Hybrid zone plates: combining high resolution with enhanced efficiency

At XRnanotech, a spin-off from the Paul Scherrer Institut (PSI) in Switzerland, we develop diffractive X-ray optics to enable experiments at many large-scale research facilities. Our goal is to push the limits of diffractive X-ray optics by continuously, improving the resolution and efficiency enabling new applications in microscopy to make the invisible visible. In X-ray microscopy, Fresnel zone plates (FZPs) are used as high-resolution lenses. Their resolution depends mainly on the size of their smallest outermost zones and many years of development were required in order to push this value into ever-smaller regimes. Recently, the 10 nm barrier was broken with the line-doubling approach. While the zone-doubling approach is very helpful for achieving ultra-high resolutions, it has a fundamental disadvantage compared to conventional FZPs in terms of efficiency. The efficiency of a FZP depends not only on the wavelength, the material and its optical thickness but also on the duty cycle. For a FZP consisting of structures with a continuously changing period along its radius, the duty cycle of the Ir structures changes accordingly, because the deposited Ir thickness by ALD is uniform. Consequently, the efficiency along the radius changes as a function of the duty cycle and drops to zero towards the center of the zone plate. An elegant method to mitigate this effect is to fabricate hybrid FZPs that consist of a conventional center region with structures made e.g. of gold by electroplating and a zone-doubled outer part, where the deviation in duty cycle is not as pronounced anymore. Here, by fabricating such a hybrid structure, we achieved 7 nm resolution in direct imaging with soft X-rays and are able to fabricate X-ray optics with ultra-high aspect ratios.