Design of Reflective Relay for Soft X-ray Lithography

Abstract

Optical lithography has demonstrated a resolution capability of about 0.35 micron. Systems using deep-ultraviolet light sources (e.g., excimer lasers) are anticipated to be able to resolve 0.25 micron features. For optical/UV sources, however, the resolution limit appears to be in the range of 0.15-0.25 microns, due to the upper limit of 1.0 on numerical aperture. One way to resolve smaller features, e.g. 0.1 micron, is to use soft x-ray sources. With such a source (with a wavelength, for example, of 13 nm), an optical system with a modest numerical aperture (e.g., 0.08) can be designed to provide resolution significantly better than is possible with optical/UV sources. An optical design which could be used in the formation of images over a 10x10 mm format in the soft x-ray spectral band is described in this paper.