Alternating Phase Shift Mask in Extreme Ultra Violet Lithography

Pattern printability by alternating phase shift mask (alt-PSM) is discussed by simulation. The alt-PSM with an additive structure that provides a completely flat surface structure on a multilayer mask blank is considered. The additive structure is constituted of Si, Ru, TaN and Mo materials that have been generally used for binary masks in extreme ultraviolet lithography so far. The alt-PSM has sufficient capability to resolve an 18 nm line pattern on a wafer due to strong resolution enhancement. Pattern edge contrast of an aerial image by the alt-PSM becomes 1.5 times larger than that by the conventional binary mask. Depth of focus of the alt-PSM increases up to 300 nm that is three times larger than that of the binary mask. The mask error enhanced factor becomes small and changes gradually with defocusing. The alt-PSM enhances pattern edge contrast even for dense lines at a large angle of the off-axis incident light of 7.05 deg.