High-resolution through-focus metric with two-pitch grating target

Through-focus focus-metric is a method that is used to analyze images on a fixed plane with the help of focus-metric values when the target moves through the focus. In this study, an optical microscope having a magnification of 250× is constructed. A two-pitch grating target is imaged on a CCD detector and through-focus image data are measured. In addition, the optical field on the CCD detector is simulated using the boundary element method and physical optics propagation. By comparing the measured focus-metric signatures with the simulated signatures, information regarding the linewidth is obtained. The results show that the through-focus focus-metric method is a very sensitive method that can be used for measuring parameters such as the linewidth. Thus, a nanoscale resolution can be achieved.