Combining diffraction efficiency measurements with RCWA for high-precision grating characterization

The precise characterization of multilayer dielectric gratings is crucial to improve their performance in high-power laser systems. Scanning electron microscopy requires metallic coating over non-conducting materials to extract the profile information. In the case of optical profilers, the resolving power is limited by the microscope objective. Also, these techniques may require invasive procedures to extract key geometric information of fabricated grating like actual height and tooth angle, especially for high-aspect-ratio structures. In this work, we propose a non-destructive metrology technique that uses the data from experimental diffraction efficiency along with rigorous coupled-wave analysis (RCWA) to predict fabricated grating geometry without damaging it. Diffraction efficiencies were measured for different angles of incidence at a fixed wavelength of 1064nm, and tolerance analysis was conducted through the RCWA method. The contour plots are used to map diffraction efficiency as a function of grating geometries, and the solutions were identified by locating intersections in these contours, enabling accurate parameter estimation. The accuracy of the RCWA model is improved by incorporating the feedback of the multilayer thickness variation data from the ellipsometry analysis. We have coated 20 bilayers of dielectric material, with four additional layers of different thicknesses. Measurements were taken using an ellipsometer with an FTIR spectrometer in the 800–2500nm range, with a spectral resolution of 4cm⁻¹ at 70° angle of incidence. The grating height, tooth angle, and duty obtained from the proposed model are validated using atomic force microscopy with specialized high-aspect-ratio probes, confirming the reliability of the technique used. This approach provides a robust, non-invasive solution for characterizing high-aspect-ratio gratings, advancing metrology for precision optical devices in high-performance applications.