A high efficiency metrology for asymmetric grating structure based on scalar diffraction theory and intelligence optimization

Gratings, as important optical elements, are widely applied in the field of optical metrology as well as spectral purity. With the gradual decrease in the measuring wavelength to ensure high sensitivity, the scalar diffraction model has once again gained attention as a fast method for obtaining diffraction results and analyzing physical processes. This paper provides a detailed deduction of the reflected diffraction efficiency of an asymmetric trapezoid grating using the scalar diffraction theory, and demonstrates its validity and accuracy by comparing it with the vector diffraction theory. Building upon the scalar model, we further developed a rapid reconstruction model to determine the grating structure using an intelligence optimization method, namely an improved particle swarm optimization algorithm with random perturbation. The method eliminates the need for creating a large dataset for model training, while still providing a relatively accurate measurement result. It can be seen as a step towards narrowing the search range of feasible solutions, which significantly improves the efficiency of metrology.