Characterization of dual-periodic structures with optical scattering

Laser scattering method as a non-contact method provides an opportunity to real-time monitor the evolution of periodic nanostructures during fabrication, e.g. when the surface is irradiated with a focused ion beam. Using the method, the diffraction angle needs calibrating according to the grating equation. In this paper, we used scalar analysis to demonstrate the use of a dual-period structure for formation of dual spectral peaks at the 1st diffraction order. We then made use of numerical study based on rigorous coupled-wave analysis to optimize these Al-grating structures in terms of depth. It is found that dual-peak wavelengths can be selected by using different angles of incidence and low-loss reflection is obtained using an optimized structure. It is further proposed that these wavelengths can be used to determine the diffraction angle during fabrication without the need for pre-calibration of an optical scattering system.