Recent advances in the fabrication of diffractive X-ray optics have boosted hard X-ray microscopy into spatial resolutions of 30 nm and below. Here, we demonstrate the fabrication of zone-doubled Fresnel zone plates for multi-keV photon energies (4-12 keV) with outermost zone widths down to 20 nm. However, the characterization of such elements is not straightforward using conventional methods such as knife edge scans on well-characterized test objects. To overcome this limitation, we have used ptychographic coherent diffractive imaging to characterize a 20 nm-wide X-ray focus produced by a zone-doubled Fresnel zone plate at a photon energy of 6.2 keV. An ordinary scanning transmission X-ray microscope was modified to acquire the ptychographic data from a strongly scattering test object. The ptychographic algorithms allowed for the reconstruction of the image of the test object as well as for the reconstruction of the focused hard X-ray beam waist, with high spatial resolution and dynamic range. This method yields a full description of the focusing performance of the Fresnel zone plate and we demonstrate the usefulness ptychographic coherent diffractive imaging for metrology and alignment of nanofocusing diffractive X-ray lenses.
We have employed ptychographic coherent diffractive imaging to completely characterize the focal spot wavefield and wavefront aberrations of a high-resolution diffractive X-ray lens. The ptychographic data from a strongly scattering object was acquired using the radiation cone emanating from a coherently illuminated Fresnel zone plate at a photon energy of 6.2 keV. Reconstructed images of the object were retrieved with a spatial resolution of 8 nm by combining the difference-map phase retrieval algorithm with a non-linear optimization refinement. By numerically propagating the reconstructed illumination function, we have obtained the X-ray wavefield profile of the 23 nm round focus of the Fresnel zone plate (outermost zone width, Δr = 20 nm) as well as the X-ray wavefront at the exit pupil of the lens. The measurements of the wavefront aberrations were repeatable to within a root mean square error of 0.006 waves, and we demonstrate that they can be related to manufacturing aspects of the diffractive optical element and to errors on the incident X-ray wavefront introduced by the upstream beamline optics.