Asymmetric properties of the aerial image in EUVL

In extreme ultraviolet lithography (EUVL), the reflective type mask is obliquely illuminated, and the oblique angle must be larger with increasing the NA of projection optics. EUV mask has the very thick structure. When such mask is illuminated obliquely, asymmetric diffraction waves occur and these waves make the asymmetric aerial image. This oblique illumination effect depends on the thickness of absorber and the illumination angle. We have been investigated this problem precisely by using the vector diffraction theory and optimized the parameters of the mask and the illumination system.