Abstract
This paper describes a mask defect inspection system using 13.5 nm light for extreme ultraviolet lithography (EUVL). The Schwarzschild optics which is employed as a microscope optics has a numerical aperture (NA) of 0.3 and a magnification of 30. Furthermore, it has a potential of detecting defects as small as 22 nm on a mask. In order to inspect defects with the phase change induced by swelling of the multilayer, a Mirau interferometer is employed. It is developed that the performance of the optical system, the focal position detection mechanism for image detection while scanning, and the driving mechanism of the reference mirror for the Mirau interference. Since the accuracy of the driving mechanism of the reference was found to be 0.1 nm, enough performance of the ring-shaped piezo actuator for interference measurement was confirmed. Moreover, examples of the mask inspection by this system are shown.
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