ABSTRACT
Extreme Ultraviolet Lithography (EUVL) is the favored technology by the IC in dustry for the 45 nm critical-dimension node and beyond [1]. Compared to lithog raphy in current production, EUVL represents a 10-times wavelength decrease to X = 13.4 nm, i.e., in the soft X-ray or extreme ultraviolet regime. Since all lens
materials absorb at this wavelength, EUVL is based on reflective optical surfaces, which are coated with an 81-layer Mo/Si quarter-wave stack for enhanced reflec tivity. Most of these surfaces are not on an image plane of the EUVL lithographic stepper, such as, for example, the condenser optics in the illuminator or the reduc ing optics after the reticle. These surfaces away from the image plane are not sen sitive to modest levels of particle contamination, provided that the flare, or total intensity scattered by the particles, is negligible. But the EUVL reticle, which is also reflective and coated with the Mo/Si multilayer, is imaged directly onto the wafer. Therefore, any particles in the reticle coating that are larger than the reso lution of the optical system will be imaged as well. For this reason, a coating process with unprecedented cleanliness is required to apply the multilayer coating to the reticle.
