Highly sensitive detection technique of buried defects in extreme ultraviolet masks using at-wavelength scanning dark-field microscopy

Abstract

A technique to probe defects buried inside extreme ultraviolet (EUV) masks has been implemented using a dark-field microscopy detection setup. Specific samples have been fabricated to evaluate the sensitivity of this technique. They consist of silicon oxide gratings of a few nanometers height, coated with 40 layer pairs of molybdenum-silicon. We observed images with a good contrast on samples with defects as low as 3 nm. However, the imaging mechanism of scanning dark-field microscopy is not linear and can produce image distortions. Conditions of correct imaging have been analyzed, and simulations have been performed that show good agreement with the experimental data. This work opens the way for a better understanding of the capability of at-wavelength inspection technique for EUV mask

Rigorous Simulation of Line-Defects in Extreme UV Masks

“…Indeed, visible or even DUV inspections are only sensitive to the blank top surface topography. At wavelength inspection 13) could be necessary to track all kind of defects in the EUV masks. Moreover, following the trend observed in the DUV lithography, at wavelength aerial image microscope is likely to be used in addition to faster inspection tools.…”

“…This is illustrated in the two different situations for which corresponding near fields (intensity and phase) are shown in Figs. 13 and 14. A 40 nm circular seed is considered, covered by a 40-layer-pair mirror.…”