Two basic types of interferometer, a point diffraction interferometer (PDI) and a lateral shearing interferometer (LSI) suitable for operation in the extreme-ultraviolet (EUV) wavelength region, are described. To address the challenges of wavefront measurement with an accuracy of 0.1 nm rms, we present a calibration method for the PDI that places a mask with two large windows at the image plane of the illumination point light source and a general approach to deriving the phase-shift algorithm series that eliminates the undesired zeroth-order effect in the LSI. These approaches to improving the measurement accuracy were experimentally verified by the wavefront measurements of a Schwarzschild-type EUV projection lens.
Comparisons between several at-wavelength metrological methods are reported. The comparisons are performed by measuring one test optic with several kinds of measurement methods from the viewpoints of accuracy, precision and practicality. According to our investigation, we found that the PDI, the LDI, and the CGLSI are the most suitable methods for evaluating optics for EUV lithography.
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