Fabrication of silicon kinoform lenses for hard x-ray focusing by electron beam lithography and deep reactive ion etching

Abstract

The focusing of subnanometer wavelength x rays is limited by the ability to fabricate high-quality optics. In general, the resolution is of the order of the smallest feature of the optic, so nanometer spot sizes are extremely difficult to achieve with lenses made by traditional fabrication methods. In addition, gains in resolution for a given lithography limit are often made at a sacrifice of focusing efficiency. Kinoform lenses offer a compromise position of high resolution and efficiency. The object of this work is to describe the fabrication of kinoform lenses and to show how their unique properties could provide a path toward nanometer scale focusing. Fabrication is made easier by using higher order focusing and larger features. By combining 100keV electron beam lithography and deep reactive ion etching, the authors have fabricated cylindrical kinoform lenses in silicon. These lenses can be used in a crossed pair to produce a two-dimensional focus, but to maintain a large aperture and high resolution this requires etch depths of up to 100μm. Such large etch depths require careful consideration of lens design—feature sizes and densities can be changed with some latitude in the kinoform lens pattern without affecting the lens performance. Multiple lenses are fabricated in serial stacks to increase the resolution and provide a path forward to nanometer resolution.