Spiral zone plate imaging for soft x-ray microscopy

Phase sensitive x-ray microscopy techniques are important in the study of samples that exhibit phase contrast. One way to detect these phase effects is to optically implement the radial Hilbert transform by using spiral zone plates (SZPs), resulting in the imaging of the amplitude and phase gradient in a sample. This is similar to differential interference contrast imaging in light microscopy. Soft x-ray microscopy using a SZP as a single element objective lens was demonstrated through the imaging of a 1 μm circular aperture at a wavelength of 2.73 nm. A regular zone plate, a charge 1 SZP, and a charge 2 SZP were fabricated on a silicon nitride membrane using electron beam lithography. The negative e-beam resist hydrogen silsesquioxane (HSQ) was used for patterning, and the zone plates were electroplated with nickel. These zone plates were then used as the imaging optic in a soft x-ray microscopy setup.