  Diffractive optical elements such as periodic gratings are fundamental
devices in X-ray imaging - a technique that medical, material science and
security scans rely upon. Fabrication of such structures with high aspect
ratios at the nanoscale creates opportunities to further advance such
applications, especially in terms of relaxing X-ray source coherence
requirements. This is because typical grating-based X-ray phase imaging
techniques (e.g., Talbot self-imaging) require a coherence length of at least
one grating period and ideally longer. In this paper, the fabrication
challenges in achieving high aspect-ratio nanogratings filled with gold are
addressed by a combination of laser interference and nanoimprint lithography,
physical vapor deposition, metal assisted chemical etching (MACE), and
electroplating. This relatively simple and cost-efficient approach is unlocked
by an innovative post-MACE drying step with hexamethyldisilazane, which
effectively minimizes the stiction of the nanostructures. The theoretical
limits of the approach are discussed and, experimentally, X-ray nanogratings
with aspect ratios >40 demonstrated. Finally, their excellent diffractive
abilities are shown when exposed to a hard (12.2 keV) monochromatic x-ray beam
at a synchrotron facility, and thus potential applicability in phase-based
X-ray imaging.
