Asymmetric Properties of the Aerial Image in Extreme Ultraviolet Lithography

The extreme ultra violet lithography (EUVL) mask is reflective and has a very thick structure. When such a mask is illuminated obliquely, the diffracted waves occur asymmetrically. We have investigated the influence of asymmetric diffraction on the image properties by numerical simulation. We have shown that the shadowing caused by oblique illumination reduces the image intensity and deteriorates the telecentricity. It has been concluded that the desirable illumination angle is less than 6 deg and the desirable absorber thickness is less than 100 nm. We have shown that the diffraction pattern has a nonuniform intensity and have pointed out that the nonuniformity of the diffraction pattern must be considered in order to discuss telecentricity.