Abstract

The path to high resolution leads to shorter wavelengths. In the extreme ultraviolet (EUV) spectral region, imaging systems are possible using normal incidence reflective optics. As the diffraction limit is reduced, the errors in the optical surfaces often limit performance. Figure errors with a spatial period ranging from the size of the mirror down to about a tenth of the mirror size limit the resolution. Finish errors or roughness on a shorter length scale leads to non-specular scattering. Scattering will reduce the throughput of an optical system by removing light from the image. In addition, light scattered within the field of view (flare) can reduce the contrast of the image. For a lithographic imaging system this will reduce the process latitude. As the wavelength of light is reduced, the total integrated scatter increases as 1/λ², which places a much more stringent demand on optical finish.

© 1998 Optical Society of America