Ultra-high-aspect multilayer zone plates for even higher x-ray energies

Abstract

Penetration lengths in the millimetre range make hard x-rays above 60 keV a well-suited tool for non-invasive probing of small specimens buried deep inside their surroundings, and enable studying individual components inside assembled, complex devices (solar cells, batteries etc.). The real-space resolution of typical imaging modalities like fluorescence mapping, scanning SAXS and WAXS depend on the available beam size. Although routine in the 5-25 keV regime [1-4], spot sizes below 50 nm are very challenging at x-ray energies above 50 keV: Compound refractive lenses lack in refractive power, the multilayer thickness of coated mirrors is bounded by interfacial diffusion, and lithographic Fresnel Zone Plates loose their efficiency in the two-digit keV regime. Multilayer Laue Lenses and Multilayer Zone Plates (MZP) are promising candidates for high-keV focusing to small spot sizes; compared to Fresnel Zone Plates, the aspect ratio comparing outermost layer width (~focal spot size) to optical thickness (efficiency) is virtually unlimited by the fabrication. Using Pulsed Laser Deposition on a rotating wire (several millimetre long), we have fabricated an MZP with 10 nm outermost zone widths and optical thickness of 30 µm (optimum phase shift at 60 keV), yielding an unprecedented ultra-high aspect ratio of 1:3000 (outermost zone width compared to optical thickness). We present experimental results obtained at ESRF's high energy beamline ID31, where for the first time scanning experiments with real-space resolutions below 50 nm even at x-ray energies ranging from 60 keV to above 100 keV have been achieved.

Hard X-ray Microscopy Enhanced by Coherent Image Reconstruction

“…However the focal length of currently 1 mm at 15 keV results in short working distances. On the other hand, the fabrication allows the usage of MZPs at energies up to 100 keV [Ost+17b].…”

“…A first STXM experiment was realised in 1972 using a simple pinhole[HH72] but soon, in 1980, improved by zone plates as focusing optics in combination with piezoelectric translators[Rar+80] for a precise sample movement.The imaging method of STXM is today still used especially in combination with new high-resolution focusing optics such as the multilayer zone plates (MZP) presented in this thesis or multialyer laue lenses (MLL). These newly developed optics can focus an x-ray beam to a size of sub-10 nm (see chapter 4 and [Dör+13; Baj+18]) and have shown to operate at photon energies of up to even 100 keV[Ost+17b]. The improvements in fabrication of zone plates can be seen in Fig.2.3.…”

“…photon energies of 100 keV[Dör+13;Ost+17b]. For a photon energy of 13.8 keV, the focal length f is about 1 mm.…”