Phase correcting layers in EUV imaging systems for microlithography

Abstract

The requirements on the shape accuracy of reflecting surfaces in a projection system for EUV (extreme UV) microlithography are very severe. We propose the use of thin correcting layers with a strongly reduced optical contrast in the wavelength region of interest. These thin layers can be applied with relatively large tolerances to tune the reflecting mirror surface to its required shape. The application of such phase-correcting layers is useful too when defects on the reflecting reticle (mask) need to be repaired.