High‐Resolution Kinoform X‐Ray Optics Printed via 405 nm 3D Laser Lithography

Efficient focusing of X‐rays is essential for high‐resolution X‐ray microscopy. Diffractive X‐ray optics called kinoforms offer the highest focusing efficiencies in theory. However, they have long remained unavailable due to their challenging nanofabrication. Recently, various X‐ray optic geometries including kinoforms have been realized using 3D laser lithography at near‐infrared wavelengths. As the smallest features (period) of the kinoform determines the resolving power, there is a natural drive to find ways to fabricate kinoforms with ever smaller features. Here, a custom‐built 3D laser lithography setup with an excitation wavelength of 405 nm is used, which allows to half the smallest period of the kinoforms compared to previous work. A 40% improvement in scanning transmission X‐ray microscopy image resolution, that is, a cutoff resolution of 145 nm, and an efficiency of 7.6% at 700 eV is achieved. A reconstructed pixel size of 18.5 nm, reaching the limit imposed by the design of the microscopy set‐up, is demonstrated through ptychographic imaging of a magnetic sample which has a strongly reduced contrast mechanism. Moreover, X‐ray lenses manufactured by 405 nm 3D laser lithography have the potential to become much less expensive than X‐ray lenses made by other means.