Advances of EUV interferometry in EUVA

The high-accuracy wavefront metrology at wavelength is a key technology for assembling EWV projection optics, In EUVA, we have been developing the EUV Wavefront Metrology System (EWMS), which is an interferometer for a high NA, 6-mirror projection system. We are studying EUV interferometry by using the Experimental EUV Interferometer (EEI) to make the best choice of measurement methods for EWMS. Usually, the resolution and the capture range of measurement system contradict each other. For instance, Point Diffraction Interferometer (PDI) is expected to have high accuracy but its capture range of aberrations is rather narrow. While Lateral Shearing Interferometer (LSI) might have larger error factors than PDI, it has the ability to deal with larger aberrations. It is probably necessary to select an appropriate measurement method in accordance with the degree of the adjustment level of the projection optics system. LSI is most likely to be adopted into the EWMS because of its wide capture range of aberration and its easiness of alignment.