Orthogonality measurement of cross-grating based on image processing of multi-order diffraction patterns

Cross-gratings or two-dimensional gratings have found applications in precision displacement measurement for nano-scale machine tools or mask-wafer alignment in photo-lithography. The orthogonality of displacement measurement or alignment in two perpendicular directions is assured by the orthogonality of the grating grooves. Once the grooves are not exactly normal to each other, it should be calibrated before measurement or alignment to guarantee the accuracy. Scanning probe microscopy (SPM) and optical diffractometry (OD) are commonly used for highly accurate calibration of cross-grating orthogonality. In this paper, two methods based on diffraction are proposed for middle accuracy measurement at relative higher speed. At first, we briefly explain the configuration of each method. The calibration results of cross-gratings are shown. We also discuss the results of uncertainty analysis and the cause of main uncertainty sources.