photonic crystals, and metamaterials.
Recently, new efforts have been made to incorporate the FDTD algorithms into the scatterometry overlay technology
(SCOL) toolkit. In SCOL, multilayered grating targets on silicon wafers are illuminated with polarised light at a
particular angle of incidence; reflected signal of the 0th diffraction order is processed to extract the information about
misalignment between grating layers. Since the illumination spot size typically covers tens or even hundreds of grating
periods, direct 3D FDTD modelling of such an electrically large problem needs long computing times. The periodic
FDTD algorithm discussed herein, built upon Floquet theorem, allows reduction of the modelling problem to one or just
a few periods. As a consequence, it substantially speeds up the simulation. The incident wave is modelled as a plane
wave. The reflected wave is extracted via near-to-far (NTF) transformation as in antenna analysis. We cross-calibrate the
FDTD algorithm against other numerical techniques better established in optical metrology, like Rigorous Coupled
Wave Analysis (RCWA). For a benchmark of multilayered rectangular grating composition illuminated with light within
the 500 to 700 nm spectrum, we show that the FDTD and RCWA results for the 0th diffraction order reflection
coefficient are in excellent agreement. The FDTD approach is more flexible as it further allows quantitative
characterisation of non-rectangular periodic structures, higher-order diffraction rays, and periodicity violation. This work
was done in the framework of the SOCOT Consortium [18], sponsored by the European Commission under the IST 6th
Framework Programme, Contract No. 016403.
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