Distortion Free Projection Lithography

Soft x-ray projection lithography (SXPL) may be used to fabricate high resolution structures for future devices, but will require an all-reflecting optical system with ~ 100 nm resolution and <10 nm image distortion over large fields-of-view. In present designs, the lithographic tool for SXPL is envisioned as a "ring-field" scanning system with multiple (3-5), possibly aspheric, imaging optics fabricated to ~ < 1 nm figure precision. In its present form, several technologies must be developed before this tool can become practical. A simple, non-scanning optical system with less expensive optics, reduced mirror reflection losses and lower source power requirements would be very attractive. We have developed a technique, called Encoded Mask Lithography (EML), which allows for distortion free, high resolution reticle replication over a large field-of-view while using an imaging system with substantial inherent distortion. When applied to SXPL, EML allows us to use a simple, two spherical mirror imaging system. The simplified optical system used in EML eases optic fabrication requirements, obviates the need for mask-to-wafer scanning, and decreases multilayer mirror reflection losses and source power requirements. Although developed for SXPL, this concept is applicable to all forms of projection lithography (i.e. ion beam projection lithography, e-beam projection lithography, etc.) where distortion over large fields may be a problem.