All projection optics in an EUV lithography (EUVL) system must be reflective and are coated with thin films consisting of alternating layers of materials termed “multilayers (MLs),” at a total thickness of about 280 nm. These coatings act as Bragg reflectors and are designed to operate at the EUVL wavelengths of illumination (centered at ∼13.4 to 13.5 nm) at near-normal-incidence angles. The ability to deposit such highly reflective ML coatings with extremely precise thickness control across the optic surface (in order to preserve the optic figure) has been the enabling technology for EUVL. Since ML coatings are essential, every EUVL projection system must be ML-compatible. The theoretical reflectance properties of a normal-incidence molybdenum-silicon (Mo/Si) ML suited for EUVL are shown in Fig. 4B.1.
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