XUV Projection Lithography System Design Based on Single-Surface Reflecting Optics*

Optical projection lithography using exposure wavelengths less than 100 nm is being developed to produce integrated circuits with feature sizes less than 0.2 µm while providing a total depth of focus (DOF) of ~1 µm. With such short wavelengths, all-reflective projection systems with reflective masks will be required. Since six to seven reflections at normal incidence will be necessary to attain large, diffraction-limited images ≥1 cm 2 , high mirror reflectance is very important for future high- volume production. As a result, present attempts to develop soft-x-ray projection lithography are focused mainly around 13 nm [1-3] where relatively high reflectance ~60% has been attained with Mo/Si multilayer mirrors.