1× Reflective X-Ray Optics

Abstract

Using soft X-rays with a wavelength of 13 nm, a diffraction limited projection system with an NA of about 0.09 would have a resolution of about 0.1 μm. The corresponding Rayleigh depth of focus would be about 1.6 μm. Several catoptric reduction designs for use in this regime have been proposed, but they are limited to very small field sizes, because of the difficulty of controlling geometric aberrations. In addition, central obscuration in some designs (e.g. the Schwarzschild), further reduces image modulation. The use of aspheric mirrors can improve the performance, but the figuring and in particular testing of such surfaces at these wavelengths are major challenges. In addition, the alignment of such an aspheric system is a far from trivial undertaking. An alternative approach is to adopt a unit magnification optical system, and make use of the inherent freedom from aberrations such a configuration can posess. Such an approach, using the Offner ring field design, has been proposed by Wood et al [1]. An objection that is often raised to 1× lithography in general is the perceived difficulty of making masks. It is shown that the assumptions customarily made to extrapolate 5× mask specifications down to the 1× level are fallacious. In particular, evidence is presented that error distributions do not, as is commonly assumed, lie on a Gaussian. Even more importantly, the errors are not distributed evenly over the mask: it is shown how this fact, in particular, greatly eases the supposed difficulty of making 1× masks.