High aperture diffractive x-ray and extreme ultraviolet optical elements for microscopy and lithography applications

Microscopy and lithography using isotropic plasma x-ray and extreme ultraviolet sources require condenser elements with high numerical aperture. As shown by coupled-wave theory, zone structures with high aspect ratios can diffract a sufficiently large proportion of incident radiation into a single high order. This enables new diffractive optical elements for these applications with significantly increased numerical aperture. Suitable zone plate materials are nickel for water window wavelengths and organic polymers for λ=13 nm radiation. Copolymer and nickel zone structures have been produced with aspect ratios of 15 and 11, respectively. For optimization of the etched zone profile, the reactive ion etching process with ${\mathrm{O}}_2$ was periodically interrupted. In a first experiment at λ=3.4 nm a nickel linear zone plate with 75 nm outermost zone width and 500 nm height showed a diffraction efficiency of 1% in the sixth diffraction order.