Design optimization of phase-shifting point diffraction interferometer

摘要

The phase-shifting point diffraction interferometer (PS/PDI) is so far the most accurate measurement tool in at-wavelength interferometry of projection optics for extreme ultraviolet lithography (EUVL). The complicate interrelationships between configuration parameters of PS/PDI call for an optimization to achieve high accuracy of PS/PDI. In this paper, a novel system-level modeling approach is proposed to optimize the parameters of PS/PDI designed for visible light (λ=632.8nm) concept proof experiment. The optimal reference pinhole size selection is performed by modeling pinhole spatial filtering effect using Diffraction-Based Beam Propagation (BPR) module of CODE V and in house software. The result shows that various pinhole diameters ranging from 1.6um to 2.2um should be used in our PS/PDI experiment. The test window size and grating duty cycle optimization, which is based on the spatial frequency domain analysis of PS/PDI, is conducted by modeling the entire PS/PDI system using Physical Optics Propagation (POP) module of Zemax and in house software. The optimal window size is approximately 62um for a given window-pinhole separation of 63.3um. The optimal duty cycle of grating is calculated to be 83% to obtain the maximum fringe contrast of 0.879.