Design of phase-shift masks in extreme ultraviolet lithography

An attenuated phase-shift mask (att-PSM) and an alternating phase-shift mask (alt-PSM) were designed for extreme ultraviolet lithography (EUVL). For an att-PSM, a bilayer structure is newly designed. The bilayer structure provides appropriate attenuated reflectance by choosing thicknesses and ensures sufficient thickness latitude. For the alt-PSM, the additive structure is newly introduced. The new additive structure generates 180 degrees of phase shift and uniform reflectance simultaneously. The new alt-PSM is constituted of materials of TaN, Ru, Si and Mo that have been already made available in a binary mask. An appropriate combination of materials in the additive structure also gives a flat surface structure. The new PSMs enable the fabrication of smaller gates with lengths of 18 nm or below.