Refractive X-ray optics: towards wavelengths of 13.5 nm and beyond

Abstract

In this work we analyze the possibility of using refractive lenses for X-ray lithography in the wavelength range λ=2-20 nm, where the absorption of radiation in the lens material has a significant impact on the image quality. The general scheme of image formation in projection refractive X-ray optics using a composite objective lens consisting of biconcave elementary lenses is considered. By means of the analytical solution and finite-element simulation model, possible ways to obtain the lens transmission function taking into account the absorption of X-rays were investigated. A lens based on a beryllium (Be) thin film with a complex refractive index n=0.989+0.0015i was chosen to estimate its optical properties at a wavelength of 13.5 nm. Two mechanisms of blurring of the X-ray beam at the lens focus were studied: a mechanism associated with X-ray diffraction and a mechanism based on the absorption of X-ray radiation in the lens material. The possibility of reducing X-ray absorption by switching to diffractive and kinoform lenses is being explored. A method for constructing an image in a diffraction lens with a 4-fold reduction taking into account X-ray absorption is presented. It was shown that to obtain image resolution in the range of 20-40 nm, composite kinoform lenses consisting of a small number (N=3-6) of elementary lenses can be used.